Vibrational spectroscopy and theory of alkali metal...

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Surface Science Reports 68 (2013) 305–389

Vibrational spectroscopy and theory of alkali metal adsorptionand co-adsorption on single-crystal surfaces

A. Politanoa,n, G. Chiarelloa,b, G. Benedekc,d, E.V. Chulkovc,e,f, P.M. Echeniquec,e

aUniversità degli Studi della Calabria, Dipartimento di Fisica, 87036 Rende (Cs), ItalybConsorzio Interuniversitario di Scienze Fisiche per la Materia (CNISM), Via della Vasca Navale, 84, 00146 Roma, Italy

cDonostia International Physics Center (DIPC), P. Manuel de Lardizabal 4, 20018 San Sebastián—Donostia, SpaindUniversità Milano-Bicocca, Dipartimento di Scienza dei Materiali, 20125 Milano, Italy

eDepartamento de Física de Materiales and Centro Mixto CSIC-UPV/EHU, Facultad de Ciencias Químicas, Universidad del País Vasco, Apdo. 1072, 20080 SanSebastián—Donostia, Spain

fTomsk State University, Pr. Lenin 36, 634050 Tomsk, Russian Federation

Received in revised form 1 March 2013; accepted 30 June 2013

Abstract

Alkali-metal (AM) atoms adsorbed on single-crystal surfaces are a model system for understanding the properties of adsorption. AMadsorption, besides introducing new overlayer vibrational states, induces significant modifications in the surface vibrational structure of the metalsubstrate. Several studies of the vibrational properties of AM on metal surfaces have been carried out in last decades. Most of these investigationshave been performed for low coverages of AM in order to make the lateral interaction among co-adsorbates negligible. The adsorbed phase ischaracterized by a stretch (S) vibrational mode, with a polarization normal to the surface, and by other two modes polarized in the surface plane,known as frustrated translation (T) modes. The frequencies and intensities of these modes depend on the coverage, thus providing a spectroscopicsignature for the characterization of the adsorbed phases.The vibrational spectroscopy joined to an ab-initio theoretical analysis can provide useful information about surface charge re-distribution and

the nature of the adatom–surface bond, establishing, e.g., its partial ionicity and polarization. Gaining this information implies a significantadvancement in our knowledge on surface chemical bonds and on catalytic reactions occurring in AM co-adsorption with other chemical species.Hence, systematic studies of co-adsorption systems are essential for a more complete understanding of heterogeneous catalysis.The two principal experimental techniques for studying the vibrations of AM adsorbed phases are high-resolution electron energy loss

spectroscopy (HREELS) and inelastic helium atom scattering (HAS), the former being better suited to the analysis of the higher part of thevibrational spectrum, while the latter exploits its better resolution in the study of slower dynamics, e.g., T modes, surface acoustic phonons anddiffusive phenomena. Concerning AM co-adsorption systems, reflection–absorption infrared spectroscopy (RAIRS) has been also used (as well asHREELS) for obtaining information on the influence of AM adsorption on the vibrational properties of co-adsorbates.

www.elsevier.com/locate/surfrep

0167-5729/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.surfrep.2013.07.001

Abbreviations: 2DEG, two-dimensional electron gas; 3HeSE, 3He spin echo (spectroscopy); AES, Auger electron spectroscopy; AM, alkali metal; amu, atomicmass units; ASP, acoustic surface Plasmon; CDO, charge density oscillation; DFPT, density functional perturbation theory; DFT, density functional theory; DME,dimethyl ether; e–p, electron–phonon; EAM, embedded atom method; FC, force constant model; FK, Fuchs–Kliewer; HAS, helium atom scattering (spectroscopy);HREELS, high-resolution electron energy loss spectroscopy; INS, inelastic neutron scattering; IRAS, infrared reflection absorption spectroscopy; LDOS, localdensity of states; LEED, low-energy electron diffraction; ML, monolayer; OP, organ pipe mode; OR, overlayer resonance; QHAS, quasi-elastic He atom scattering;QWS, quantum well state; RAIRS, reflection–absorption infrared spectroscopy; RT, room temperature; S, perpendicular stretch mode (2, 3 indicate 2nd and 3rdovertones); SERS, surface enhanced Raman scattering; SHG, second harmonic generation; SFG, sum frequency generation; STM, scanning tunneling microscopy;T, frustrated translation; TOF, time-of-flight; TPD, thermal programmed desorption; TRSHG, time resolved second-harmonic generation; XPS, x-ray photoemissionspectroscopy

nCorresponding author. Tel.: +39 0984 496107; fax: +39 0984 494401.E-mail address: [email protected] (A. Politano).

In this review an extended survey is presented over:

a) the existing HREELS and HAS vibrational spectroscopic studies for AM adsorbed on single-crystal metal surfaces;b) the theoretical studies based on semi-empirical and ab-initio methods of vibrational structure of AM atoms on metal surfaces;c) the vibrational (HREELS, RAIRS, TRSHG) characterization of the co-adsorption on metal surfaces of AM atoms with reactive species.

& 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3082. A brief survey of surface vibrational spectroscopies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

2.1. HREELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3102.1.1. Low-energy HREELS spectrometers for investigation of elementary surface excitations . . . . . . . . . . . . . . . . . . . 311

2.2. HAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3112.3. QHAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3152.4. 3HeSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3172.5. RAIRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3192.6. SERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.7. SFG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

3. Basic concepts in the theory of surface excitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3223.1. Surface phonons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3223.2. Vibrational excitations of adsorbates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3243.3. Dynamics of AM on metals with the EAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

3.3.1. AM/Al(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3253.3.2. Na/ Cu(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3263.3.3. AM/Pt(111). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

3.4. Dynamics of AM on metal surfaces with ab-initio methods (DFPT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3304. Vibrational spectroscopy of adsorbed AM atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333

4.1. Single-adatom properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3334.1.1. Mass dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3334.1.2. Dependence on the adsorption site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3334.1.3. Dependence on surface indices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3344.1.4. Dependence on temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

4.2. From low coverage to one monolayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3354.2.1. Adatom–adatom interaction: dipolar and Lau–Kohn forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3354.2.2. Cs/Cu(100) monolayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3374.2.3. SHG studies on AM/Cu(111). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3384.2.4. HREELS studies on AM/copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3384.2.5. AM/Al(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3404.2.6. AM/Ni(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3414.2.7. AM/Pt(111). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3424.2.8. AM/Mo(100). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3434.2.9. AM/Ru(0001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3434.2.10. AM/graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3444.2.11. K/Si(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3454.2.12. K/GaAs(110). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346

4.3. AM multilayers: organ-pipe modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3465. Binary co-adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348

5.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3485.1.1. General considerations on AM co-adsorption with carbon monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3495.1.2. General considerations on AM co-adsorption with oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3495.1.3. General considerations on AM co-adsorption with water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3505.1.4. General consideration on AM co-adsorption with carbon dioxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350

5.2. On copper. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3505.2.1. AM+CO/Cu(111). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3505.2.2. AM+CO/Cu(100). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3525.2.3. AM+O/Cu(111). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

A. Politano et al. / Surface Science Reports 68 (2013) 305–389306

5.2.4. Na+O/Cu(110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3535.2.5. Na+OH/Cu(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3545.2.6. Li+H2O/Cu(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3545.2.7. K+CO2/Cu(110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354

5.3. On aluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3555.3.1. Cs+H/Al(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

5.4. On nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3555.4.1. AM+CO/Ni(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3555.4.2. AM+O/Ni(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3565.4.3. K+Ethylene oxide/Ni(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358

5.5. On platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3605.5.1. K+CO/Pt(111). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3605.5.2. K+OH/Pt(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3605.5.3. K+H2O/Pt(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3605.5.4. K+C2H4/Pt(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360

5.6. On ruthenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3615.6.1. K+CO/Ru(0001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3615.6.2. Cs+CO/Ru(0001). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3625.6.3. Cs+O/Ru(0001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3625.6.4. AM+H2O/Ru(0001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3635.6.5. K+CH3OH/Ru(0001) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363

5.7. On iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3645.7.1. K+CO/Fe(110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3645.7.2. K+N2/Fe(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364

5.8. On cobalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3645.8.1. K+CO2/Coð1010Þ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364

5.9. On rhodium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3655.9.1. K+NO/Rh(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3655.9.2. K+dimethyl ether on Rh(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365

5.10. On palladium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3665.10.1. Na+CO2/Pd(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3665.10.2. K+NO/Pd(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3675.10.3. K+C2N2/Pd(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3675.10.4. Cs+C2H4/Pd(110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

5.11. On gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3675.11.1. K+CO2/Au(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3675.11.2. K+nitriles/Au(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

5.12. On molybdenum carbide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3675.12.1. K+CO2/Mo2C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367

5.13. On graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3685.13.1. K+O2/graphite. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3685.13.2. AM+H2O/graphite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

5.14. On diamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3705.14.1. K+O/C(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3705.14.2. K+CO/C(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370

5.15. On silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3705.15.1. AM co-adsorption with organic molecules on Si(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370

6. Ternary co-adsorption systems and multilayered substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3726.1. K+CO2+H2O/Cu(111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3726.2. AM+CO+O/Ni(111). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3736.3. K+perylene-tetracarboxylic-dianhydride films on Ag(110). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3746.4. K+CO2 on copper films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3756.5. K+CO2 and CO on silver films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3756.6. AM co-adsorption on Mo2C/Mo(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

6.6.1. K co-adsorption with alcohols on Mo2C/Mo(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3756.6.2. K co-adsorption with C3H7 on Mo2C/Mo(100) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376

6.7. AM co-adsorption on Cr2O3(0001)/Cr(110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3766.7.1. Na+CO2/Cr2O3(0001)/Cr(110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3766.7.2. Na+NO/Cr2O3(111)/Cr(110) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376

6.1. AM co-adsorption with CO on bimetallic surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3776.2. AM co-adsorption on epitaxial graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

7. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

A. Politano et al. / Surface Science Reports 68 (2013) 305–389 307

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380

1. Introduction

The physical and chemical properties of supported AMlayers continue to attract the attention of researchers in surfacescience [1–41]. At the basis of this interest there are severalfundamental and practical motivations. Due to their simpleelectronic structure, AM have been used as model system todescribe surface chemical bonds [42–53], the interactionsbetween adsorbates [54–56], the electronic properties ofmetallic layers [13,19,27,57–68] and surface diffusion [69–79]. Moreover, long-lived excited states have been reported forAM overlayers on metal substrates [80–85]. The adsorption ofAM on metal substrates influences electron–electron [86–96]and electron–phonon [97–108] scattering in excited electronand hole states leading often to a change of the decaymechanisms of an excited electron or/and hole [109–112].

AM adsorption may induce a rearrangement of surfaceatoms of the substrate [4,113–124]. This leads to the modifica-tion of the substrate surface electronic states [125–127] andgives rise to adsorbate-induced electron states [10,128–143].The work function of the system [144–159] is significantlyinfluenced by the presence of AM. Adsorbed AM producechanges in surface dipoles and electric fields at surfaces [160–162] and charge transfers to the substrate [85,163–175]. Theunderstanding of AM adsorption should also imply a remark-able advancement in the comprehension of surface chemicalbonds and of catalytic reactions occurring in co-adsorption ofAM atoms with other chemical species [15,42,51,117,176–193]. AM-induced electrostatic fields have a crucial impact onthe activity and selectivity of a number of technologicallyrelevant catalytic processes [194]. Hence, systematic studies ofco-adsorption systems are essential for a more completeunderstanding of heterogeneous catalysis. Co-adsorption stu-dies of lithium [116,195–200], sodium [201–212], potassium[46,57,149,213–246], cesium [149,228,247–262] and rubi-dium [263–266] with simple molecules (CO, CO2, H2O, NOetc.) have been carried out in order to shed the light on the AMpromotion effect on surface chemical reactivity. A microscopicunderstanding of these and related phenomena is importantbecause of their impact on processes such as vibrationalexcitations, surface scattering, photochemical reactions andcharge and energy transport on surfaces.

Several previous reviews [267,268] or monographs [269]appeared on various aspects of this topic. The review byBonzel [268] is focused on adsorption energetics and kinetics,while Kiskinova [267] reviewed the promotional effects of AMadsorption. The last one (dated 1996) is by Diehl and McGrath[121] which reviewed the structure of AM co-adsorptionsystems. No review exists on vibrational studies on AMadsorption and their co-adsorption with molecules on metalsurfaces. In particular, our review is a natural continuation ofthat one by Diehl and McGrath [121]. In fact, structural

information on AM systems contained therein is used in thepresent review paper to finely analyze lattice and adsorbatedynamics in AM/metal systems.Several techniques have been used for studying AM vibrations

at metal surfaces: HREELS [1,16,26,190,205,212,270–284],SHG [107,272,285–288], RAIRS [289–296], SERS [297,298],HAS [5,73,74,299–309], and, more recently, SE-3He [69,310](see Table 1). They provide direct evidence of bond formation orbreaking and charge transfers at metal surfaces.Similar to the creation of QWS, AM adsorption gives rise to

new vibrational states related to the AM overlayer (adatoms)[11,34,69,278,340–342]. In the last decade extensive studies ofvibrational properties have been performed for AM on metalsurfaces. Many of these studies have been carried out for lowcoverages of AM in order to reduce the influence of lateralinteractions among adsorbates. A stretch (z-polarized) vibra-tional mode arises [314,343] upon AM adsorption. This modedoes not normally change its energy with increasing coverage[323], even if its intensity decreases [343]. On the other hand,the frustrated translation (T) mode [300,323], polarized in theplane parallel to the surface, changes its energy with thecoverage increase.Furthermore, significant advancement has been made in our

understanding of AM co-adsorption systems which could leadto a theoretical remodeling of AM interactions with reactiveco-adsorbates.Moreover, calculations of vibration spectra has been an

important method to determine the adsorption structures bycomparison with the experimental spectra. Herein we willattempt to provide a comprehensive guide to all the work inthis area of which we are aware.The review is organized as follows. In Sections 2 and 3

basic concepts in surface vibrational spectroscopies and in thetheory of surface excitations are reported, respectively. InSections 4 and 5 we discuss AM adsorption and binary co-adsorption on single-crystal surfaces, respectively. The litera-ture is organized by substrate and then further sub-divided intospecific adsorbate and co-adsorbate systems. The review ofeach system begins with a brief discussion of availableinformation on the electronic, structural or chemical propertiesof the system in order to help the reader. In Section 6 wereview available vibrational studies on ternary co-adsorptionand AM adsorption on multilayered substrates. Conclusionsand outlook are given in Section 7.Here the phonon energies (or the corresponding frequencies)

are normally given in meV or, in some cases, through thecorresponding wave-number in cm�1 (1 meV¼8.1 cm�1;1 cm�1¼0.124 meV). Less used are frequencies in THz(1 THz¼4.13 meV), angular frequencies in 1013 rad/s (¼6.55meV) and energies in degrees K (10 K¼0.86 meV). Theore-ticians often give electronic energies in atomic units (1 a.u.¼2 Ry¼1 hartree¼27.2 eV).


2. A brief survey of surface vibrational spectroscopies

Vibrational spectroscopy is unrivaled by any other methodsconcerning the chemical analysis of surface species due to itshigh sensitivity. As a matter of fact, depending on the vibrationalspectroscopy and the nature of the molecular vibration, the

minimum surface coverage required for the detection of a speciesranges from 0.001 to 0.1 of a monolayer.Adsorbed AM vibrational modes which have polarizations

perpendicular to the surface have been mainly studied usingHREELS, while measurements of the modes with polarizationsparallel to the surface have been investigated by HAS.

Table 1Adsorption structures and phonons in AM adsorbed systems.

Systems Coverages and structures Adsorption sites Phonons

Exp. Calc.

Li/Al(001) c(2� 2) Subst. EAM [25]Na/Al(001) c(2� 2) Hollow LT EAM [25]Li/Al(111) 0.03–1.0 ML Subst. RT Subst. HREELS [311] EAM [25]

(√3�√3) EAM [312]Na/Al(111) (√3�√3) Subst. HREELS [311,313] EAM [312],K/Al(111) (√3�√3) Subst. HAS [5]

DFT [314], FC [313], EAM [312], DFT [314]HAS [5]

Cs/Al(111) (√3�√3) Top low temp. HAS [5](2√3� 2√3) Subst. RT HAS [5]

Li/Cu(001) 0.04–0.8 ML On-surf. HREELS [315]c(2� 2) Hollow EAM

Na/Cu(001) 0.1 ML Hollow HAS [303]0.0–0.4 ML HREELS [316]0.0–0.5 ML HAS [11,300,307]0.05 ML HAS [55]c(2� 2) Hollow EAM

Quasi-hex HAS [304,317]K/Cu(001) 0.07 ML Hollow HAS [55]

0.02–0.11 ML HREELS [316]Quasi-hex HAS [318]

Cs/Cu(001) 0.08 ML On-surf. HAS [55]0.27 ML Quasi-hex. HAS [319]

Li/Cu(110) 0.04–0.8 ML On-surf., reconstr. HREELS [315]Na/Cu(110) 0.13 ML On-surf., reconstr. HREELS [320]K/Cu(110) 0.02–0.32 ML On-surf., reconstr. HREELS [320]Li/Cu(111) 0.025–0.5 ML On-surf. HREELS [321]Na/Cu(111) 0.075–0.3 Hollow HREELS [321,322]

p(3� 3), p(2� 2) Hollow EAM [323](√3�√3)(3/2� 3/2)1 ML Jellium model [324], DFT [325]

K/Cu(111) 0.02–0.27 ML On-top HREELS [316]0.08–0.4 ML On-top HREELS [321,322]

Cs/Cu(111) 2–5 ML Quasi-hex. HAS [299]K/Ni(001) 2–5 ML HAS [299] [326] [327]K/Ni(110) 0.2 ML On-surf. HREELS [328]Na/Ni(111) 0–0.20 ML On-surf. HREELS [329]K/Ni(111) 0–0.20 ML On-surf HREELS [329]Na/Pt(111) 0.05 ML On-surf. HAS [302]K/Pt(111) 0.02–0.15 ML Hollow HREELS [330,331]

(2� 2) EAM(3� 3) TRSHG [332] EAM, DFT [333]

Cs/Pt(111) 0.22–0.41 ML On-surf. TRSHG [285]Na/Mo(001) 0.0–0.45 ML On-surf. HREELS [334]Li/Mo(001) (1� 1) On-surf. HREELS [335]Cs/Ru(0001) c(2� 2) On-top RT

0.03–0.24 ML HREELS [336]0.08–0.25 ML HREELS [337]

Li/W(110) HAS [338]Cs/HOPG(0001) p(2� 2) HAS [309,339]K/HOPG(0001) p(2� 2) HAS [309,339]Rb/HOPG(0001) p(2� 2) HAS [309,339]


Perpendicular modes have large dynamic dipole componentsso as to render more suitable HREELS, while parallel modeshave lower energies which are more accessible with HAS.

2.1. HREELS

In HREELS experiments, a primary electron beam impingeswith energy Ep onto a metal or semiconductor surface and thescattered beam emerges with energy Ep�Eloss where Eloss isthe loss energy, that is the energy lost by the electron towardselementary excitations of the surface. They include one ormore phonons of the surface or of the adsorbed species, as wellas single-particle transitions or surface plasmons (see [344–346] for a review).

For moderate incident energies and small deflections of thescattered beam from the specular direction the inelastic event isinduced at fairly large distances from the surface. This processis called dipole scattering as the long-range electric field ofprimary electrons interacts with the fluctuating dipolar fieldassociated with the induced surface charges. At larger incidentenergies the energy loss occurs mainly through impactscattering, which takes place in the close vicinities of ioncores and the scattering intensity is not peaked in the speculardirection.

Specular energy loss intensities in HREELS arise mainly fromdipole scattering. Hence, HREELS performed in the speculargeometry gives basically the same information as RAIRS. For thedipole scattering mechanism the parallel components of dipolemoments are perfectly screened by their image dipoles on metalsurface. Thus, only vibrations that bear a dipole momentperpendicular to the surface could be excited. In the frameworkof group theory, the surface selection rule therefore states thatonly the modes belonging to the total symmetric representation A′(Cs-group) and A1 (Cnv groups) are active in inelastic electronscattering via the dipole scattering or in surface IR-spectroscopy.For all practical purposes, the selection rule applies also tosemiconductor surfaces.

The inelastic interaction could be treated as a classicalenergy loss of a charged particle reflected from a surfacewithin the framework of the dielectric theory of inelasticelectron scattering. The system is represented by its complexdielectric functions ε(ω) or its complex dynamic polarizabil-ities α(ω), respectively. The loss probability P(Q, ω) isproportional to

PðQ;ωÞp ε2ðωÞ��εðωÞ þ 1��2

¼ Im�1

εðωÞ þ 1ð1Þ

where Q is the parallel wave-vector of the elementaryexcitation. Conservation of both energy and parallel momen-tum leads to

Eloss ¼ Ei�Ef ð2Þ

ℏQ ¼ ℏðKi�Kf þGÞ ð3Þwhere Ef is the energy of the scattered electron beam, Ki and Kf

the parallel components of the incident and final wave-vectors

ki=(Ki, kiz) and kf=(Kf, kfz), respectively, and G is a surfacereciprocal lattice vector. Under the kinematic conditions dis-cussed below no umklapp scattering process is considered(G=0). For planar scattering, Ki=ki sin θi and Kf=kf sin θf, withθi the incidence and θf the final angles with respect to the surfacenormal, conventionally fixed along the z-axis. (Fig. 1). From theabove equations it is possible to calculate the wave-vectortransfer Q for planar scattering as a function of the energy loss:

Q¼ffiffiffiffiffiffiffiffiffiffiffi2mEi

p

ℏsin θi�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1�Eloss

Ei

rsin θf

� �ð4Þ

According to Mills [347], the differential cross sectiond2S=dωdΩ, is given by

d2S

dωdΩ¼ ðmev? Þ2

2π2ℏ5

Kf

kiz

PðQ;ωÞQ2

��v?QðRf þ RiÞ þ iðRf�RiÞðω�vjjQÞ��2

½ðv?QÞ2 þ ðω�vjjQÞ2�2ð5Þ

where vjj are v? the parallel and perpendicular components ofthe velocity of impinging electrons with respect to the surface,respectively, and P(Q;ω) is the surface loss function. Ri and Rf

are the amplitude of the complex reflectivity for initial andfinal energies. The maximum inelastic scattering occurs forω¼ νjjQ, in correspondence of a minimum in the denominatorof (5). Such condition corresponds to the interaction ofelectrons with partial waves with phase velocity νjj ¼ω=Q.By defining δ as the deviation from trajectory of electronsinelastically scattered from specular direction, thus for ℏω{Ei

and δ{1, the denominator of (5) may be written as

ðv?QÞ2 þ ðω�vjjQÞ2 ¼ 4E2i ðδ2 þ Ψ 2

EÞ cos 2θi ð6Þ

where ΨE ¼ ℏω=2Ei. Eq. (6) determines the angular depen-dence of dipole scattering and its concentrations in a lobe withsemi-amplitude ΨE along specular directions. Dipole scatteringdominates for small transfer momenta. As shown in Fig. 2, aprincipal maximum exists, corresponding to the conditionω¼ vjjqjj, although also a secondary maximum does occur.For short-range interactions impact scattering occurs.

Within this scattering mechanism, electrons are diffused inevery possible solid angle, even beyond the incidence plane.Both perpendicular and parallel component of the wave-vector(with respect to the sample normal) are not conserved. As aconsequence of such complexity, theory which describes suchinteractions is not deeply developed. The cross section is

Fig. 1. Scattering geometry in HREELS experiments.


defined as [345]

dsdΩ

¼ mEi cos 2θf2π2ℏ2 cos θi

��M��2 ð7Þ

where M is the matrix element for the transition and m is themass of electrons. The cross section presents only minimalchanges with scattering angle.

2.1.1. Low-energy HREELS spectrometers for investigation ofelementary surface excitations

HREELS is the most powerful tool for investigating thedispersion and the damping of collective excitations at metalsurfaces (phonons [349–351] and plasmons [352–368]) as wellas the adsorption of chemisorbed atoms and molecules[369,370]. Spectrometers of the last generation have beendesigned by Harald Ibach [371] and shown in Fig. 3, areconstituted by a two-step monochromator and by a rotatinganalyzer with 1511 cylindrical deflectors. This results in anultimate resolution of 0.5 meV and a significant increase ofintensity in the high resolution range. The basic concept of thisspectrometer is a fixed double stage monochromator and arotatable single stage analyzer. In order to allow the probing ofthe largest possible fraction of the surface BZ the maximumrotation angle of the analyzer stage was increased to 4 901.The impact energy is variable from 0 to 250 eV.

Dispersion of the collective mode, i.e. Eloss(Q) is usuallymeasured by moving the analyzer while keeping the sampleand the monochromator in a fixed position, but whenever theanalyzer cannot rotate, the SP dispersion is measured throughchanging the incident angle.The quality of spectra is determined also by the angular

acceptance of the spectrometer which in turn affects Qresolution. This is achieved by low impinging energies (below50 eV) and for grazing scattering conditions. An interestingpossibility to reduce the window in the reciprocal space isgiven by ELS-LEED, which applies the spot profile analysiscommonly used in LEED to the inelastic signal so as toachieve a Q resolution of 10�3 Å�1 [37,372–377].

2.2. HAS

Like HREELS in the impact regime, HAS is a powerful toolfor investigating the dispersion relation of surface phonons[378–395]. Conventional supersonic 4He beams with incidentenergy Ei in the 10 to 100 meV range allow for a resolution ofthe order of Ei/ΔE�102, which allows to explore the lowacoustic range of surface phonons, hardly accessible toHREELS [396]. Although atom scattering spectroscopy basedon supersonic nozzle sources at thermal energies is suited toexplore the lower part of the phonon spectrum, there ishowever no conceptual obstacle to produce high-energyneutral atom beams with a good speed ratio, even in the keVrange [397–399], for a surface spectroscopy of more energeticexcitations [399]. In a phonon creation process the upper limitof the phonon energy detectable by HAS is the incident energyitself; energy transfers as large as 80% of the incident energyare distinctly observable [400]. On the other side of the energyspectrum, the recent development of spin-echo 3He atomspectroscopy allows to investigate slow surface dynamicalprocesses like molecular translational (T) modes and diffusion,with a spectacular resolution of 20 neV [401].There is another important difference between HAS and

HREELS. While electrons in the impact regime are scatteredfrom the high electron density at ion cores, He atoms at

Fig. 3. HREELS spectrometer Delta 0.5, designed by Ibach [371].

Fig. 2. Schematic plot of the kinematic factor in the dipole scatteringprobability function, showing that the inelastically scattered electrons areconfined within two lobes near the specular reflection direction. Adaptedfrom Ref. [348].


thermal energies are repelled from a much lower electrondensity (in the range of 10�4 a.u.) a few Å outside the surface.Thus He atoms transmit energy to the phonons of the systemthrough the phonon-induced modulation of the surface chargedensity, i.e., through the e–p interaction [378]. In metals, duethe long range nature of the electron–phonon interaction, Heatoms can perceive the oscillations of deep atoms beneath thesurface which are transmitted by electron density wavesthrough the Fermi sea (quantum sonar), and measure the e–pcoupling constant for each individual phonon (mode-lambdaspectroscopy) [402]. Similar information can now be obtainedfor AM overlayers via the phonon-induced oscillations of theQW charge density [403].

Helium has a number of important advantages for surfacescattering experiments [404,405]. First of all He atoms atthermal energies have a de Broglie wave-length just compar-able with the atomic dimensions (between 0.5 Å and 1.5 Å).Moreover it is extremely inert and is not expected to stick toany surface at temperatures greater than about 5–10 K. Free jetexpansions, which are generally used to produce intensemolecular beams, show particularly advantageous propertiesin the case of helium [406]. For one because of its weakinteraction potential it has the lowest of all heats of vaporiza-tion and therefore still has a sizeable vapor pressure at 1 bar at

4.2 K. In addition rather unique quantum effects arise becauseof the very weak He–He inter-molecular potential. As aconsequence inside the adiabatically expanded beam gas thescattering cross section increases from about 30 Å2 at RT toabout 2.6� 105 Å2 as the ambient translational temperature ofthe beam gas (associated with the velocity fluctuations withrespect to the mean translational energy) is cooled to ultra lowtemperatures approaching T1 ¼ 10�3 K. These large crosssections are, in fact, responsible for driving the expansion in itsfinal stages to much lower temperatures than in other gases. Atthe same time these same weak interactions also inhibit clusterformation which, because of the heat released, can spoil theexpansion and contaminate the atom beam with clusters. Thesetwo effects, giant cross sections and reduced clustering, bothexplain the extremely low ambient translational temperaturesin the range of 1 to 10 mK which can easily be achieved insidethe expanded helium gas. Of primary importance for thesurface scattering experiments are the resulting very narrowvelocity half-widths of about Δv/v E (T1 /T0)

1/2E0.5%[406]. Thus energy widths of 0.2 meV and 0.08 meV havebeen achieved for gas cells operating at liquid nitrogen andliquid helium temperature, respectively.Another important advantage of helium is that it is easily

detected in a mass spectrometer with an especially low

Fig. 4. Schematic view of the HUGO helium scattering apparatus used in Göttingen [379]. (a) The nearly mono-energetic helium beam is emitted from the nozzle(1) and after passing through a skimmer (2) and scattering from the sample (4) a part of the scattered intensity enters the time-of-flight chambers (from P5 to P8) andis detected by the mass spectrometer ionizer (5). Rotation of the sample and/or the detector arm allows the angular distribution of scattered He atoms to be mappedout. For time-of-flight studies the beam is pulsed by passing it through a rapidly rotating chopper disc with a few equally spaced narrow slits (3). P1 to P9 aredifferential pumping stages which provide for a reduction of the helium partial pressure in the detector chamber with respect to the source chamber by over 12orders of magnitude. The scattering chamber is equipped with various in-situ surface treatment and analysis devices. The inset (b) shows the (in-plane) scatteringgeometry. An incident beam of wave-vector ki and incidence angle θI with respect to the normal to the surface n is scattered into a final direction of final angle θfwith a wave-vector kf. While the initial energy distribution is sharply peaked at Ei the final distribution, besides the elastic scattering peak at Ei, shows inelasticfeatures corresponding to phonon creation and annihilation processes.


background signal, which is the reason why it is extensivelyused in commercial leak detectors. Finally, because of its lowmass and the possibility to go to low source temperatures andbeam energies, scattering conditions especially favorable to thedominant excitation of only single phonons are easilyachieved.

The energy of the beam can be calculated by thermody-namics. During the expansion process, energy must be con-served. Since the pressure inside the source chamber is keptconstant, in the limit that the velocity perpendicular to thebeam direction is vanishingly small, the kinetic energy peratom in the beam is equal to the enthalpy per atom in thesource:

Ei ¼Mv2J2

¼ 52kBT0 ð8Þ

where M is the mass of the atom, v J the axial component ofvelocity, and T0 the source temperature. A liquid-nitrogencooled nozzle source yields a beam of 16.6 meV.

The analysis of the diffraction peak intensities observed inthe angular distribution of scattered He atoms provides thecorrugation function of the surface, which essentially corre-sponds to the profile of the atom–surface repulsive potential[407,408]. In general a corrugated surface provides thereciprocal surface lattice vectors which allow He atoms to betrapped into a surface bound state – a process which leaves asignature in the angular distribution (surface resonances) andpermits to determine the attractive part of the atom–surfacepotential. The combination of the diffraction data with thenormal surface profile derived from bound-state resonancesyields a detailed knowledge of the surface structure as appearsat the turning points of the scattered atoms [407]. Usefulinformation on the ordered structures of AM submonolayerson a metal substrate, as well as on the related re-arrangementof the surface electron charge density, has been obtained in thisway with HAS. A few examples are illustrated below inSection 4.2.1. Once the surface structure and the scatteringpotential have been characterized, the surface phonon disper-sion curves can then be obtained from the TOF spectra of thescattered He atoms, after conversion to an energy-transfer scale[342,404,409,410]. Also the surface phonon dispersion curvesfor various ordered submono-, mono- and multi-layers of AMatoms on metallic substrates have been measured in this way(Refs. [299,319] and Table 1).

A schematic view of a typical HAS apparatus is displayed inFig. 4(a), together with a description of the kinematics (inset(b)). The HAS kinematics is similar to that of HREELS, exceptthat He atoms at thermal energies carry sufficiently largemomentum to allow also for umklapp scattering processeswhere surface reciprocal lattice vectors G may be exchanged.In this case the energy and parallel momentum conservationlaws for one-phonon processes read

ΔE� ℏ2

2Mðk2f �k2i Þ ¼�ℏωðQÞ ð9Þ

ΔK�Kf�Ki ¼�ðGþ QÞ ð10Þ

where kf and ki are the final and incident wave-vectors of theHe atoms, Kf and Ki are the respective components projectedon the surface plane (Fig. 4(b)), and ℏωðQÞ is the energy of aphonon of wave-vector Q defined within the first surfaceBrillouin zone. The events with ℏωðQÞ>0 and ℏωðQÞo0 arereferred to as phonon creation and annihilation scatteringprocesses, respectively. For the interpretation of the HASexperiments it has proven useful to consider the so-called scancurves connecting all possible values of ΔE¼�ℏω andΔK¼�ðQþGÞ which are accessible for a given incidentenergy and set of incident and final scattering angles. Thecorresponding equation, similar to Eq. (3), is obtained bycombining Eqs. (5) and (6) in the form

ΔE

Eiþ 1 ¼ sin 2θi

sin 2θf1þ ΔK

Ki

� �2

ð11Þ

As shown in the example of Fig. 5 for a quasi-hexagonalmonolayer of Cs on Cu(001) [319], the scan curves corre-sponding to the experimental HAS energy-transfer spectra ofFig. 5(b) allow to plot the data point in the (Q, ℏω)-plane(Fig. 5(a)) associated with the observed peaks. In this waythree acoustical branches and an optical resonance of theoverlayer/substrate system can be resolved with a resolutionbetter than 1 meV.

Fig. 5. (a) The experimental HAS dispersion curves (○) for a monolayer ofquasi-hexagonal close packed Cs on Cu(001) in the ⟨110⟩ direction of thesubstrate [319]. The data points (�) intersected by the three scan curves,labeled by the respective incident angles θi¼411, 401 and 381, correspond tothe peaks L, R and S observed in the corresponding energy-transfer spectra (b).The diffused elastic scattering peaks E in (b) originate from surface defects.The calculated scan curves correspond to the experimental incident energyEi¼29 meV and to a planar 901 scattering geometry (θi+θf¼901). Thebranches L and S are assigned to the longitudinal and shear-vertical modesof the Cs monolayer, respectively. The R branch tends in the long-wave limitto the Rayleigh wave of the clean surface (RW), whereas at short waves, due toan avoided crossing with the S branch, converts into a shear vertical Cs mode,while the S branch tends asymptotically to the RW branch. The vertical dash-dotted line indicates the zone boundary of the Cs-layer quasi-hexagonalBrillouin zone in the ⟨110⟩ direction; the full lines interpolating the data pointsare obtained from a force-constant fit. Adapted from Ref. [319].


In the distorted-wave Born approximation (DWBA) thetransition probability of a He atom from an initial state ofwavevector ki to a final state of wavevector kf for a scatteringprocess from a flat monoatomic surface at T¼0 K, in whichone phonon of energy εQv, parallel wavevector Q and branchindex ν is created, is given by (up to a constant factor)[411,412]

Pðki; kf Þp kfjkizj

∑QvjFf i UuQvj2δðΔE þ ℏωQvÞ ð12Þ

where uQv is the surface phonon displacement vector. In themore general case of He-surface interaction extending tosubsurface layers (labeled by index l>0) of a polyatomiccrystals, uQv is defined in the extended vector space (l,κ,α),where α is a Cartesian index, l labels the atomic layers, and κthe atoms in the crystal unit cell. The force Ffi exerted by theHe atom on the crystal atoms acts in the same space, and thescalar product Ff i UuQv is the atom–phonon coupling energy.

It should be remarked that the interaction between the probeatom and the phonon atomic displacements has to be treated onthe same foot as the interactions between the crystal atomswhich govern surface dynamics. In closed-shell ionic crystalsor rare-gas solids, lattice dynamics is based on two-bodyinteratomic forces, only weakly perturbed by ion (atom)polarization effects, which may be accounted for by additionalvalence electron shell coordinates (shell models). Similarly theatom scattering from ionic (or rare-gas solid) surfaces is welldescribed by direct two-body interatomic potentials and thecorresponding force

Fatf i ¼ kf ; z�� iQ;

∂∂z

� �vlκðQ; zÞ

��ki; z� �

ð13Þ

where��k; zi are the distorted He-atom wavefunction compo-

nents for the motion normal to the surface and vlκðQ; zÞ thetwo-dimensional Fourier transform of the two-bodypotentialvlκðrÞ between He and the κth ion (atom) in the lthlayer. The coupling force Fatf i is fast decaying for increasing l,so that the interaction with insulator surfaces is normallyrestricted to the surface layer (l=0). Moreover Fatf i decaysexponentially for an increase of both parallel (ΔK) and normal(Δkz) components of the momentum transfer, the faster decayoccurring for a softer interatomic potentialvðrÞ. This fact,already established by Zener [413,414] and by Jackson andMott in the thirties for an exponential soft wall potential [415],causes a similar decay of Pðki; kf Þ at larger energy transfers.The relationship of the decay with the nature of the interatomicpotentials is not trivial, however. For example the addition of avan der Waals attraction to the hard-core exponential repul-sion, while extending the range of the potential, yields a slowerdecay of Pðki; kf Þ due to the acceleration of the probe atomswhen entering the attractive well (Beeby effect) [411,412,416–418]. In a shell-model picture of surface dynamics therepulsive interaction of the He atoms with the surface is dueto the overlap with the surface ion (atom) shell and theirinteraction with phonons should be mediated by the shell–coreforce constants (inverse polarizabilities). This does not appearto be necessary in the calculation of HAS intensities, other

effects like bound-state inelastic resonances being considerablymore important in the scattering from corrugated surfaces likethose of closed-shell solids.Not so on the other extreme of He scattering from metal

surfaces. In the specific case of free-electron surfaces and, asfar as concerns the present review, alkali overlayers on ametallic substrate, the conventional HAS theory based ondirect atom–atom interactions is irrealistic. Surface atoms areshielded by a quasi 2DEG. The flying-by He atoms just ticklethe surface about 0.3 nm above the first atomic layer, and cantherefore transmit vibrational energy to the surface atomsmostly via the interposed free electrons, very much in thesame way effective interionic forces are described in the latticedynamics of free-electron metals. In this case the inelastic Hescattering intensities are essentially weighed by the electron–phonon interaction, and the range of interaction of the He atomwith the crystal atom displacements is that of the electron–phonon interaction. This fact explains the otherwise surprisingability of HAS to detect in thin metal films also subsurfacephonons eventually localized several layers beneath the surface[402,403]. The mechanism is schematically illustrated in thediagram of Fig. 6: the atomic motion in a deep atomic layer(phonon Qν) produces, via the electron–phonon interaction, acharge density oscillation extending up to the surface, which isperceived by a flying-by atom. As a consequence the atom isinelastically scattered from the initial state ki into the finalstate kf.

Fig. 6. The mechanism by which a He atom impinging on the metal surfacewith incident momentum ki is inelastically scattered into a final state kf bycreating a phonon Qν several layers beneath the surface. The atomic motion ina deep atomic layer generates, via the electron–phonon interaction g, a virtualelectron–hole pair, i.e., an electronic transition around the Fermi level fromstate Kn to state K′n′. The associated charge density oscillation (red and bluecontour lines) extends up to the surface and causes the inelastic scattering of aflying-by atom from the initial state ki into the final state kf. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)


Thus the HAS one-phonon transition probability due to thiselectron-mediated mechanism retains the form of Eq. (12)where the He-atom–phonon coupling can be expressed by[402]��Felf i UuQν

��2 ¼ IðΔEÞλQν ð14Þ

the coefficient λQν is the electron–phonon coupling strength forthe specific phonon (Q,ν) (the mode-lambda) [419] and I(ε) isa slowly-varying function of the energy. Their dimensionlessproduct is given by

IðεÞ λQνffi2

NðEFÞε2∑Kn

��∑n0gnn0 ðK;KþQ; νÞ

⟨f jψn

KnðrÞψKþQn0 ðrÞji⟩��2 ð15Þ

where N(EF) is the density of states at the Fermi energy EF,and

gnn′ðK;Kþ Q; νÞ ¼ Vðqnn′Þqnn′ UuQνexp½�12⟨ qnn′ UuQνj2⟩��

ð16Þis the electron–phonon matrix element connecting an electronicstate of band index n, wavevector (K, kn) and wavefunction ψKn toa state of band index n′, wavevector (K+Q, kn′) and wavefunctionψKþQn′ [420]. V(qnn′) is the Fourier transform of the electronpseudopotential and the wavevector qnn′ � ðQ; kn�kn′Þexp½iðkn�kn′Þzlκ� acts on the (l,κ,α)-space, with zlκ the depth of the (lκ)thatomic layer. The electronic wavefunction product inside the

matrix element between the final f��Dand initial

��ii states of the

He atom acts as an effective scattering potential for the inelasticprocesses. Thus the inelastic scattering probability for a givenphonon is weighed by the respective mode-selected electron–phonon coupling. Due to the particular structure of the effectivepotential, the inelastic scattering intensities may drop at largerenergy transfers more slowly than for the direct atom–atomcollision model.

All intermediate cases between closed-shell surfaces andfree-electron metal surfaces, like those of covalent semicon-ductors, transition metal compounds or even transition metals,may require for a better approximation the inclusion of bothdirect and electron-mediated He-phonon forces:

jFf i UuQvj2 ¼ jFatf i UuQvj2 þ jFelf i UuQvj2 ð17Þbefore the advent of DFPT, consistent calculations of surfacedynamics and inelastic HAS amplitudes could be made fornoble metal surfaces with the multipole expansion method andits parametrized form known as the pseudocharge model[421,422]. In this approach both direct and indirect (viaconduction electrons) terms of Eq. (17) were shown to benecessary for an accurate description of the HAS intensities.

2.3. QHAS

At a low coverage of adsorbed atoms surface diffusion is animportant component of their dynamics intervening in manyimportant phenomena. Diffusion is triggered by surface vibra-tions, mostly by the adatom vibrations parallel to the surface

(frustrated translational modes). Frenken, Toennies and Wöllhave shown in 1988 [423,424] that HAS also provides informa-tion on the microscopic diffusion of single atoms along thesurface. The presence of random adsorbates yields a diffuseelastic peak (E-peak in Fig. 5(b)) of intensity proportional to thecoverage. The non-instrumental part of the E-peak energeticbroadening Γ (FWHM) of the is due to the Doppler shift in thescattering from single moving adsorbed atoms, and its depen-dence on temperature and parallel momentum transfer allow todetermine the adatom diffusion coefficient and its temperaturedependence. This kind of spectroscopy is generally referred to asquasi-elastic HAS (QHAS).The QHAS method is an adaptation to the surface of the

well-known method of neutron quasi-elastic scattering, whichhas been extensively applied to studying diffusion in the bulk[425–427]. It should be noted that both techniques do notmeasure directly the diffusion coefficient but rather a correla-tion function S(ΔK,ΔE). This is the Fourier transform of thetime-dependent pair correlation function G(R,t) which pro-vides a complete description of the dynamical behavior of theensemble of diffusing particles. G(R,t) is the sum of the self-correlation function Gs(R,t), describing the behavior of aspecific particle, and a second term Gd(R,t), which describesthe pair correlation between distinct particles. Gs(R,t) is ofparticular interest since it provides information on the micro-scopic tracer diffusion coefficient. The extension of the theoryfor the neutron case [426–428] to helium scattering fromspecies diffusing along a crystal surface was first developed byLevi and co-workers [429] and reviewed by Frenken andHinch [430]. The resulting expression for the transitionprobability is given by

Pðki; kf ÞpndF2∬GðR; tÞeiðΔK UR�ωtÞd2Rdt

pndF2SðΔK;ΔEÞ ð18Þ

where nd is the surface concentration of diffusing particles andF is a constant atomic form factor. Thus, by measuring theenergetic broadening Γ(ΔK) for a wide range of wave-vectors,the Fourier transform S(ΔK,ΔE) of the time-dependent paircorrelation function is determined directly. Measurements atsmall wave-vector transfers therefore provide information oncorrelations over long distances ℓ�2π=ΔK and, similarly,events with small energy transfers ΔE provide informationover long times th/Γ. With the present resolution of a HASapparatus broadenings down to ΓE10 meV (to4� 10�12 s)and ΔK>0.03 Å�1 (ℓo200 Å) can be explored [431].Due to the large scattering cross sections from single AM

atoms on smooth metal surfaces [432], even coverages as lowas 1% can be studied with the QHAS technique. At lowcoverages only the self-correlation function Gs(R,t) is impor-tant and in this case simple formulas are available relating theenergetic half-width Γ to the adatom diffusion coefficient D.The half-widths for jump diffusion is given by [428]

ΓðΔKÞ ¼ 4ℏ∑iτ�1i sin 2ðΔKUai=2Þ ð19Þ

where τi is the inverse of the jump rate in the direction of thejump length vector aj. Clearly the method is sensitive to the


direction of the diffusive jumps since the jump rate τ�1i

depends very much on both the jump length and thecorresponding potential barrier height. For one preferentialjump direction and small momentum transfers in that direction,Eq. (19) reduces to

ΓðΔKÞffiℏa2i τ�1i ðΔKÞ2 ¼ 2ℏDðΔKÞ2 ð20Þ

This corresponds to the continuous diffusion limit [428] andallows for a direct estimation of the diffusion coefficientD¼ a2i =2τi from the measurement of ΓðΔKÞ. The activationbarrier for diffusive motion Ediff is then obtained from thetemperature dependence of D expressed by the Arrhenius law:

D¼Doexpð�Edif f =kTÞ ð21ÞFig. 7 shows some examples of QHAS measurements for

the diffusion of Na adatoms on Cu(001) at low coverage,where a nearly pure single jump mechanism is observed[301,303,306]. The change in width Γ(ΔK) of the quasi-elastic peak with the parallel momentum transfer (Fig. 7(a)) isobtained by deconvoluting the peak shape measured at thegiven temperature with the peak shape measured at a lowtemperature, where the diffusion-induced peak broadening isvanishingly small and only the intrinsic instrumental widthremains [431]. The deconvoluted Γ(ΔK), plotted as function ofΔK in Fig. 7(b), illustrates the diffusion dynamics of Naadatoms on Cu(001) at different temperatures, for a coverageof 0.028 ML for the ΓΜ direction and 0.047 ML for the ΓΧdirection. The solid lines show the result of a Langevinmolecular dynamics simulation of Γ(ΔK) as given by Eq. (20).

A more detailed analysis of the Na/Cu(001) experimentsrelied on a molecular dynamics simulation based on theLangevin equation for an assumed model for the potentialhypersurface describing the lateral motion of the Na atomswithin a unit cell [433,434]. The simulations reveal that thedata are very sensitive to the detailed shape of the lateralpotential. The reliability of the potential fitted to the diffusiondata (Fig. 7) was subsequently confirmed by successfullysimulating both the measured temperature dependence of theT-mode energy and its width without an additional adjustmentof the potential parameters (see Section 4.2) [301,307].At high temperatures the diffusion barriers measured by

QHAS agree with those deduced from other more conventionalmethods. In all cases in which the measurements are carriedout at RT or below, the activation barriers are consistentlyquite smaller than in previous measurements. For example forNa on Cu(001) from the dependence of the peak width on thesurface temperature between 180 K and 450 K the diffusionbarrier was found to be only Ediff=54 meV. This barrier ismuch smaller than the barrier energies of about 1 eV found forother alkali-metal systems [435]. The pre-exponential diffusionconstants Do, Eq. (21), are generally much larger thanpreviously reported, providing some compensation for thedifferences in the barrier energies. These differences toprevious results are attributed to the relative insensitivity ofthe QHAS method to the impeding effect of surface defects.For example, the much smaller value of Ediff for Na on Cu(001) was confirmed for potassium on Pd(111) by a photo-electron emission microscopy technique. With this technique it

Fig. 7. (a) A series of HAS energy-transfer spectra for an incident energy of 11.2 meV showing only the quasi-elastic central peak from a 2.8% monolayer coverageof Na atoms on Cu(001), for increasing parallel momentum transfers ΔK along the symmetry direction at T¼300 K. The open circles show the experimental points.The solid line through the data points is a convolution of the instrument response function (dashed line) with a Lorentian-shaped peak fitting the quasi-elasticbroadening of the central peak. (b) The FWHM of the deconvoluted quasi-elastic peak as a function of ΔK illustrates the diffusion dynamics of Na adsorbates on Cu(001) at different temperatures T¼200 K, 250 K and 300 K; the coverage was 0.028 ML for the ΓΜ direction and 0.047 ML for the ΓΧ direction. The solid linesshow the result of a Langevin molecular dynamics simulation of Γ(ΔK) as given by Eq. (20). (Adapted from Refs. [301,306].)


is possible to observe regions on the surface where diffusionoccurs on smooth defect free terraces without encounteringstep edges or other defects [436]. The relative insensitivity ofthe QHAS method to defects is related to special features ofthe QHAS technique. For one it is only sensitive to the actualmotion of the particles as reflected by the integral over time inEq. (18). Moreover, since only small distances of the order ofE100 Å or less are probed, defects, which are generally morewidely spaced, will not significantly influence the results. Theimportant role of defects which generally lead to an increase inthe effective barrier energy in the conventional diffusionmeasurements is, on the other hand, well documented [437].This explanation is also consistent with the good agreement inthe high temperature measurements. At high surface tempera-tures the defects are, on the one hand, much more mobile and,at the same time, the surface is continually being annealed.

In conclusion, QHAS experiments have provided for thefirst time a complete microscopic description of the diffusionof atoms on nearly ideal terraces of single crystal surfaces.Now that all the vibrational parameters of both the substrateand that of the adsorbate are known for many systems, evenmore detailed calculations of the microscopic dynamics can befully justified. Such calculations combined with further refineddata will lead to a better understanding of the nature andrelative importance of the phonon and electronic couplingbetween adsorbate particles and the substrate degrees offreedom. These latter processes are fundamental for under-standing many microscopic processes occurring on surfacessuch as diffusion and inelastic scattering of molecules, andultimately for understanding problems of technical importancesuch as friction and energy accommodation coefficients.

With regard to ultra-slow dynamics characterizing thediffusion or drift of heavier atoms like Cs or AM clusters,etc. the 3He spin-echo scattering spectroscopy, illustrated inthe next sub-section, with its much greater resolution of about20 neV, holds great promise, and a few QHAS experimentshave already been published [306,438–443] (Table 2).

2.4. 3HeSE

On the experimental side further progress in the high-resolution surface spectroscopy of low-energy processes wasmade in the late nineties by the group of DeKieviet inHeidelberg with the development of 3HeSE spectroscopy[444,445]. 3HeSE is a successful adaptation of the spin-echomethod invented in the early seventies by Ferenc Mezei for

high-resolution inelastic neutron scattering spectroscopy [446–448]. The 3HeSE Heidelberg apparatus was however restrictedto incident beam energies of about 3 meV, which is too low foraccessing the full range of energy transfer needed for measur-ing the full surface phonon dispersion curves at least in theacoustic region. More recently in Cambridge, UK, and soon atTechnion in Haifa, a substantial upgrading of the 3HeSEapparatus has been achieved, with an incident beam energywhich has been by now raised up to about 14 meV [310,449–451]. This is sufficient for measuring the dispersion curves ofsurface acoustic modes in most surfaces, and particularly forweakly bound adsorbate layers [451].

3HeSE exploits the effective time-reversal imposed to thenuclear spins by a surface reflection. The spin dephasing intime of an incident spin-polarized 3He beam due to unequalatom velocities, and the consequent loss of magnetization, arerestored by time reversal after reflection (rephasing). If thespin-polarized incident and reflected beams travel through twoidentical magnetic fields (Fig. 9), the Larmor precessionrespectively encodes and decodes the atom velocities beforeand after reflection. For elastic reflections no change ofvelocity occurs and no change in magnetization is observedat the detector with respect to the initial incident beammagnetization, whereas any change of magnetization signalsan inelastic scattering process. Here, the time-reversal Larmorprecession of the nuclear spins, rather than the actual time offlight, works as a “clock”, and the change of magnetization

Table 2Diffusion barriers of isolated AM adatoms on Cu(100) as derived from QHAS measurements and DFT calculations.

Na K Cs

Bridge: expt. 75 meVa [301] – 2072 meV [69]DFT 79 meV [21] 31 meV [70] 13 meV [21]Top: expt. 84 meV [301] – 2072 meV [69]DFT 143 meV [21] 52 meV [70] 25 meV [21]

aHere the diffusion barrier is 52.5+0.9 meV whereas our fit gave 56 meV.

Fig. 8. The microscopic potential energy surface V(x,y) for Na/Cu(001)determined from quasi-elastic broadening measurements. The Cu atoms aresituated at the four points (x,y)=(71.28,71.28) and the minima correspondto the hollow sites. The classical barrier for motion along the ⟨110⟩ azimuths isof 71 meV, and for motion along ⟨010⟩ azimuths is of 84 meV. Even the slightminima of the four-fold sites postulated from LEED data are reproduced (fromRef. [306]).


allows to measure the inelastic scattering energy loss withunprecedented resolution. Before starting the clock the 3Heatoms first have to be spin polarized. In the Cambridgeapparatus this is achieved in a carefully designed 30 cm longhexapole magnet [449]. The polarized atoms are then lined upperpendicular to the flight direction before they start to precesson entering a nearly 1 m long solenoid. The solenoid isdesigned to provide a homogeneous longitudinal magneticfield in which the nuclear magnetic moment of the 3Heprecesses several thousand times before arriving at the target.After scattering the atoms are passed through a secondoppositely directed but otherwise identical magnetic field,where they undergo a similar number of precessions, resultingin an “unwinding” of the polarization acquired in the firstsolenoid. Finally the polarization of the beam leaving thesecond solenoid is determined by passing the beam throughanother hexapole field before the beam arrives at the detector.

The resolution achieved with the spin-echo technique isnearly four orders of magnitude greater than with the conven-tional TOF apparatus. In the Cambridge apparatus a precessionphase shift of about 81 (0.14 rad) can be resolved. The totalphase amounts typically to about 1.9� 104 rad (3000 revolu-tions). Thus the fractional phase resolution is about Δφ/φffi7.4� 10�6. The actual energy resolution is even some-what more favorable by about a factor 3 [449]. Thus for anincident beam energy of, e.g., 8 meV an energy resolution ofabout δE/E¼2.5� 10�6 corresponds to a resolvable change inenergy of δE¼20 neV [450]. Compared to the best resolvableδE¼300 μeV achievable with the TOF technique [431], theimprovement is about 1.5� 104. The resolution is largelyindependent of the incident beam velocity spread since thewinding and unwinding of the spin precession is always thesame The dramatic improvement in resolution is acquired,however, at a high price. Because of the many stages and largeoverall dimensions of the apparatus, the intensities tend to be

less than with the conventional apparatus. To compensate forthis a new type of detector with a greatly lengthened ionizationregion of about 5 cm compared to the short 5 mm ionizer in theTOF apparatus was developed for the Heidelberg apparatus[452]. Another demanding requirement in the construction of a3HeSE apparatus is the great precision and advanced machin-ing technology in the construction of the hexapoles and theprecession magnets [453]. Moreover the use of the much moreexpensive rare isotope 3He requires a special gas handling andrecycling system to keep losses to a minimum. On the otherhand the spin-echo technique has the advantage that theresolution is largely independent of the beam velocity, so thatexperiments with any atomic species carrying a magneticmoment, like 4Hen(23S1) metastable atoms, as well as o-H2

and p-D2, and even fermion alkali atoms, can be envisaged.The superior energy resolution of 3HeSE spectroscopy has

been demonstrated in a few remarkable experiments, one onselective adsorption [401,454] and the others for quasi-elasticscattering from diffusing alkali metals, notably Na [71], K[70], and Cs atoms on Cu(001) [69]. Some notable results forthe latter two adsorbates, extracted from Jardine et al. [68] andHedgeland et al. [69] are illustrated in Figs. 10 and 11,respectively. Fig. 10(a) shows the dispersion curves of thefrustrated translation (T-) mode for ordered phases of Csadsorbed on Cu(001) at three different low coverages (∎:0.056 ML; �: 0.044 ML;▲: 0.028 ML) measured with 3HeSEalong the 〈100〉 direction at a surface temperature of 130 K[68]. The full lines are sine-law fits, while the broken linerepresents the RW dispersion curve of the clean Cu(001)surfaces. The calculated curves are interrupted below �0.1meV because it is not clear from the experiment whether theexperimental dispersion curves terminates at a finite energy forΔK-0, as found for the lighter AM's due to a residual shearforce constant between the AM ion and the surface. Despitethe superior resolution of 3HeSE the T-modes of the Cs-adatoms in the ΔK-0 limit can hardly be distinguished fromthe RW dispersion curve. The comparison with the HASdispersion curves of 1 ML of Cs on Cu(001) reproduced inFig. 5 illustrates the ability of the 3HeSE technique to measurephonon energies one order of magnitude smaller. This is evenmore evident from the 3HeSE measurements in the μeV rangeof the quasi-elastic (QE) peak width plotted in Fig. 10(b) as afunction of the parallel momentum transfer for the 0.044 MLof Cs on Cu(001). The data, referring to the ⟨100⟩ direction anda surface temperature of 130 K, show the characteristic deGennes dip at the G-vector of the periodic adatom lattice (hereat 0.55 Å�1) [68]. The fitting curve is calculated from Eq. (19)with the inclusion of only 1st and 2nd neighbor jumps, withℏ=τ1¼6.9 meV and ℏ=τ2¼4.9 meV, respectively; it providesan apparently better fit than that obtained from a moleculardynamics simulation in Ref. [68]. The measured temperaturedependence of the QE peak width for this coverage hasprovided an effective activation energy of 31+2 meV [68].The panels (a) and (b) of Fig. 10 provide a convincingdemonstration that 3HeSE spectroscopy can fully characterizethe ultra-slow dynamical processes of weakly bound heavyadsorbates occurring at metal surfaces.

Fig. 9. Schematic diagram of a 3He spin echo spectrometer. The 3He atomsform a free jet expansion are first polarized on passing through a magnetichexapole magnet. In a longitudinal magnetic field the atoms then press severalthousand times before being scattered. In a second identical, but oppositelypoled field, the atoms precess in the opposite direction. The final polarization isanalyzed by a hexapole field mounted in front of the detector. Smalldifferences in the precession phase provide information on the change invelocity after scattering.


The thorough 3HeSE study of K overlayers on Cu(001)performed by Hedgeland et al [69] reports the QE peak width(Fig. 11) as a function of the parallel momentum transfer alongthe two symmetry directions ⟨100⟩ and ⟨110⟩. The plotsindicate a more complex structure of de Gennes dip whichrequires the inclusion of diffusive jumps up to the thirdneighbor sites in Eq. (19) for a reasonable fit of the QE peakwidths. Also in this case Eq. (19) seems to make a better jobthan molecular dynamics simulation.

An important piece of information obtained from 3HeSEmeasurements is the vertical motion of the adsorbates leadingto an increase of the adsorption height z0 (the distance of theadatom from the first surface atomic layer) with the increase ofthe coverage. This effect is well accounted for by the Lau–Kohn forces originated from the electrons donated by the AMatom to the surface and stored in the surface states of the Cu(001) surface at the four X symmetry points. It is expressed by

the equation [506]:

ðq=z0Þ2 ¼ Aβnðz0Þ=½1þ θ3=2=ð1þ θÞ1=2� ð22Þwhere q is the effective AM ion charge, A is a constant, β is theinverse decay length of the metal surface charge density n(z),and θ is the adatom coverage. Clearly for a given q, z0 mustincrease for increasing θ. It is somewhat surprising that theaddition of charge to the Cu(001) X surface states, resulting forlow coverages in the appearance of a small Cu(001) surfacecorrugation growing with coverage (see Section 4.2.1), hasapparently not been investigated so far with 3HeSE, nor thepeculiar effects of the Lau–Kohn forces, as distinguished fromthe dipolar forces, have been considered in the mentionedstudies of AMs on the Cu(001) surface [68–70]. From thisbrief analysis 3HeSE appears to be a choice method for thestudy a slow surface diffusive processes as well as of thedispersion relations of low-energy phonon and librationalmodes of large adsorbed molecules. As concerns AM onmetal surface, a more complete discussion is presented in thenext sections.

2.5. RAIRS

RAIRS is a reflection-based technique which can be appliedalso for surfaces in contact with a gas phase, provided that thedensity of the gas phase is not so high to block the IR-beam inthe spectral ranges of interest. In particular, it can be used forinvestigating adsorption at metal surfaces. RAIRS involves animpinging IR beam penetrating the thin film, followed byreflection at a metal surface and the subsequent retransmission

Fig. 10. (a) Dispersion curves of the frustrated translation mode for orderedphases of Cs adsorbed on Cu(001) at low coverages (∎: 0.056 ML; �:0.044 ML; ▲: 0.028 ML) as measured with 3HeSE spectroscopy along the⟨100⟩ direction at a surface temperature of 130 K [68]; the full lines are sine-law fits, the broken line is the RW dispersion curve of the clean Cu(001)surfaces. (b) The quasi-elastic (QE) peak width measured with 3HeSE as afunction of the parallel momentum (wavevector) transfer for a 0.044 ML of Cson Cu(001) in the ⟨100⟩ direction at a surface temperature of 130 K [68]. Thefitting curve is based on Eq. (19) including only 1st and 2nd neighbor jumps(ℏ=τ1¼6.9 μeV, ℏ=τ2¼4.9 μeV); the temperature dependence of the QE peakwidth for this coverage provides an effective activation energy of 31+2 meV.

Fig. 11. The quasi-elastic (QE) peak width measured with 3HeSE as a functionof the parallel momentum (wavevector) transfer for a 0.018 ML of K on Cu(001) in the ⟨100⟩ ○ and ⟨110⟩ (□) directions at a surface temperature of 155 K[69]. The fitting curves are based on Eq. (17) including 1st, 2nd and 3rdneighbor jumps (ℏ=τ1¼ℏ=τ2¼1.9 μeV, ℏ=τ3¼0.63 μeV for ⟨100⟩ andℏ=τ1¼1.2 μeV ℏ=τ2¼1.5 μeV, ℏ=τ3¼0.40 μeV for ⟨110⟩); the temperaturedependence of the QE peak width measured in the ⟨100⟩ direction for0.056 ML provides an effective activation energy of 26+2 meV.


through the film, before the beam enters the detector. Ananalysis of the reflected beam provides information on themolecular vibrations in the surface film, and can be used toidentify the surface species. In particular, adsorbed and gas-phase species are distinguished by taking the difference of thespectral response for s- and p-polarized light.

The IR process is governed by a direct absorption of aphoton (usually from the vibrational ground level) to anexcited state. As the electric field is perpendicular on a metalsurface, only modes with a perpendicular dipole moment (thetotally symmetric modes, Fig. 12) can be excited (for a reviewsee Ref. [455]). For the same reason the absorption occurs onlyin the reflectivity of p-polarized light. As a matter of fact, thedipole moment μ must change with respect to the normalcoordinate Q during a vibration, that is

∂μ∂Q

� �a0 ð23Þ

In the case of adsorbates on metal surfaces, only vibrationalmodes with a non-zero component of the dynamic dipolemoment perpendicular to the surface can be observed. Thisoriginates from the screening effect of the electrons of themetal which produces an image dipole within the metal. Theresulting dipole will vanish if the molecular dipole is parallelto the surface.

RAIRS spectra are commonly collected using Fouriertransform infrared spectrometers because of their high-resolu-tion, high sensitivity, ease of use, and commercial availability[455]. In a typical RAIRS experiment (the usual experimentalapparatus for RAIRS is shown in Fig. 13), the IR beam isreflected off the crystal surface at grazing angles in order tomaximize sensitivity.

Theoretical foundations of RAIRS could be found in Ref.[456]. Since the electric field is perpendicular to the metalsurface, the absorption occurs only in the reflectivity of p-polarized light. The angular dependence of the change in thereflectivity due to surface absorption can be calculated byapplying the Fresnel-boundary conditions of conventionaloptics to a three-layer system consisting of vacuum, theadsorbate layer and the metal substrate [456,457]:

ΔRp ¼8πλ

sin 2θ

cos θdIm

�1ε? ðωÞ

ffi 32π2

λ

sin 2θ

cos θnsImα? ðωÞ ð24Þ

where λ is the wave-length, d is the film thickness, ns is thesurface concentration of adsorbates while ε┴ and α┴ are theperpendicular component of the dielectric function and ofpolarizability at the surface, respectively [458].The spectral range of RAIRS is limited by window materials

(such as NaCl which has a lower limit of 75 meV and KBrwhich has a lower limit of 50 meV). The spectral region up to50 meV is therefore difficult to investigate by FT-RAIRS,while infrared spectroscopy has a high inherent resolution formodes with vibrational energies up to 300 meV [459]. Evenwith the newly available synchrotron light sources the verylow-energetic frustrated translational modes with typical vibra-tional energies lower than 10 meV are not accessible to thistechnique.

2.6. SERS

Raman spectroscopy is a powerful optical tool for providinginformation about the vibrational properties of molecules.However, the application of Raman spectroscopy in biologicaldetection is impeded by the relative low efficiency of Ramanscattering due to the small optical cross section of molecules(typical Rayleigh scattering cross sections of molecules are inthe range of 10�26 cm2 and typical Raman scattering crosssections are in the range of 10�29 cm2) [461]. Such problemscould be crossed by using the SERS technique which is able toincrease the Raman signals from a molecule by factors of 106–1012 (a review on SERS could be found in Refs. [462,463]).

Fig. 12. Vibrations of diatomic molecules adsorbed in bridge geometry on asolid surface. Only A modes are dipole active, while B modes are notobservable in dipole scattering.

Fig. 13. Typical experimental apparatus for RAIRS. The experiments areperformed by focusing the IR beam from a commercial Fourier-transforminfrared spectrometer through a polarizer and a KBr (or NaCl) window onto thesample at grazing incidence. The reflected beam passes through a second KBrwindow and it is refocused onto either a mercury-cadmium-telluride or anindium-antimonide detector. Such configuration for IR in UHV conditions hasbeen developed by several groups [460] with intensities as low as 2–3� 10�6

absorbance units.


This great enhancement of Raman intensities is generallyachieved by exciting vibrational transitions in moleculesdirectly or in close vicinity to a roughened metal electrode.In general, the enhancement of Raman signals through SERSis given from two contributing mechanisms, namely theelectromagnetic mechanism and the chemical mechanism.

The electromagnetic mechanism which is believed to be thedominant mechanism responsible for the enhancements foundin SERS, explains enhancements primarily due to the collec-tive electromagnetic resonance (or localized plasmon reso-nances), which refers to the excitation of collective oscillationof free electrons shared by the material in conduction bands[462–464]. A strongly localized plasmon resonance, supportedby a metallic nanostructure, is followed by modifications in thelocal electromagnetic density.

This can occur, for example, when a small metal sphere isinfluenced by electromagnetic field provided that the radius ofthe sphere is much smaller than the wave-length of electro-magnetic radiation. Fig. 14 shows that a Raman activemolecule placed at a distance, d, away from a metal nano-particle of radius, r, will experience a total electromagneticfield which is the superposition of the incoming field (E0) andthe electromagnetic field of dipole (Esp) induced by the metalsphere. The electromagnetic field (Esp) on the metal particle'ssurface is a surface plasmon which is expressed as [465]

EspðvÞ ¼ ε�ε0εþ 2ε0

r

r þ d

� �3

E0 ð25Þ

When the incident electromagnetic wave (E0) resonates withthe electromagnetic of the dipole field induced from the surfaceof the nanoparticle (Esp), the incident field and the surfaceplasmon reinforce each other resulting in a large enhancementof the electromagnetic field. This increase in the field intensityexperienced by the molecule will lead to an increase in itsRaman scattered signals.

The field enhancement factor A(ν) (at a specific frequencyν), for molecule in the vicinity of the sphere at a distance d isthe ratio of the field at the position of the molecule and the

incident field:

AðvÞ ¼ EMðνÞE0ðνÞ

ε�ε0εþ 2ε0

r

r þ d

� �3

: ð26Þ

In this equation A(ν) is the strongest when the real part ofthe dielectric function ε(ν) is equal to �2ε0. In addition, theimaginary part of ε(ν) should be small. This condition occursat the resonance excitation wave-length of the surface plasmonand assumes that an embedding medium does not have animaginary portion in its dielectric constant (i.e., the metalnanoparticle is embedded in a non-absorbing medium).An additional enhancement factor of the order of 10–102

[462] is represented by a chemical enhancement as a conse-quence of electronic coupling. It implies a charge transfer,resulting in new electronic transitions between molecules andthe metal surface, so as to create a molecule–surface complex.

2.7. SFG

SFG is a nonlinear optical phenomenon that occurs whenhigh intensity radiation interacts with a medium that, as aresult, radiates at the sum of the frequency of the incidentradiations [466]. SFG can be considered as the conversion oftwo photons with energy ω1 and ω2 into a single photon withenergy ω¼ω1+ω2. Symmetry considerations imply that,within the electric dipole approximation, SFG is forbidden inthe bulk of centrosymmetric media, whereas at surfaces orinterfaces the symmetry is broken and SFG becomes allowed[467]. Accordingly, for centrosymmetric media such as Si andamorphous oxides SFG possesses surface and interface speci-ficity of unusual purity and generality.In local optics the polarization P(r, t) at the position r and at

time t depends only on the field E(r, t) at the same position andtime. The Taylor-expansion of the polarization in terms of thefield is

Pαðr; tÞ ¼ ε0½∑βχð1ÞαβEβðr; tÞ þ∑βγχ

ð2ÞαβγEβðr; tÞEγðr; tÞ

þ⋯� ð27Þwhere α, β, γ denote the components in Cartesian coordinates,χð1Þα;β is the conventional susceptibility tensor, and χð2Þα;β is thesecond-order susceptibility tensor. A special case of sum-frequency generation is SHG, in which ω1¼ω2¼ (1/2)ω3. Thisis the most common type of sum-frequency generation inexperimental physics as only one input light beam is required.When the photon energy of either the fundamental or the

SHG radiation coincides with an optical transition in themedium the SHG response is resonantly enhanced. InFig. 15 the generation of SHG radiation at the surface andburied interface of a thin film is illustrated.Moreover, the SHG technique is a suitable probe for a broad

range of physical properties such as electronic surface andinterface states (e.g., dangling bonds) [469], surface andinterface roughness [470], externally applied and internalelectric fields [471], fixed and trapped charge [472], surfaceand interface strain [471,473], contaminants and adsorbedspecies [469,474], and surface and interface symmetry [468].

Fig. 14. A simple schematic illustrating the concept of the electromagneticSERS enhancement for a Raman active molecule at a distance d from thesurface. EM is the field experienced by the Raman molecule and it is acombination of the incident field E0 and the induced dipole field in the metalnanoparticles ESP. Hence, EM¼E0+ESP.


This broad range of detectable physical properties shows theversatility of SHG and illustrates the high potential of thetechnique in contributing to an enhanced understanding ofsurface and interface properties during processing. However,because of the sensitivity of SHG to such a broad range ofphysical properties the interpretation of SHG experiments canbe difficult. In this respect, the combination with linear opticaltechniques that also probe bulk properties such as spectro-scopic ellipsometry is very useful.

A comparison between RAIRS, HREELS and SFG spectrahas been reported for potassium-doped fullerenes in Ref. [475](Fig. 16).

It should be mentioned that the SHG technique is a suitableprobe for a broad range of physical properties such aselectronic surface and interface roughness [469] surface andinterface roughness [470] fixed and trapped charge [471] fixedand trapped charge [472], surface and interface strain[471,473], contaminants and adsorbed species [469,474], andsurface and interface symmetry [468]. This illustrates theversatility of SHG and clarifies its high potential. However,the interpretation of SHG experiments is quite complicated andthus it is particularly useful its combined use with linearoptical techniques such as spectroscopic ellipsometry, whichalso probe bulk properties.

AM overlayers enhance the conversion efficiency of SHGby a few orders of magnitude in comparison with clean metalsurfaces [476]. There are two major origins of the SHenhancement associated with alkali adsorption: interbandtransitions between surface electronic states and multipoleplasmon excitation [477–479]. Fig. 17 shows the dependenceof the SH intensity of 800 nm (hν¼1.5 eV) photons as afunction of the AM coverage on Cu(111).

3. Basic concepts in the theory of surface excitations

Vibrational spectra of adsorbed species on surfaces canprovide important information on surface chemical bonds, as

the vibration frequency spectrum is characteristic for thestrength and the type of the bonds [345,458]. The developmentof experimental techniques for studying surface vibrations wasthus essential for the advancement of surface chemistry.

3.1. Surface phonons

The vibrational excitations of clean, two-dimensional peri-odic surfaces, besides providing essential information for theircomplete characterization as substrates, are also relevant fortheir coupling to the elementary excitations of the adsorbedspecies as well as for their possible role in adsorption/desorption phenomena, diffusion and surface chemicalproperties.It is worth remembering that the quantized plane-wave

solutions of the equation of motion of all atoms in the solidare the phonons of the 3D-solid. A flat surface or interface

Fig. 15. Schematic representation of the generation of SHG in a thin filmsystem consisting of a centro-symmetric film and substrate: an incident laserpulse with frequency ω induces the generation of second-harmonic radiationwith frequency 2ω at the film surface and the buried film/substrate interface.From Ref. [468].

Fig. 16. Comparison of RAIRS (upper panels), HREELS (center panels) andSFG (lower panels) spectra recorded in the frequency range 161–186 meV(1300–1500 cm�1). Left panels: α-phase, C60 monolayer on Ag( 111). Centralpanels: β-phase, intermediate K-doping. Right panels: γ-phase, saturationdoping. Adapted from Ref. [475].


breaks the 3D-translational symmetry of the solid in onedirection (conventionally taken as the z-direction), whichmay give rise to solutions that are localized at the surface inthe sense that the vibrational amplitude decays exponentially inthe interior of the solid. These modes are called surface(interface) phonons and are classified by a parallel wave-vector Q and a branch index ν. A schematic overview over thespectrum of eigenmodes at a surface is shown in Fig. 18. In thelimit of a semi-infinite solid or of an infinitely thick slab withtwo parallel surfaces the bulk modes form a continuumcorresponding, for each parallel wave-vector Q, to all possiblevalues of the third wave-vector component qz contained in thesurface-adapted Brillouin zone of the 3D solid. There may besolutions of the surface dynamical problem whose frequenciesfall into the bulk continuum. In this case one has surfaceresonances, whose eigenvectors can propagate inside the solidbut have a strongly enhanced amplitude in the surface region.In some symmetry direction of the surface there may besurface solutions inside a band of the bulk continuum whoseeigenvector is orthogonal to the eigenvectors of the bandmodes. In this special case the surface mode is exponentiallydecaying inside the solid, thus retaining a surface localizedcharacter, but only along the symmetry direction. Any devia-tion of the propagation from the symmetry direction transformsthe local mode into a resonance. These special solutions aretermed embedded surface modes.

As for bulk modes, surface localized modes and resonancesare classified according to their character (acoustic or optical)and their polarization, the latter referring to the direction of theparallel wave-vector Q and to the plane containing Q and thez-direction (the sagittal plane). When the sagittal planecoincides with a mirror plane of the structure (e.g., the [112]direction on the (111) surface of a monoatomic fcc crystal), thesurface phonons are even or odd with respect to the sagittal

plane. The odd modes are polarized perpendicular to the mirrorplane and are therefore shear-horizontal (SH) transverse modes,whereas the polarization vectors of the even modes lie in thesagittal plane. Since the surface breaks the symmetry in the z-direction, even in the symmetry directions where the sagittalplane is a mirror plane the modes are neither perfectly long-itudinal (L) nor perfectly z-polarized (shear-vertical, SV). Ingeneral the two components are mixed and out of phase so as togive an elliptical polarization in the sagittal plane (sagittalpolarization) However at certain symmetry points where thegroup velocity is zero the sagittal modes becomes purely L orSV, and the L and SH modes may be degenerate (e.g. at the Xpoint of a hexagonal surface lattice). Away from these specialpoints the coupling between L and SV displacements remains inmost cases fairly weak so that the modes retain a quasi-SV (e.g.,the Rayleigh waves (RW)) or quasi-L character (e.g., the S3resonance at long waves).Another important feature of surface modes, and more often

of resonances, is their maximum amplitude in the z-direction:this is in general at the top surface layer, but also sub-surfacemodes with the maximum amplitude in the second or thirdlayer may exist, as a consequence of the surface relaxation.This occurs quite often in metals, due to the peculiaroscillating interplanar spacing near the surface, and in deeplyreconstructed semiconductor surfaces.

Fig. 17. Second harmonic intensity as a function of coverage of (top panel)potassium and (bottom panel) cesium on Cu(111) surface. The excitationwave-length is 800 nm (E¼1.55 eV). Adapted from Ref. [476]. Fig. 18. The surface phonon dispersion curves (heavy full lines) of sagittal

polarization for the Cu(111) surface along the ΓΜ symmetry direction,superimposed to the surface-projected bulk phonon bands (light gray area)are shown as a paradigmatic example of a surface phonon structure for a metalsurface. The broken lines indicate the expected dispersion curves in theabsence of avoided crossing between modes of equal symmetry. Quasi-shear-vertical (SV1, SV2) and quasi-longitudinal (L1, L2) modes have the largestamplitude in the first or second surface atomic layer, respectively. The portionsof the phonon branches inside the bulk continuum describe surface resonances.Some important modes are conventionally labeled by Sj (S1 is the Rayleighwave (RW); S2 the longitudinal gap mode; S3 the so-called L resonance). Allsagittal branches have been measured by HREELS or HAS. (Adapted fromRef. [299]; the HAS data for S2 have been measured with a room temperatureHe beam of energy Ei¼63 meV (G. Zhang, unpublished)).


Fig. 16 shows the surface phonon spectrum of Cu(111)calculated with DFPT in the high symmetry direction ΓΜ withall surface branches of sagittal polarization in the first andsecond surface layers on top of the bulk continuum (light grayarea). All branches have been measured by either HREELS orHAS and assigned by the theoretical analysis to either quasi-SV modes or to quasi-L modes, both in the first (SV1, L1) andsecond (SV2, L2) layer. Some modes which are usually foundin all surfaces of equal structure have some conventional labelsSj: S1 labels the Rayleigh wave, S2 a gap quasi-L mode, S3 astrong resonance which is quasi-L at long waves and trans-forms into a SV2 mode at the zone boundary, etc.

In the long-wave limit (Q-0) there must be three phononbranches whose phonon frequencies go linearly to zero – thethree acoustic branches imposed by the translational invarianceconditions. Besides the S1 and S3 acoustic modes of quasi-SVand quasi-L polarization, respectively, there is a SH acousticmode, which has in general a bulk character and only at finiteQ may acquire a surface character (S7 mode) [480]. For a morecomplete information on surface phonons the reader is referredto the chapters of Ref. [409].

3.2. Vibrational excitations of adsorbates

Other vibrational excitations of interest here are the vibra-tion modes of isolated adsorbed atoms or molecules. Someexamples are depicted in Fig. 19. Single adsorbed atoms mayform a strong chemical bond with the substrate or may bebound to the surface by weak forces. Their stretching (S) orfrustrated translation (T) modes can alone inform about thebonding nature since a strong bond may locate the two modesabove the maximum frequency of the substrate, while thecorresponding modes of a weakly bound atom may fall in thelow acoustic region of the substrate. In the former case the twomodes are localized, in the latter they acquire a resonantcharacter. It should be noted that the actual position of theadatom frequencies depends not only on the bond strengthbut also on the ratio of the adatom mass to that of thesubstrate atoms.

When the same atoms form a monolayer, say for simplicitya (1� 1), the system recovers the translational symmetry of thesubstrate and the modes of the adsorbed layer contribute threeadditional dispersion curves associated with the three degreesof freedom of the adatom. They are schematically shown inFig. 19 for the S mode and for the two T modes by two heavylines. While at the zone center (Q¼0) the adatoms move all inphase, at the zone boundary (Q¼π/a with a the lattice spacing)the nearest neighbor adatoms move in opposite phase. In thecase of strong adatom–surface bond the lateral interaction isnegligible so as to give two dispersionless branches localizedabove the substrate continuum. For a weak bond the lateralinteraction may not be negligible and the correspondingbranches (the lowest two in Fig. 19) show some weakdispersion. When the lateral interactions among adatomsbecome comparable or stronger than the adatom–substratebonds a substantial dispersion occurs, as, e.g., for the case of aCs monolayer on Cu(001) shown in Fig. 5. Moreover the

additional adatom dispersion curves will exhibit avoidedcrossing and consequent hybridization with the surface disper-sion curves of the substrate of equal symmetry, as shown inFig. 19 and also in Fig. 5 for the S branch with the substrateRW modes.In the case of adsorbed molecules, the modes associated with

their internal degrees of freedom fall often outside the range ofsubstrate phonons due to the strength of the chemical bonds, andare therefore localized modes. Moreover the rotational andtranslational degrees of freedom of the molecule, being hinderedby the surface potential and possibly by the formation of a bond,turn into vibrational modes. Fig. 19 shows the case of diatomicmolecule standing vertically on the substrate surface a formingwith the substrate a bond weaker than the internal one. Ahypothetical distribution of the molecular mode frequencies isalso shown, where the internal stretching (S) is localized abovethe substrate maximum, whereas the external stretching (S′), thefrustrated rotation (R) and the frustrated translation (T), orrocking (R′) modes are shown as resonances inside the substratecontinuum. In case of a molecular monolayer there is a numberof additional dispersion curves equal to the number of themolecular degrees of freedom, exactly as discussed above forthe atomic overlayers.Both internal and external modes provide a rich information

about the chemical state of the molecule and its chemicalprocesses in the adsorbed phase. For example vibrationalspectroscopy can be used for observing the decomposition ofcomplex molecules into fragments and the appearance ofreaction intermediates on the surface. Moreover, the presenceor absence of a stretching frequency which characterizes agiven molecular bond can give unambiguous indication aboutthe possible dissociation of a diatomic molecule.

Fig. 19. A schematic representation of the vibrational modes of singleadatoms, either strongly or weakly bound to the substrate, and of an adsorbeddiatomic molecule. Their frequencies are superimposed to the surface phonondispersion curves and bulk phonon bands of the substrate (light gray area) inorder to separate the high-frequency localized modes from the low frequencymodes, which respectively fall above and inside the substrate continuum, thelatter acquiring a resonant character. The heavy lines represent the branches ofan atomic monolayer. For a strong adatom-surface bond the lateral interactionis negligible so as to give two dispersionless branches localized above thesubstrate continuum. For a week bond the two lowest branches show somedispersion and an avoided crossing with the surface phonon branches of thesubstrate.


The frequencies of stretching modes also depend on theadsorption site. As the coverage increases, the vibration modesof the isolated species may couple with each other giving riseto a dispersion, i.e., to a dependence of the vibrationalfrequency on the parallel wave-vector. The lateral couplingbetween adsorbates can be direct, like for example the dipole–dipole interaction between polar molecules, as well as indirectthrough the surface electronic states of the substrate. Examplesof indirect interaction among adsorbed species are the Lau–Kohn long-range forces occurring on certain metallic sub-strates [300]. Both kinds of lateral interactions affect theoverlayer dispersion curves and cause frequency shifts whichdepend on the local environment around the species. Otherfrequency shifts are caused by the anharmonicity of thepotential, which can be enhanced at the surface due to lackof inversion symmetry. The main effect of anharmonicity invibrational spectroscopy is the reduction of spacing betweentwo consecutive vibrational levels for increasing quantumnumbers. Thus the observation of the overtones of externalmodes can provide further information on the molecule–substrate interaction potentials [306,336].

3.3. Dynamics of AM on metals with the EAM

Before the advent of the DFPT [481], various semi-empirical methods have been suggested in order to accountfor the fundamental role of free electrons in the phonondynamics of metals and metal surfaces. Among these methodsone of the most effective and most largely used is the EAM,for which the reader is referred to the abundant existingliterature [482–484] The EAM, when directly compared toDFPT (see for example [378]) qualifies as an expedient tool fora fast and reliable, albeit semi-empirical, analysis of metalsurface dynamics, especially for extended surface cells likethose occurring in low-coverage adsorbate phases on metals,which would require a large computational effort with DFPT.

3.3.1. AM/Al(111)Lithium and sodium adsorbed on the Al(111) surface in the

(√3�√3)R301 phase have the peculiarity of occupyingsubstitutional sites [311]. The surface vibrations for this specialgeometry of the adsorbate, shown in Fig. 20 a side view

(a) together with the corresponding SBZ (b), have beenthoroughly investigated with EAM by Chulkov et al. [312].The calculated phonon dispersion curves are displayed inFig. 21(b,c, and d) for the Li, Na and K adsorbed phases,respectively, and compared with the phonon dispersion curvesof the clean Al(111) surface, plotted in Fig. 21(a).It is important to note the smaller size of the SBZ (Fig. 20

(b)) for the adsorbate phases (irreducible part: ΓΜ′Κ′) withrespect to that of the clean surface (irreducible part: ΓΜΚ), andthe corresponding folding of the external portion of ΓΜΚ intoΓΜ′Κ′. In particular the symmetry point Κ is folded into thezone center Γ, whereas Μ becomes equivalent to Μ′. This isreflected in the folding of the original surface dispersion curvesof Al(111) at both points Μ′ and Κ′ into branches eventuallyseparated by gaps: this is best appreciated for Na (Fig. 21(c))which has the smallest mass difference with respect to Al, sothat the perturbation on the phonon branches is mostly due tothe local change of force constants. For Na the lowest branch,corresponding to the Rayleigh wave (here labeled S1) near thezone boundary ðΜ′�Κ′Þ is well localized below the acousticbulk edge and is degenerate with the upper branch R2 at Μ′.A third branch just above, also partially localized around Κ′

and having a shear-vertical (SV) polarization, is single. Thisbranch descends below the (S1,R2) pair at the zone boundary forboth Li and K adsorbates, acquiring a strong localized character.The softening is moderate for Li, though interesting as itindicates that the expected frequency increase due to the lightermass is overcompensated by the weakening of the local forceconstants. For K both the mass increase and the force constantweakening, the latter being due to the large outward relaxation ofthe K ions, concur in giving a very soft and flat SV branch at lessthan 4 meV. In is interesting to remark that for K at the zoneboundary this SV branch is more than twice softer than thelongitudinal (L) and shear horizontal (SH) acoustic branches.This behavior is also found to occur for K overlayers on

graphite [485] or Be(0001) (see below and Ref. [403]), whereit has been associated with the formation of a surface quantumwell with a regular array of alkali atoms floating on thesubstrate surface and dipped into an almost uniform free-electron gas. This picture does not apply to the substitutionalconfiguration of Li and Na, but is approximately valid for Kdue to the substantial outward relaxation [312,486].

Fig. 20. (a) The substitutional positions of AM adsorbates on Al(111) in the (√3�√3)301 structure as seen in a side view in the ½112� direction and (b) thecorresponding SBZ drawn inside that of Al(111) with the respective irreducible parts in gray.


The large changes with respect to the clean Al(111) surfaceof the phonon LDOS projected onto the alkali sublattice andon the first and second Al substrate layers are illustrated inFig. 22 for both z (SV), and x+y (SH+L) polarizations. Thesharp peaks appearing in the z-polarized alkali-projected DOSreceive the largest contribution from the zone-boundarylocalized branches discussed above. As appears fromFig. 22, the participation of the substrate atoms of the firstand second layers in the displacement field of these localizedbranches is practically null.

HREELS measurements on Li/Al(111) for coverages ran-ging from 0.03 to 1 [5,311] indicate a SV mode of energybetween 17.6 and 18.1 meV. This can be better associated withthe folded SV surface mode at Γ (Fig. 20(b)) than with thesharp peak at 21 meV in the calculated LDOS projected on theLi overlayer displayed in Fig. 22. On the other hand theHREELS data for Na/Al(111) (θNa¼0.33) [5,311] give a SVmode at 10 meV, which can be readily associated with thesharp peak in the LDOS for the Na overlayer in Fig. 22,corresponding to third flat zone-boundary branch of SVpolarization.

3.3.2. Na/ Cu(111)The dynamics of Na overlayers on the Cu(111) has been

thoroughly studied theoretical by means of EAM for four differentcoverages corresponding to the adsorbate superstructures p(3� 3)(coverage θNa¼0.11), p(2� 2) (coverage θNa¼0.25) and (√3�

√3)301 (coverage θNa¼0.33) and (3/2� 3/2) (coverage θNa¼0.44). The respective hexagonal arrays of Na atoms and corre-sponding SBZ's are shown in Fig. 23 on top of those for the cleanCu(111) surface. Note that the rhombic unit cell for the whole(adsorbate+substrate) system in the p(3� 3) configuration (one Naatom per unit cell) also holds for the (√3�√3)301 (three atomsper unit cell) and (3/2� 3/2) (four atoms per unit cell) super-structures, so that the SBZ for the whole systems is the same forthe three coverages. Thus it is convenient to represent thecalculated phonon dispersion curves within the reduced SBZ's ofthe rhombic cells, which are those depicted in Fig. 23(a), valid forthe p(3� 3), (√3�√3)301 and (3/2� 3/2) superstructures, andin Fig. 23(b) for the p(2� 2) superstructure.With respect to the phonon branches for the clean surfaces of the

Cu(111) slab, plotted in Fig. 24(a), the same phonon branches ofthe clean Cu(111) slab represented on the two extended rhombiccells (Fig. 24(c) and (e)) show a dense folding pattern with acomplex array of folded surface branches, all (except the Rayleighwave) falling onto the surface-projected bulk continuum and takingtherefore a resonant character. When the Na atoms are added atdifferent coverages (Fig. 24(d,f,g, and h)), new surface phononbranches are introduced, accompanied by a perturbation of theintrinsic Cu(111) surface phonon branches. This can be assessedby a careful comparison of the phonon structure of the adsorbedphases to that of the clean surface represented on the same rhombiccell. However the most important feature is the pair of flat andalmost degenerate (degenerate at Κ′) acoustic branches appearing

Fig. 21. (a) Calculated surface phonon dispersion curves of the Al(111)�-(√3�√3)301 clean surface and with substitutional Li (b), Na (c) and K (d) adsorbates.The surface phonon branches are marked by full circles. Adapted from Ref. [312].


well below the bulk edge for p(3� 3) and p(2� 2) superstructures(Fig. 24(d and f)). Unlike substitutional K on Al(111) discussedabove, where the flat localized branch has SV polarization, here thetwo flat branches have L and SH polarizations like the lowestzone-boundary branches of substitutional Na on Al(111).

On the contrary at the higher coverages (√3�√3)301 and(3/2� 3/2) the lowest acoustic branches are only weaklylocalized as an effect of the increased repulsive interactionbetween neighbor adatoms.

All these features are clearly reflected in the Na-projectedDOS plotted in Fig. 25 for the four coverages and from theircomparison with the DOS of the clean Cu(111) surface plottedin Fig. 24(b). The latter is clearly independent of the chosenunit cell representation, and serves as a reference for all fourNa superstructures.

HREELS data on Na/Cu(111) [322,343] at θNa=0.3 MLindicate a SV mode at 21 meV which clearly corresponds tothe peak at about that energy for the (√3�√3)301 coveragein Fig. 25. More precisely at this energy there is a flat opticalsurface branch crossing the whole SBZ (Fig. 24(g)), whichinvolves to a large extent the SV motion of the Na atoms.

3.3.3. AM/Pt(111)An EAM calculation for two different coverages, p(2� 2)

and (√3�√3)301, of potassium on the (111) surface ofplatinum [487–489], shown in Fig. 26, indicates a fairly goodlocalization of the flat zone-boundary acoustic branches of

parallel (x,y) polarizations for the lower coverage, but a weakerlocalization and a substantial dispersion for the higher cover-age. The latter effects can certainly be ascribed to a largermutual interaction of the adatoms. However the strikingdifference with (√3�√3)301�K/Al(111), Fig. 21(d), wherethe lowest, sharply localized and dispersionless K branch has aSV polarization is due to other factors. While the K branch onPt(111) is about at the same energy as in Al(111), the heaviermass of Pt brings the lower acoustic edge at Μ′ down to about6 meV, thus dramatically reducing the localization of the Kbranches. Another, perhaps more important factor, is thedifference between the surface of a metal with sp electronbands like Al(111) and that of a d-band surface: the latterallows for a tighter binding of the K ion to the surface, yieldinga stiffening of the SV mode. This implies that the lowest Kbranches have parallel polarizations, whereas the lowestbranch in Al(111) has a SV polarization.The phonon LDOS for the two K/Pt(111) superstructures are

plotted in Fig. 27 and compared with those projected onto thesurface Pt layer and the bulk DOS. Despite the stronger K–Ptbinding the LDOS's of K and Pt appear largely decoupled dueto mass difference and the strong anisotropy between verticaland parallel force constants: the LDOS for K parallel-polarizedmodes is sharply peaked at about 6 meV, much below the mainPt LDOS features, whereas the LDOS of potassium SV modesis peaked at the top of the bulk Pt phonon spectrum. TheHREELS spectrum for a K coverage of 0.15 gives a mode at

Fig. 22. Calculated phonon DOS for the four systems of Fig. 20 Adapted from Ref. [312].


Fig. 23. Hexagonal arrays of sodium adsorbed on Cu(111) for increasing coverage θNa and corresponding two-dimensional BZ (gray areas; hatched triangles are theirreducible parts) inscribed into the Cu(111) SBZ: (a) p(3� 3), i.e., θNa¼0.11; (b) p(2� 2), θNa¼0.25; (c) (√3�√3)301, i.e., θNa¼0.33; (d), (3/2� 3/2), i.e.,θNa¼0.44. Note that the rhombic unit cell for the adsorbate+substrate system is the same in (a), (b) and (c) and therefore the SBZ for the whole systems are thesame for the three coverages.


Fig. 24. (a) Surface dispersion curves of Cu(111) calculated with the EAM: the open circles describe the surface phonon branches while the continuous lines are the bulkdispersion curves for a 31-layer slab. The corresponding surface-projected phonon density of states are shown in (b) for the SV and the parallel components (L,SH) of surfaceatom displacements. (c,d) dispersion curves of the Cu(111) slab folded into the p(3� 3) unit cell and modifications induced by the adsorption of one Na atom per p(3� 3) unitcell as in Fig. 23. (e,f) same as (c,d) for the p(2� 2) structure. (g,h) phonon dispersion curves for the (√3�√3)301 and (3/2� 3/2) Na adsorption phases. From [323].


22 meV of SV polarization [330,331] in agreement with thementioned flat branch of the SV modes of potassium at20.5 meV calculated with EAM.

3.4. Dynamics of AM on metal surfaces with ab-initio methods(DFPT)

Nowadays density functional perturbation theory (DFPT) islargely used for ab-initio calculations of the surface–phonondispersion curves of clean metal surfaces, thanks to develop-ments of efficient codes [481]. Density functional methods

have also been applied to the study of AM on metal surfaces asregards adsorbate diffusion and local modes at low coveragesand surface charge re-distributions (see Section 4.2.1) [69–71],whereas little has been done about their phonon dispersioncurves [305]. HAS studies of metal surfaces revealed acomplex dynamic response of the surface charge density tophonon displacements [402]. These effects were found to bepronounced in supported metal multilayers characterized byfree-electron QW states [304]. A special case is represented byalkali overlayers, due to their ability in enhancing certainsurface reactions and field emission [121,490]. On a more

Fig. 25. Phonon density of states projected onto the Na adsorbed layer on Cu(111) for the four coverages considered in Figs. 23 and 24 and for SV (full lines) andin-plane (L,SH) polarizations (broken lines) calculated with the EAM. Adapted from Ref. [323].

Fig. 26. EAM calculations of the surface phonon dispersion curves for the p(2� 2) and (√3�√3)301 potassium overlayers on Pt(111). Adapted from Ref. [488].


fundamental side, in these systems the coupling of the over-layer phonons to electronic transitions between states of the2DEG allows to study the effects of a quasi-2D electron–phonon interaction [105,491,492]. 2DEG associated with apotassium layer adsorbed on Be(0001) is presently attractingmuch interest of theoreticians also for the occurrence ofcollective electronic excitations of acoustic type [493–495]and their involvement in photoemission [134,496], as a naturalfollow-up to the recent discovery of surface acoustic plasmonsin Be(0001) [497,498]. The recent DFPT investigation of thesurface phonon structure and e–p interaction of K/Be(0001)complements these studies.

Fig. 28 shows a side view (a) with the calculated electroncharge density and a top view (b) of p(2� 2)K/Be(0001)[403]. The charge density associated with the electronic statesof the K-overlayer quantum well fills rather uniformly theempty space between the K ions. The DFPT lowest dispersioncurves, associated with longitudinal (L), shear-horizontal (SH)and shear-vertical (SV) modes of the K overlayer, are shown inFig. 29. The DFPT approach also allows to calculate the HASspectral intensities for these modes at different wave-vectors

(for planar scattering only for SV and L components). Asshown in Section 2, the intensity of the inelastic HAS from aphonon of wave-vector Q and branch index ν is proportional tothe square of the phonon-induced CDOs at the turning point of

Fig. 27. Phonon localized density of states of p(2� 2) (a) and (√3�√3)301(b) K/Pt(111) projected on the potassium (top panels) and the Pt(111) surfacelayer (middle panels) compared with the Pt bulk DOS (bottom panels) for theplanar (x+y) and normal (z) polarizations. Adapted from Ref. [488].

Fig. 28. (a) Side view of p(2� 2)K/Be(0001) showing the K monolayer andthe first three Be atomic layers together with the electron charge density: thecharge density associated with the K-overlayer quantum well states fills ratheruniformly the empty space between the K ions [403]. (b) Top view of the Koverlayer and of the substrate Be atoms: the dotted atoms are on the firstsurface layer, the other atoms are in the second layer (adapted from [403]).

Fig. 29. Contour plots of the spectral intensities of the longitudinal (L), shear-horizontal (SH) and shear-vertical (SV) components of the surface modes of p(2� 2)K/Be(0001) projected onto the alkali overlayer. The upper part of thespectrum with the substrate modes is not shown since the projection of thesemodes on the substrate coordinates gives negligible intensities [403].


the scattering He atom. This squared amplitude is in turnproportional to the electron–phonon coupling strength λQν forthat specific mode, which can therefore be directly obtainedfrom inelastic HAS measurements. While no HAS study of K/Be(0001) is available yet, the DFPT results for the CDOs canshed light on the puzzling HAS results for the K/graphitesystem [309,339,499]. The calculated CDOs induced by thealkali SV and L modes at Γ and Μ are plotted in Fig. 30, andshow quite peculiar properties as the K atom oscillates aboutits equilibrium position.

The SV mode at Γ, whose CDO is shown in Fig. 30(a),actually is a weak resonance with a uniform displacement fieldpropagating inside the Be substrate and a strong enhancementat the K layer, which however generates no modulation of thesurface charge density. For an outward vertical motion of theK atoms the valence charge donated to the 2DEG is calledback, so as to give a positive spot in front of the ion at theexpense of the delocalized electron gas charge, which shows auniform depletion. Altogether, there is practically no spacemodulation in the low density region. Of course the inwarddisplacement of the K atoms yields the opposite change in the2DEG density. Since the K atom is in the on-top position adownward motion of the K ion attracts electronic chargebetween the two ions to reinforce the K–Be bonding and tobetter screen the positive ion charges.

Also at the Μ point the SV modes (Fig. 30(b)) produces amodest spatial modulation of the CDO, almost vanishing at theK plane, with no change of sign. The small modulation in

space of the 2DEG density all over the BZ may explain thenegligible dispersion for the K-ion SV branch (Fig. 29). On thecontrary the 2DEG responds dramatically to the L motion ofthe K atoms (Fig. 30(c and d)). Since the L motion of the Kions is opposite to the L motion of the first Be layer, the chargedensity is compressed on one side and is squeezed outward,compensated by the charge depletion on the other side. Thestrongly modulated outward CDO lobes occurring at Μ are dueto the part of the quantum-well wave-function provided by theBe layer, while the smaller lobes near the K ion following itsmotion are due to the part of the wave-function provided bythe K ion itself. The L mode of the K ion at Γ is insteadstrongly localized, almost decoupled from the substrate, and nomodulation of the surface charge density is produced above theK overlayer. The important change in the charge densityresponse in moving from Γ to Μ is associated with anappreciable dispersion of this mode (Fig. 29).The present analysis may be applied to the intriguing case of

the inelastic HAS data for the (2� 2) alkali overlayers ongraphite [309,339,499–501], where only the longitudinal over-layer branch was observed, besides its avoided crossing withthe substrate RW. It should be noted however that theoverlayer density on the honeycomb structure of graphite issmaller than on the closed-packed Be(0001) surface. This factcombined with the larger inertness of graphite suggests that theeffects described above should be emphasized for the graphitesubstrate. In particular the CDO associated with the SV alkalimode should remain rather small all over the BZ, whereas the

Fig. 30. Contour plots of the charge density oscillations (CDOs) in the surface region, including the K adsorbed layer and the three first substrate Be layers, asfunctions of normal (z) and parallel (x) coordinates, induced by frozen-phonon displacements of the low-energy modes of the K layer with SV and L polarizations atthe Γ (panels (a) and (c)) and Μ (panels (b)and (d)) symmetry points. The insets indicate the respective phonon energies. The contour lines correspond to CDOvalues in units of 10�4 a.u. ranging from 71 to 7128, each step corresponding to a factor 1/2. Red (blue) lines correspond to positive (negative) modulations.(Adapted from Ref. [338].) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


alkali L branch, which is less dispersed, should be character-ized by a comparatively large CDO for wave-vectors beyondthe region of hybridization with the RW, thus giving the onlyrelevant feature in HAS time-of-flight spectra.

4. Vibrational spectroscopy of adsorbed AM atoms

4.1. Single-adatom properties

The few theoretical examples of phonon dispersion curvesof AM on different close-packed surfaces discussed in theprevious Chapter have shown fairly flat dispersion curves forcoverages lower than or equal to p(2� 2). Thus at sufficientlylow coverage the mutual direct interaction between adsorbedatoms can be neglected, and their vibrational frequencies can,in a first approximation, be viewed as single-adatom proper-ties. This allows for a first assignment of adatom vibrationalfrequencies for a given mono-crystalline substrate once theirgeneral dependence on the adatom mass, adsorption site andsurface orientation is qualitatively established.

4.1.1. Mass dependenceJacobi et al. [337] and Finberg et al. [5] have shown from

the analysis of the experimental data that the energies of thevertical vibration mode are scaled approximately as the inversesquare root of the AM adatom mass. In order to analyze therole of the mass factor itself, we consider vibrations ofdifferent AM adatoms in the same conditions (adsorption sites,coverage, and surface orientation).

In Fig. 31 the experimental energies of vertical (? ) and in-plane (||) polarized modes, as well as the theoretical valuesobtained from EAM and DFT calculations, are plotted asfunctions of the inverse square root of the adatom mass (M�1/2

law) for the substitutional AM on Al(111) (cf. Section 3.3.1)and compared to hollow-site AM on Cu(001).

As seen in Fig. 31(a), for the substitutional atoms thevertical modes fulfill only roughly the M�1/2 law, whereas thevibrational energies of adatoms in the hollow site followquite well the M�1/2 law, indicating for this configuration asubstantial equality of the bond strength. The deviationsobserved for substitutional adatoms cannot however beexplained as exclusively due to the difference in their atomicradii: the exceedingly soft frequency of K can rather be relatedto its larger outward position with respect to the idealsubstitutional site. This effect is even more pronounced forthe in-plane modes.The energies of the in-plane modes of adatoms in the hollow

sites (also termed frustrated translations, or T modes) deviateeven more strongly from the M�1/2 law, showing rather aninverse proportionality to M. As discussed below, an AM atomadsorbed on a perfectly flat surface like that of Cu(001) buildsits own potential and induces a corrugation by transferringelectrons into zone-boundary surface state pockets. Thismechanism (equivalent to 2D Friedel oscillations for anisolated adatom), raising a long-range (R�2) interactionbetween distant adatoms via surface states (Lau–Kohn forces),can also explain the observed deviations from the M�1/2 lawfor both T- and vertical-modes.

4.1.2. Dependence on the adsorption siteAM adatom can be adsorbed in different positions of the

surface lattice, the most common ones being the symmetrichollow, bridge, on-top or substitutional sites. Since eachadsorption site of a given surface and for a given AM adatomis characterized by its vertical and in-plane vibrationalenergies, a comparison with theoretical prediction may helpdetermining the adsorption sites and their probabilities and todistinguish adatom spectra from those of surface defects. Forexample, a calculation by Hannon et al. [331] of the potentialenergy function of K on Pt(111) and of the vertical vibrationalfrequencies at different sites gives a vertical mode at 17 meV

Fig. 31. Vibration energy as a function of the inverse square root of the adatoms mass for (a) the substitutional adsorbed AM/Al(111) and (b) the on-surface hollow-site adsorbed AM/Cu(001). Solid blue and green solid lines represent linear fits for vertical and in-plane modes, respectively. The computed EAM results for AM/Al(111) are taken from [312] and the experimental and DFT data from [5,311,313,314]. The experimental data for AM/Cu(001) are taken from[55,303,315,316,319,502]. The theoretical data for AM/Cu(001) are adapted from Ref. [11]. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)


for the hollow site, in excellent agreement with HREELSexperimental value [331], 19 meV for the on-top site, 12 meVfor K bonded in a single Pt-atom vacancy, and 15 meV for Kbonded in the center of a three-Pt-atom vacancy island. On thisbasis unambiguous information is obtained that K on Pt(111)at low coverages accommodates in the hollow site [331].

Fig. 31 also shows that in systems with AM atoms in thehollow-site position the vertical vibrations are, as expected,stiffer than the corresponding in-plane vibrations, while theopposite holds for the substitutional adsorption site, due to themissing nearest neighbors above the vibrating AM atom. Thusthe ratio of the in-plane to vertical energies can also provideinformation on the adsorption site through its dependence onthe adsorbate coordination. This is better seen by comparingvibration energies of the same AM atom on different sites ofthe same surface. As seen in Fig. 32(a and b), the calculateddispersion curves of a c(2� 2) Na overlayer on Al(111)arranged in either the hollow sites (coordination 3) or thesubstitutional sites (coordination 9) show important differencesin the Na vibrational branches. The corresponding phonon

DOSs projected onto the Na overlayer for the vertical and in-plane components (Fig. 32(c and d)), show indeed that for thehollow position the in-plane modes are softer than the verticalones, while for the substitutional position the vertical modesare softer that the in-plane modes .Other examples of experimental vibration frequencies which

permit to assign the adsorption site of different AM adsorbateson the Al(111) surface are listed in Table 3.

4.1.3. Dependence on surface indicesFor adsorption on multi-faceted surfaces, the dependence of

vertical and in-plane vibration energies on the surface indicesis remarkable. However, it should be noticed that no studies onAM vibrations on high-index surfaces are present in literature.While AM adatoms on high-index surfaces would find severalinequivalent adsorption sites yielding rather different vibra-tional frequencies, low-index surfaces (111) and (001) of cubiccrystals look fairly uniform on the length scale of the adatom-image charge distance. While T-modes directly probe thelocal site geometry, vertical modes (also termed stretching

Fig. 32. Phonon dispersion curves and LDOS for c(2� 2)–Na/Al(001) with sodium atoms in hollow (a) and substitutional (b) adsorption sites (surface states aredenoted by open circles). The respective phonon density of states projected onto the Na overlayer are shown in (c) and (d). Adapted from Ref. [11].

Table 3Experimental vibrational energies of alkali adatoms on different adsorption sites of Al(111). Adapted from Ref. [5].

Adsorbate Structure Mode energy (meV) Polarization

Na On-surface (4� 4) (mixed sites) [503] �2.3 In-planeNa Substitutional (√3�√3)R301 [503,504] 1.4 Defect adsorptionNa Substitutional (√3�√3)R301 [503,504] 9.2 VerticalNa Intermixed [311,313,505] 4.5 –

K On-surface (√3�√3)R301 (adsorption in on-top sites at T=90 K) [503,506,507] 3.1 In-planeK Substitutional (√3�√3)R301, T=300 K [503,506,507] 6.4 VerticalCs On-surface (√3�√3)R301 (adsorption in on-top sites) [503] 1.7, 2.4 In-planeCs Intermixed (2√3� 2√3)R301 [503] 2.4 In-plane


(S) modes) do not. Thus, S-modes are expected to beinsensitive to surface indices. In fact, results summarized inTable 4 for the case of S-modes of AM (in the limit of lowcoverages) on low-index Cu surfaces indicate a substantialindependence on surface indices.

4.1.4. Dependence on temperatureAdatom vibrations can be highly anharmonic. Vertical

vibrations probe a highly asymmetric potential, since theadatoms are confined below by the rapidly (exponentially)increasing surface charge density and above by the long-rangeimage-charge Coulomb attraction. This implies a robust cubicanharmonic term in the AM-surface potential. In-plane vibra-tions probe instead more symmetric potentials. For example,the potential of Na in a hollow site of Cu(001) represented inFig. 8 is perfectly symmetric and has no cubic term. On theother hand, the comparatively low barrier height implies asizeable negative quartic term causing a substantial decrease ofthe vibrational energy and a corresponding increase of thespectral line-width for increasing temperature T. In a quasi-harmonic approximation the harmonic force constant f0 iscorrected by quartic anharmonicity, represented by the 4thderivative of the potential d4V=dx4 ��h, into

f ¼ f 0�h

12u20coth

εT02kT

; u20 �ℏ2

2MεT0ð28Þ

where εT is the T-mode harmonic energy and u20 is the mean-square displacement at T¼0. The corresponding T-dependentvibration energy and line-width (FWHM) are then given by

εT ðTÞ ¼ εT0�ΓðTÞ; ΓðTÞ ¼ h

12u40coth

εT02kT

ð29Þ

The T-mode of Na on Cu(001) shows indeed a largeanharmonicity, as resulting from HAS measurements of thevibration energy and FWHM at Q¼0 as functions oftemperature (Fig. 33). Eq. (30) provides an excellent fit ofboth experimental quantities with h¼61.4 meV/Å4.

4.2. From low coverage to one monolayer

4.2.1. Adatom–adatom interaction: dipolar and Lau–KohnforcesAM adsorption on copper surfaces was found to induce a

rearrangement of the substrate via relaxation, reconstruction,and faceting, as suggested by STM and LEED measure-ments [511]. Some of the phenomena can be interpreted asdue to the charge donation of AM atoms to the substrate. Atlow coverage there are however more subtle effects involvingthe substrate surface states which have been elucidated byHAS phonon spectroscopy. High-resolution inelastic HASmeasurements of the vibrations parallel to the surface for

Table 4Summary of S-modes energies for various AM atoms adsorbed on low-index Cu surfaces in the limit of low AM coverage.

Surface AM S-mode energy (meV) Method

Cu(111)

Li 38 HREELS [321]

Na21

HREELS [315,316,508]DFT [325]

22 EAM [323]

K13 HREELS [322]12–13n SHG [509]

Cs 7.44 SHG [510]

Cu(110)

Li 33–35 HREELS [315]Na 18 HREELS [315]K 13.7 HREELS [315,316]

Cu(100)Na 18 HREELS [316]K 10 HREELS [316]

Fig. 33. The temperature dependence of the frustrated translation frequency ofNa on Cu(001) and its spectral width (FWHM) as measured by HAS[301,307]. Eq. (29), expressing the effect of quartic anharmonicity, providean excellent fit of both the energy and FWHM with the quartic anharmonicconstant h¼61.4 meV/Å4.


low coverages of alkali atoms Na, K and Cs on Cu(001) showa strong fall off in frequencies from 5.6 meV (Na) to 0.65 meV(Cs) which cannot be explained in terms of the massdifference. It corresponds in fact to a substantial reduction ofthe effective force constant from 172 meV/Å2 for Na to76 meV/Å2 for K and only 14 meV/Å2 for Cs (Table 5). Thedispersion of the T-mode energy measured by HAS as afunction of parallel wave-vector Q appears to be negligible fora coverage of 0.05 ML of Na and 0.07 ML of K, whereas for0.08 ML of Cs a sizeable dispersion is observed (Fig. 34) [55].A mean-field theory [55] suggests that the T-mode forceconstants depend on the adatom size: for small atoms thefrequency is determined by the local substrate holdingpotential, whereas in the case of the larger adatom (Cs) themain restoring force is the electrostatic adsorbate–adsorbateinteraction. A model based on dipole–dipole interaction pre-dicts an increase of the dispersion amplitude with coverage(inset of Fig. 34) in apparent agreement with observation, but itis at odds with the observation of an increase with coverage ofthe Q¼0 T-mode energy.

It is indeed an intriguing fact, pointed out by Graham et al.[514], that even at Q¼0, i.e., when all adsorbates oscillates inphase and lateral interactions do not play any role, the T-modefrequency shows an unexpected increase with coverage.

An example is illustrated in Fig. 35, for Na coverages on Cu(001) varying from 0.008 to 0.125 ML, corresponding to aninter-adatom separation decreasing from 31 Å to about 8 Å(inset). In this range the T-mode frequency increases almostlinearly with coverage by about 14%! This unexpectedbehavior together with and the hexagonal ordering of theadatoms even at the lowest coverage point to the existence of along-range interaction despite the lack of dispersion. Thepuzzle was associated with another intriguing observation,illustrated in Fig. 36.Even at very low coverages (Fig. 36, above) the He

diffraction pattern shows the specular peak and the lowest-order diffraction peaks of the substrate surface; the ordering ofthe Na adatoms is signaled by the appearance of a ring ofsatellite spots around the specular peak. At the largestmeasured coverage of 0.125 (Fig. 36, below) the adatomlattice satellite peaks become as intense as the specular anddiffraction peaks of the substrate. Since the clean surfaceof Cu(001) appears perfectly flat to He atoms at thermalenergies and no diffraction is observed within the currentHAS sensitivity, the unexpected appearance of the substratesurface diffraction at very low coverages was associated to an

Table 5Summary of T-mode energies and related physical parameters of alkali atoms adsorbed on Cu(001). Adapted from Ref. [55].

AM atom θML μ(0) (Debye) Adsorption height (Å) ℏωT (meV) kT (meV/Å2)

Li 0.80 2.370.1 1.96 [512]Na 0.50 3.570.2 2.23 5.6 172K 0.37 7.570.5 2.60 [513] 2.86 76Rb 0.29 7.370.5 2.75Cs 0.27 7.570.5 2.94 [513] 0.65 14

Fig. 35. Comparison of the frustrated translation (T-mode) frequency of Naatoms on Cu(001) measured as a function of the coverage for a parallelmomentum transfer ΔKffi�2.0 Å�1 and surface temperature Ts¼50 K with acalculation based on a model with effective long-range forces originating froma charge transfer into intrinsic surface states (full line, for ΔKffi0; broken line,for ΔKffi�2.0 Å�1). The inset shows the dependence of the T-modefrequency on the separation d between adatoms.

Fig. 34. Comparison of the measured dispersion curves for the T-mode alongthe ⟨100⟩ azimuth for θNa¼0.05, θK¼0.07 and θCs¼0.08 ML on Cu(001) at asurface temperature of 100 K. The dashed lines indicate the dispersion curve ofthe Rayleigh mode R. and the longitudinal mode (L) of the clean Cu(001)surface. The inset shows the amplitude A of the dispersion of the T-modecalculated as a function of coverage (from Ref. [55]).


adatom-induced modification of the surface charge distribu-tion. Another possible mechanism as the formation of a latticegas was ruled out by the appearance of the satellite peaksindicating an ordering of the adatom lattice. As explained inRef. [514], the adsorption of Na atoms implies the injectionof electronic charge into the four X-point pockets of surfacestates at the Fermi level of Cu(001), thus building up asurface corrugation with the exact Cu(001) periodicity andthe change of the potential well of Na adatoms. The effect issimilar to the formation of surface Friedel oscillations dueto the surface states at the Fermi level and consequent long-range ðpR�2Þ interaction between adatoms, which feeleach other through the mediation of the electrons in thesurface states (Lau–Kohn forces). This mechanism provideda quantitative explanation of the observed increase of theT-mode frequency with coverage and a first clear evidenceof Lau–Kohn forces.

The theory provides an expression for the T- and S-modesquared frequencies as a function of Q and of the coverage θfor an AM atom on an fcc(001) metal surface [514]. At Q¼0and θ¼0 they are given by

ω2T ;0 ¼

12G2 AnGðz0Þ

M; ω2

S;0 ¼G G� 2z0

� �An0ðz0Þ

M; ð30Þ

where G is the shortest surface reciprocal lattice vector(2.49 Å�1 for Cu(001)), n0ðz0Þ and nGðz0Þ the 2D Fouriercomponent for Q¼0 and G of the surface charge density at theadatom position z0 above the first atomic plane. Both compo-nents depend on z0 as exp(�Gz0). A is a constant transformingthe charge density into the corresponding potential energyof the embedded AM atom. In Cu(001) the fitting of theexperimental frequencies gives An0ðz0Þ¼0.555 eV andAnGðz0Þ¼0.056 eV. Eq. (31) permits to compare the K-to-Nafrequency ratios and to explain the important deviations from

the mass formula:

ω2T ;0ðNaÞω2T ;0ðKÞ

¼ MK

MNaeGΔz0 ¼ 4:21;

ω2S;0ðNaÞω2S;0ðKÞ

¼ MK

MNa

G�2z�10 ðNaÞ

G�2z�10 ðKÞ eGΔz0 ¼ 3:88 ð31Þ

with Δz0 � z0ðKÞ�z0ðNaÞ ¼ 0:37Âe (Table 6). Both calculatedratios are in reasonable agreement with the experimentalvalues derived from the HAS data of Fig. 35 for the T-modeand the HREELS data of Fig. 37 for the S-mode.

4.2.2. Cs/Cu(100) monolayerThe phonon dispersion curves of a well-ordered Cs mono-

layer epitaxially grown on Cu(001) show (in addition to theRayleigh mode) a perpendicular resonance near the Γ pointand a longitudinal film mode (Fig. 38). The appearance of adistinct longitudinal mode, which was not observed for thecorresponding monolayer films of Na or K, is attributed to thesmall interface corrugation seen by the Cs atoms and thephonon velocity mismatch between the film and substrate.Hence, the lateral motion of the film is effectively decoupledfrom the substrate and reveals a quasi-two-dimensional phononbehavior.

Fig. 36. HAS diffraction patterns of Na/Cu(001) at two different coveragesθ¼0.048 (above) and θ¼0.125 ML (below) (from Ref. [514]). The spot ringsaround the substrate specular and diffraction peaks (arrows) correspond toordered quasi-hexagonal adsorbate layers with two possible equivalentorientations. The diffraction peak intensities grow with coverage, indicatingthat Na adsorption also induces a surface corrugation of the substrate. For theclean Cu(001) surface the diffraction peaks would not be discernible on thepresent gray scale.

Table 6Dynamic dipole moment dμ/dz (in electrons) and energy (in meV) for the low-lying states of Cu5K and Cu25K cluster models of K/Cu(100). Adapted fromRef. [42].

Cluster Character dm/dz (e) Energy (meV)

Cu5KCovalent 0.25 11.0Mixed 0.60 11.8Ionic 0.98 12.5

Cu25KIonic 0.90 13.4Ionic 0.90 13.6Ionic 0.91 13.9

Fig. 37. HREELS data for the S-mode energy of Na and K on Cu(001) asfunctions of coverage. For Na the HREELS spectra also provide an overtonecorresponding to the creation of two quanta (adapted from Ref. [316]).


When the Cs coverage is slightly increased above the firstmonolayer an entirely different behavior was observed. Insteadof the Rayleigh mode and the longitudinal mode, characteristicnon-dispersive organ pipe phonon modes at energies ofℏω¼2.3, 1.4, and 1.0 meV appear for films of 2, 3, and4 ML thickness, respectively (see below).

4.2.3. SHG studies on AM/Cu(111)The AM overlayer at high coverages is characterized by the

presence of an OR located below the L-band gap and a QWSaround the Fermi level. These bands correlate to s-like and thepz-like bands of a free standing alkali monolayer in the vacuum[515], respectively. Since the QWS is situated inside the L-bandgap, its wave function is localized at the surface. In contrast, thewave function of the OR extends more into the substrate, becauseit is positioned below the lower edge of L-band gap.

For AM coverage ranging from θ¼0.5 to 1.0 ML, transi-tions of OR-QWS, OR-IPSs, and QWS-IPSs are possibleexcitations localized at the overlayer [476]. On the other hand,intraband and interband excitations of s, p, and d bands of bulkare involved in the substrate-mediated excitation in whichelectrons or holes created by the electronic excitation of bulkbands transiently transfer to the AM-induced electronic state.This results in a modulation of the electron density near AMadatoms. In addition, the excitation of the multipole plasmon

can produce a longitudinally oscillating electron density,which induces a coherent motion of alkali adsorbates.To distinguish the excitation mechanisms, the excitation

photon energy dependence of the initial amplitude of thecoherent motion of the S mode was measured for K/Cu(111)(Fig. 39) [509]. The excitation photon energy dependence wasfound to be similar to the absorption curve of bulk copper(solid curve in Fig. 39). Therefore, the substrate electronicexcitation is likely responsible for the coherent phononexcitation of AM on copper. Similar results have beenobtained also for Na/Cu(111) [107].Time-resolved SHG spectroscopy can observe the coherent

Cs–Cu stretching vibration for a complete single layer of Cs onCu(111). While the irradiation with ultrafast pulses at both 400and 800 nm generates the coherent Cs–Cu stretching vibrationat a frequency of 1.8 THz (60 cm�1 ), they lead to a distinctpump fluence dependence of the initial amplitude of coherentoscillation and to a different initial phase. At 400 nm excita-tion, the coherent oscillation is nearly cosine-like with respectto the pump pulse and the initial amplitude increases linearlywith pump fluence. By contrast, at 800 nm excitation, thecoherent oscillation is sine-like and the amplitude is saturatedat high fluence. These features are successfully simulated byassuming that the coherent vibration is generated by twodifferent electronic transitions: substrate d-band excitation at400 nm and the quasi-resonant excitation between adsorbatebands at 800 nm, i.e., possibly from an alkali-induced quantumwell state to an unoccupied state originating in Cs 5d bands orthe third IPS (Figs. 40 and 41).

4.2.4. HREELS studies on AM/copperFig. 42 shows HREELS spectra of the clean and AM-

covered Cu surfaces. The spectrum of the clean Cu(110)substrate (curve a) is characterized by a surface phononresonance at 20 meV [516]. After dosing Na (K) (curves b,d, f and c, e, g for Na and K respectively) an intense and broadpeak at about 18 meV (12 meV for K) appears, reaches itsmaximum intensity at about l/6 of the saturation coverage andthen decreases without frequency shifts. Loss peaks can beattributed to the dipole active adsorbate-substrate stretching

Fig. 38. (a) Measured phonon dispersion curves of a Cs monolayer along the[1 0 0] and [110] directions. L and R denote the longitudinal and Rayleigh modes,respectively, and P the perpendicular resonance. The dashed–dotted lines indicatethe Brillouin zone boundaries for the two domains of the Cs film and the graydashed line shows the Rayleigh phonon curve of the undisturbed bare coppersurface. (b) The phonon density of states of the Cs atoms calculated from a best fitof the measured dispersion curves for a c(4� 2)Cs overlayer on Cu(001) shown inthe inset. The calculated vibrational frequency spectrum has been convoluted by aGaussian with FWHM¼0.25 meV. The solid and dashed lines representdisplacements perpendicular and parallel to the surface.

Fig. 39. Behavior of the initial amplitude of the coherent vibration of the Smode of K/Cu(111) (filled circles) for a K coverage of 0.63 ML as a functionof the excitation photon energy. The solid curve represents the dependence ofthe number of photo-generated carriers within the Cu substrate [509] on theexcitation photon energy. From Ref. [476]


mode as loss amplitudes are strongly peaked in the speculardirection. The adsorbate-substrate frequency is nearly inde-pendent of the coverage, with the only exception of K on Cu(100), where a 3 meV upward shift is observed for θo0.02ML. On all the surfaces the intensity of the stretching induced

peak, normalized to the intensity of the elastic peak, increasesroughly linearly with coverage only for coverages less then l/5of the saturation coverage.The incoming metallization of the AM layer at higher

coverages induces the reduction in the amplitude of the AM-substrate stretch. This behavior can not be explained consider-ing only long range dipole coupling between the alkali atomswith a constant dynamical dipole moment. As the scatteringcross section is proportional to the square of the alkali atomdynamical dipole moment [517], the strong coverage depen-dence of the loss intensities could be due to a largeredistribution of the charge in the alkali atoms for increasingcoverage.The calculations based on electronic wave-functions for

cluster models of K/Cu(100) demonstrated that the Cu–K bondis predominantly ionic [42]. The large dynamic dipole momentof ionic K on a metal surface leads to a large observedintensity of the K vibrations for the frustrated translationalmode normal to the surface.The stretching frequencies of Na and K fall in the continuum

of the bulk and surface modes of Cu. The coupling betweenoverlayer and substrate modes can give rise to a broadeningand a shift of the stretching peak in the HREELS spectra. Inorder to evaluate these effects Rudolf et al. [315] have studiedthe modes induced by Li on Cu(110), as the Li stretchingenergy is more than 5 meV above the maximum of thesubstrate continuum. Li–Cu stretching energy is shiftedupwards by 2.5 meV with increasing coverage (Fig. 43), andthat this shift is explainable in terms of the dipole–dipoleinteraction and delocalization of the vibrational excitationsrelated to the Li stretching mode. Upon reconstruction theenergy of the Li–Cu stretching mode shifts downwards byabout 2.5 meV.

Fig. 40. Coherent Cs–Cu stretching vibration at Cs/Cu(111) surface isobserved by using time-resolved SHG spectroscopy. Irradiation with ultrafastpulses at both 400 and 800 nm produces coherent Cs–Cu stretching vibration ata frequency of 1.8 THz (60 cm–1). At 400 nm excitation, the coherentoscillation is nearly cosine-like with respect to the pump pulse and the initialamplitude increases linearly with pump fluence. In contrast, at 800 nmexcitation, the coherent oscillation is sine-like and the amplitude is saturatedat high fluence. From Ref. [510].

Fig. 41. (a) Excitation wave-length dependence of TRSHG traces taken fromCs/Cu(111) at λex¼800 nm (red open circles) and λex¼400 nm (blue opensquares). The probe wave-length is 565 nm in both cases. The Cs coverage was0.23 ML and the incident pump �uence was 8.0 mJ/cm2 at λex¼800 nm;0.25 ML and 3.6 mJ/cm2 at λex¼400 nm. Solid lines are the results of thenonlinear least-squares �tting. Dotted lines are over-damped componentscontributed by hot electrons. Inset: Fourier power spectra of the oscillatorycomponents of the TRSHG traces for λex¼800 nm (solid) and for λex¼400 nm(dotted). (b) Oscillating components obtained by subtracting the over-dampedones. Note that initial phases are very different, while the frequencies are verysimilar. From Ref. [510]. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

Fig. 42. HREELS spectra for AM on Cu surfaces. Spectra are normalized tothe amplitude of the elastic peak. Cu(110): (a) clean surface; (b) θNa¼0.13ML; (c) θK¼0.08 ML; Cu(100): (d) θNa¼0.18 ML; (e) θK¼0.01 ML; Cu(111): (f) θNa¼0.12 ML; (g) θK¼0.05 ML. Adapted from Ref. [316].


On the Cu(110) surface Rudolf et al. [315] found anadditional energy loss peak at 11.5 meV, which was attributedto a reconstruction effect of the surface under sodium adsorp-tion. Similar results were obtained for potassium, with the onlydifference that the energy of the vertical vibration of a Kadlayer on Cu(110) was found to be 14 meV in the unrecon-structed case, that is somewhat higher than for (111) and (001)surfaces, where it was estimated as 12 meV. As in the case ofsodium, an additional mode at 10 meV was also observed on areconstructed Cu(110) surface.

4.2.5. AM/Al(111)Vibrational spectroscopy studies on Na and Li adsorbed on

Al(111) have been carried out by Nagao et al. [311]. Thevibrational frequency of AM has been found to be dispersion-less (Fig. 44, 11.6–12.6 meV) indicating a strong screening ofthe Na–Na interaction due to the substitutional structure. It isworth mentioning that structural difference has been demon-strated to cause different restoring forces at Na atoms [313].

For this system also surface phonon measurements havebeen performed. For the

ffiffiffi3

p�

ffiffiffi3

pR301�Na/Al(111) the

observed S1 mode (Fig. 45) was assigned [313] to be atransverse surface mode. The surface resonant mode R2 and R1was identified as an acoustic transverse mode and its back-fold,respectively. Fig. 46 shows the perpendicular motion of Naagainst the almost fixed substrate which generates the observeddipole active surface resonant mode at the Γ point (R1).

For the (2� 2)�Na/Al(111) surface, the measured surfacephonon dispersion curves in Fig. 47 locate very close toRayleigh mode of the clean Al(111) and its back-fold atM′ point.

Concerning Li adsorption, a weak Li–Li interaction isdeduced by the occurrence of a constant Li stretching vibration(17.6–18.1 meV, Fig. 48).

Fig. 43. Energies of the stretching peaks in the loss spectra as a function ofcoverage for Li/Cu(110). Measurements were performed at both 90 (filledsquares) and 300 K (empty circles). Re-adapted from Ref. [315]

Fig. 44. A series of HREELS spectra taken at different Na coverages inspecular direction. Adapted from Ref. [311].

Fig. 45. Calculated (R1′, S1′) and measured (R1, R2, S1) phonon dispersions for√3�√3)R301�Na /Al(111) (with a threefold adsorption geometry are shown.Two z-polarized modes R1′ and S1′ (thick curves) can be fit to the experimentallyobserved R1 and S1 modes. However, it is impossible to reproduce the stronglydispersing R2 mode by this model. Adapted from Ref. [311].


4.2.6. AM/Ni(111)Very clean layers of AM could be obtained with the Ni(111)

surface held at 400 K during both deposition and measure-ments. Moreover, all spectra were recorded in a few minutes tofurther reduce contamination. Very likely, the influence of co-adsorbed CO on the A–S bond would be enhanced at low AMcoverage, i.e. at a low Na/CO ratio (Fig. 49).Loss spectra of Na deposited on Ni(111) at 400 K are shown

in Fig. 50a. The Na–Ni stretching vibration shifted (Fig. 50b)from about 25 meV for a very low coverage (0.01 ML) downto 21 meV for increasing Na coverages (0.06 ML). No furthervariation of the Na–Ni stretching energy was observed forhigher coverages, at which the adlayer undergoes a metalliza-tion because of the closer distances between Na adatoms. As aconsequence, dipole fluctuation are screened very efficientlyby the two-dimensional electron gas and the excitation of

Fig. 46. Schematic view of the calculated displacement vectors of the R1mode. Na atoms have large z-polarized displacement vectors while Al atoms inthe second layer move only slightly. The magnitudes of the displacementvectors of the second Al atoms are less than few percent of those of thetopmost Na atoms. This vibrational mode is the same mode as assumed insome ab initio frequency calculations. Adapted from Ref. [313].

Fig. 47. Measured phonon dispersions for (2� 2)�Na/Al(111) (filled circles)are shown. Calculated (solid curves) and measured (open circles) phonondispersions of clean Al(111) are shown together for a comparison. Theresemblance of the observed modes to the Rayleigh mode of the clean Al(111) surface is clear. Adapted from Ref. [313].

Fig. 48. Behavior of the loss energy for several coverages of Li deposited onAl(111). Adapted from Ref. [311].

Fig. 49. Behavior of the Na–Ni stretching energy for a clean Na adlayer andfor Na dosed onto a CO-modified Ni(111) surface at 400 and 300 K,respectively. From Ref. [329].


adatom vibrations perpendicular to the surface is no longerobservable [337].

Aside from the expectations of Ishida's covalent model[168,518] for alkali adsorption on metal surfaces, predicting asoftening of the AM-substrate bond as a function of coverage,theoretical calculations for Na/jellium [324] (Fig. 51) and K onCu(100) [42] found coverage-dependent downshifts of theAM-substrate stretching energy. In addition, the AM adsorp-tion energy decreases largely with coverage [203,519], thusimplying a bond weakening for increasing coverage.

Similar conclusions were reached for K deposited onto theNi(111) substrate. K–Ni vibration energy was found to shiftfrom 18 down to 15 meV as a function of AM coverage(Fig. 52).

4.2.7. AM/Pt(111)The vibrational frequency of adsorbed K adatoms shifts

from 17 meV at low coverages to 22 meV at a coverage of

θK¼0.16 ML, due to lateral dipole–dipole interactionsbetween the adsorbed AM atoms (Fig. 53) [330]. For Kcoverages up to θK¼0.33 ML the frequency decreases to19 meV and the loss intensity nearly vanishes, which isattributed to the metallization of the AM layer. Above Kcoverages of about θK¼0.10 ML, the adsorbed K reacts withresidual water molecules to form KOH, with the K atombonding to platinum. Vibrational modes at 15, 28, 95 and452 meV are assigned to K–O stretching, the OH bending andthe O–H stretching vibrations of KOH, respectively.K-adsorbate-induced phonon modes have been also studied

by TRSHG [332]. The K–Pt stretching mode shows a large

Fig. 50. (a) HREELS spectra of Na layers deposited on Ni(111) at 400 K (lossspectra were acquired at the same temperature). Loss peaks are due to the Na–Ni stretching vibration (b) behavior of the Na-substrate vibration as a functionof Na coverage. The intensity of all peaks was normalized to the intensity ofthe elastic peak. All spectra were multiplied by the same factor. FromRef. [281].

Fig. 51. (left scale, filled squares) Stretching energy of the Na–jellium bondfor hexagonal Na overlayers on jellium with rs¼2 as a function of Nacoverage; (right scale, empty circles) Layer binding energy for the same systemas a function of Na coverage. Adapted from Ref. [324]

Fig. 52. HREELS spectra of K layers deposited on Ni(111) at 400 K (lossspectra were acquired at the same temperature). The inset shows the behaviorof the K-substrate vibration as a function of K coverage. The intensity of allpeaks was normalized to the intensity of the elastic peak. All spectra weremultiplied by the same factor. From Ref. [281].


anharmonicity via coupling to lateral modes. In contrast, thesubstrate surface phonon modes do not show any indicationsof anharmonicity within the laser fluence used in the study inRef. [332].

The mode-selective excitation of coherent phonons at Pt(111) surfaces covered with submonolayer cesium atoms hasbeen demonstrated by TRSHG [285]. A burst of 150 fs laserpulses with the repetition rate of 2.0–2.9 Hz was synthesizedby using a spatial-light modulator, and used for the coherentsurface phonon excitation. By tuning the repetition rate, it waspossible to control the relative amplitude of the vibrationalcoherence of the Cs–Pt stretching mode (2.3–2.4 THz, Fig. 54)to that of the Pt surface Rayleigh phonon mode (2.6 or2.9 THz, depending on the Cs coverage).

4.2.8. AM/Mo(100)The vibrational and electronic excitations of Na atoms

chemisorbed on Mo(100) have been investigated withHREELS [334]. An intense feature at 19 meV is assigned to

the Na–Mo stretching (Fig. 55). The frequency of this modedoes not shift with coverage. The absence of a frequency shiftis contrary to theoretical expectations [42,324].However, the mode intensity is strongly coverage-dependent

as the dynamic charge decreases monotonically with increas-ing coverage (Fig. 56). The decrease in the dynamic chargecan be attributed to depolarization interactions between thealkali dipoles.

4.2.9. AM/Ru(0001)The upward shift of the Cs–Ru stretch frequency by 30% for

0oθCso0.19 ML (Fig. 57) is discussed in Ref. [521] interms of three different effects. The dipole–dipole interaction

Fig. 53. (a) Loss energy versus K coverage for K/Pt(111). (b) Relative lossintensity versus K coverage for K/Pt(111). The primary energy was 5 eV.Adapted from Ref. [330].

Fig. 54. TRSHG trace obtained for a Cs coverage was 0.26 ML. The spectrumhas obtained by single pulse excitation (bottom) Fourier transformed spectra ofthe oscillatory parts of TRSHG traces in the top panel. The time-domain dataused for the transformation were those for t40.45 ps, The center frequenciesfor the Cs–Pt stretching and the surface phonon modes at 0.26 ML Cs areindicated by arrows. Adapted from Ref. [285].

Fig. 55. Coverage dependence of HREELS spectra for Na/Mo(100): (a) 0.007,(b) 0.013, (c) 0.060, (d) 0.100, (e) 0.190, (f) 0.300, and (g) 0.410 ML. Allspectra are normalized to the elastic peak intensity. Adapted from Ref. [334].

Fig. 56. Coverage dependence of the dynamic effective charge at 320 K,derived from the HREELS data using Ref. [520]. Adapted from Ref. [334].


contributes by 7.5%, and the response of the substrate surfacelayer by 12%. The remaining 10% may be due to an increaseof the curvature of the potential-energy surface near to theequilibrium position. This may be due to a lateral squeezing ofthe screening charge with increasing coverage. For thesereasons, in general, a constant or even increasing AM–Mstretch frequency is observed (Fig. 58). Metallization starts atθCs¼0.19 ML. Interestingly, at this coverage a first indicationof the 2� 2 structure appears, so that the change in shift atθCs¼0.19 ML may also be put in relationship with theoccupation by Cs atoms of the on-top position, which is foundin the 2� 2 structure at θCs¼0.25 ML.

4.2.10. AM/graphiteLow-energy (2.3–4.4 meV) vibrational modes have been

observed from K, Rb, and Cs chemisorbed on the basal planeof graphite using inelastic HAS [486] (Fig. 59). These modesare interpreted as phonons propagating in the sagittal plane andpolarized parallel to the surface plane. Modes having perpen-dicular polarization were undetectable. This anomalous resultmay be due to the coupling of the He atoms to the conductionelectrons in the surface.

Information on the vibrational properties of the K/graphite,the metallization of the adlayer at submonolayer coverages,and the charge transfer from the K adatoms to the graphite

substrate could be also provided by HREELS [16] experiments(Fig. 60). A weak, but well-resolved dispersionless peakappears at about 17 meV [16]. The peak reaches its maximumintensity at θ¼0.30 (inset Fig. 60) and then decreases withincreasing coverage. No energy shift was observed withincreasing coverage and the loss intensity was strongly peakedin the specular direction.The energy loss continuum at high coverages resembles a

typical loss continuum normally seen for metal surfaces,consistent with the metallic character of the overlayer. Thelack of an energy shift with coverage is consistent with theessentially linear work function shift in this regime [16], andsuggests constant charge transfer, and weak depolarization ofthe K atoms in this coverage regime.

Fig. 57. HREELS spectra for a Ru(0001) surface [521]. The parameter is theCs coverage θCs, which is the number of Cs atoms relative to the number ofsurface atoms. Primary energy EP and sample temperature T are indicated. Theincidence angle of the electron beam is 601 with respect to the surface normal.

Fig. 58. (a) Loss energy of the Cs–Ru vibrational mode versus Cs coverage forCs/Ru(0001). (b) Intensity of the Cs–Ru stretch vibration relative to theintensity of the elastic peak for the same surface. (c) Relative dynamic chargeQ/Q0. Adapted from Ref. [521].


4.2.11. K/Si(111)AM adsorption induces a (3� 1) surface reconstruction of

the Si(111) surface. Fig. 61 shows the HREELS spectra of theSi(111)� (7� 7) clean surface (dashed line) and the K/Si(111)� (3� 1) surface (solid lines), [522]. The long tail of theelastic peak (the Drude tail), which is observed in the spectrumof the Si(111)� (7� 7) surface (characterized by a metallicbehavior) disappears together with the decrease in FWHM ofthe elastic peak after the K adsorption. The disappearance ofthe Drude tail indicates the semiconducting electronic featuresof the K/Si(111)� (3� 1) surface.

This result is in agreement with the previous angle-resolvedphotoemission studies reporting the semiconducting characterof the AM adsorbed Si(111)� (3� 1) surfaces [523–526]. Theenergy loss peak observed at 55 meV in the spectrum of the K/Si(111)� (3� 1) surface coincides with the well-known55 meV mode of the clean Si(111)� (2� 1) native surface[527] associated with the SV oscillation of the upper edge ofthe 5-membered rings (5-rings) at the reconstructed surface[528]. While the side view of the clean Si(111)� (2� 1) showsan alternate sequence of 5- and 7-rings, where the latter formsurface rows of π-bonded chains (Fig. 61(a)), a possiblestructure for the K/Si(111)� (3� 1) surface (Fig. 61(b))presents a sequence of 5-, 7-, and 6-rings, the latter havingthe surface cusps passivated by K atoms. For this structure the55 meV corresponds to the same SV motion of the upper edgeof the surface 5-ring (double arrow in Fig. 62). Similarly to

clean Si(111)� (2� 1), where the 5-ring upper edge isprotected with respect to adsorption of oxygen and it oscilla-tion frequency remains substantially unchanged for moderateexposures [527], also in K/Si(111)� (3� 1) this mode survivesto K chemisorptions. Moreover, as in Si(111)� (2� 1) [528],

Fig. 59. HAS measurement of the surface phonon dispersions for (a) the lowcoverage phase of K/graphite, and the p(2� 2) phases of (b) K/graphite, (c)Rb/graphite, and (d) Cs/graphite. The measurements were all taken along theΓ�M direction of the graphite using 17.4 meV He beams. The points at higherenergies for the p(2� 2) overlayers are believed to be overtones. Thehorizontal dashed lines are guides for the eye. DFT calculation points forthe p(2� 2) structure of K/graphite are shown as solid squares. The dashedcurves in the Cs graph indicate avoided crossing behavior near the graphiteRayleigh mode, shown as a solid curve. Adapted from Ref. [486].

Fig. 60. HREELS spectra for the clean graphite surface at T¼83 K at differentpotassium coverages ranging from 0.17 to 1.13 ML. The HREELS spectrawere obtained in the specular direction using a primary electron energy ofEi¼1.5 eV, and an incident angle of 601 from the surface normal. Inset: therelative intensity of the energy loss at 17 meV as a function of K coverage hasa maximum centered around 0.24 ML of K. Adapted from Ref. [16].

Fig. 61. HREELS spectrum of the clean Si(111)�(7� 7) surface (dashed line)and the K/Si(111)� (3� 1) surface (continuous line). The primary electronbeam energy is 5.0 eV and the incident and scattering angles are both 601 withrespect to the surface normal direction. In the inset the dispersion relation ofthe loss feature is shown. Measurements have been carried out along the alongthe ½110� direction, in the Γ�Adirection of the (3� 1) surface BZ. Adaptedfrom Ref. [522].


this mode is dispersionless in the long-period direction due tothe good spatial separation of the 5-rings.

4.2.12. K/GaAs(110)The interaction of K with the GaAs(110) surface has been

studied by Betti et al. [529]. On the clean surface a feature at35.5 meV with its first overtone have been recorded andassociated to the FK surface phonon polariton [530](Fig. 63). Due to the macroscopic nature of the FK mode, itsenergy and intensity remain practically unchanged upon the

adsorption of K ions. On the contrary a second feature, fallingat about 50 meV and associated with the surface-plasmonpolariton, rapidly fades out with K adsorption. This may beexplained as due to the fact that the formation of a positive K+

layer on the surface is screened by a negative space charge,eventually leading to a degenerate electron accumulation layerAs a consequence the HREELS signal from the surface-plasmon polariton, which is determined by the bulk carrierconcentration, is quenched while the quasi-elastic peak startsbroadening due to low-energy excitations of the free electronsaccumulated at the surface.

4.3. AM multilayers: organ-pipe modes

The phonon dispersion curves have been measured using Hescattering for Na [317] and Cs [319] films on Cu(001), and thenanalyzed within the framework of a force constant model [305].The strain due to reduced nearest-neighbor distance in film(3.61 Å) with respect to the bulk value (3.66 Å) leads to a 20%stiffening of the resonance frequencies in film compared withthe corresponding ⟨110⟩ longitudinal acoustic (LA) phonons inthe bulk material. An even larger differences occurs for theRaleigh wave velocity along the film surface as compared tothat in Na(110). The resonance frequencies for Na films of agiven thickness closely follow an odd-integer law like the lowerharmonics of an organ pipe with a maximum at the surface anda node at the first substrate layer (organ-pipe modes).The hexagonal sodium overlayers, which grow as bcc (110)

films, have one axis oriented along any of the ⟨100⟩ crystal-lographic directions of the squared substrates. They form inprinciple a domain structure due to the two possible equivalentorientations. However, the observation of a single and constantRW velocity indicates mono-domain overlayers with an uni-form orientation during growth.In Figs. 64 and 65a some selected time-of-flight HAS

spectra, converted to an energy-loss scale and taken at incidentangles close to the specular direction (451) for various Nacoverages, are shown. The RW of the substrate exists also for2 and 3 layers of Na (broken vertical bar). The RW of Naappears upon increasing Na thickness (solid vertical bar). Thetransition occurs for 5 layers, for which RW of both substrateand overlayer coexists. The other features, labeled by a fraction(2n�1)/NL (n=1,2,…,NL=number of layers), are associatedwith the confined resonances of the film. Their frequencies aregiven by:

ω ¼ ð2n�1Þ πvL2anNL

ð32Þ

where νL is the phase velocity and an is an average filminterplanar spacing. The plot of the experimental frequencies(Fig. 65b) for each thickness as a function of n shows indeedthat Eq. (32) is very well satisfied with the same value of thephase velocity. From this correspondence it was concluded thatthese modes are longitudinal standing waves normal to thesurface with wave vector qz¼π(n� (1/2))/an. Their maximumis at the surface and the nodal plane is at a distance d¼NLa

n

below the surface (Fig. 66).

Fig. 62. (a) The structure of the native Si(111)�(2� 1) surface, with the π-bonded chains (thicker lines) in the topmost atomic plane formed by the upperedges of 7-membered rings and the upper edges of the 5-membered rings in thesecond atomic layer beneath the surface, whose SV motion (double arrow)corresponds to the 55 meV mode observed by HREELS [527,528]. (b) Theadsorption of K on the stable Si(111)� (7� /7) surface determines a (3� 1)reconstruction; this can be modeled as a sequence of 7- and 5-membered ringsas in (a) with the insertion of 6-membered rings with cusps saturated by Katoms. The 5-ring upper edges retain their SV mode at 55 meV.

Fig. 63. HREELS spectra (incident energy: 10 eV) from a cleaved GaAs(110)surface for increasing K exposure time. While the Fuchs and Kliever (FK)surface phonon polariton is practically unaffected by K adsorption, the surfaceplasmon polariton peak (SP) fades out. At 2' exposure the elastic peak startsbroadening. Adapted from Ref. [529].


The organ-pipe mode frequencies for a given value of thequantum number n are inversely proportional to the filmthickness d¼anNL, as shown in Fig. 65. For increasing moreand more stationary waves are possible in the film, although thefinite resolution of HAS experiments limits the number of theorgan-pipe modes which are actually resolved in the TOFspectra. The elastic waves which are confined in the film anddo not propagate into the substrate due to a sufficiently largeacoustic mismatch have been first observed in seismology andare known as Sezawa waves [531]. The organ-pipe modes are

just Sezawa waves at zero parallel wave-vector. The Sezawawaves have however a dispersion in their propagation along thesurface, so that their frequencies depend in principle also on theparallel wavevector Q. In the case of organ-pipe modes for AMoverlayers on metal substrates, their dispersion, shown in Fig. 67for various ML numbers, is, however, fairly small, except in theregions of avoided crossing with either the RW dispersion curveof the substrate or that of the film, provided the latter is thickenough to sustain a RW. Since the RW penetration length isapproximately given by ΛT ¼ ð1�v2R=v

2T Þ�1=2=Q [532], where vR

Fig. 64. (a) Inelastic HAS spectra from 2 to 20 ML film of Na on Cu(001), incident energy Ei¼22 meV and a surface temperature of 60 K. For some ML numbersthe spectra of different incident angles θi, are shown. The arrows indicate features corresponding to film eigenmodes of vertical polarization with a node at theinterface and a maximum amplitude on the first layer (organ-pipe modes) [317].


and vT are the RW and transverse acoustic sound velocities, thecross-over from the substrate to the film RW branch occurs whenΛT,film>d. The data reported in Fig. 68 indicate a cross-over atabout 5 ML at phonon energies corresponding to the lowestorgan-pipe mode (�2.5 meV).

It is interesting to note that in the set of HAS dispersioncurves for 2 ML shown in Fig. 68 there are a few points atsmall Q along the anomalous longitudinal surface branch ofthe substrate (dotted line) [378]. The observation of thisbranch, in which the substrate atoms move parallel to thesurface in the third atomic layer beneath the Na surface and atleast 7 Å below the classical turning point of the scattering Heatoms is a further demonstration of the basic inelastic HASmechanism from metal surfaces. As mentioned in Section 3.4,this is based on the phonon-induced surface charge-densityoscillations [402] with two important implications: the inelasticHAS amplitude from a given phonon of wavevector Qand branch index j is proportional to its electron–phonon

interaction coupling strength (the so-called mode-selectedlambda, λQj), while the depth of the detected phononsmeasures its range [402]. This is precious information becausethe coupling of the substrate phonons to the quantum-wellelectrons of the AM overlayer may have important implica-tions in phonon-assisted surface diffusion and surface chem-istry [533].

5. Binary co-adsorption

5.1. Introduction

AM are effective promoters in many industrially importantreactions, such as ammonia synthesis, CO hydrogenation orcatalytic transformation of n-hexane [267,534–537]. Recently,

Fig. 65. (a) Inelastic HAS spectra from a 10 ML film of Na on Cu(001) at three different incident angles θi,, incident energy Ei¼22 meV and a surface temperatureof 60 K. The arrows indicate the fetaures corresponding to organ-pipe modes [317]. (b) The organ-pipe mode frequencies at zero parallel wavevector as functions ofthe wavevector qz in the normal direction for different numbers NL of monolayers. The series of observed quantum numbers n are indicated for each thickness;different symbols correspond to different thicknesses. The experimental points lie along a film dispersion curve with a slope which is about 20% larger than that ofthe corresponding LA bulk branch in the (110) direction [317].

Fig. 66. Schematic diagram showing different possible overtones of organ pipemodes in a 10 ML thin film and a ball and spring model for a row of 10 atomsperpendicular to the plane of the layers.

Fig. 67. The organ-pipe frequencies measured with HAS for Na films on Cu(001) [317] are plotted as functions of the film thickness (same symbols as inFig. 65(b)) and compared with the predicted frequencies of Sezawa wavesSWn (full lines) [531].


AM are commonly used to tailor the activity of noble-metalcatalysts toward hydrocarbon oxidation and to improve thedurability of automobile three-ways catalysts [534,538]. Theability of AM additives to alter the adsorption properties oftransition-metal surfaces is also promising for finely tuning theworking range for catalytic reactions as CO oxidation onplatinum. Thus, the co-adsorption of AM with reactive speciesis a topic of surface science, in view of the fundamentalinterest in understanding the mechanisms of heterogeneouscatalysis and other properties of technological importance suchas the oxidation processes and the enhanced electron emissionrates [47,245,508,539–543].

The catalytic activity of AM promoters is a consequence ofthe AM-induced changes in the adsorption enthalpies ofadsorbates and the activation energies of chemical reactionsinvolving these species [49,544–550] and of the associatedreduction of the work function of the system [551–555].

5.1.1. General considerations on AM co-adsorption withcarbon monoxide

The co-adsorption of AM and carbon monoxide moleculeson single-crystal metal surfaces is characterized by a largedecrease of the C–O stretching frequency [278,556–558], anincrease in the heat of adsorption of both AM and carbonmonoxide [559,560], and changes in core and valence levelbinding energies of CO [561,562]. The CO desorption

temperature increased [120], as a consequence of the co-adsorption-induced mutual stabilization of both AM and CO.The general picture of the AM-induced promotion effect is thatthe intra-molecular C–O bond is weakened, while the metal-CO bond becomes stronger in the presence of AM [563].The detailed description of the above effects involves

various models including substrate-mediated charge transfers[564,565], direct bond through complex formation [566],electrostatic interactions [162,567,568], and the nonlocalAM-induced enhancement of the electronic surface polariz-ability [2,3]. The presence of AM on the surface induces astrong polarization of conduction band electrons of thesubstrate toward CO which therefore can overlap moreefficiently with CO accepting orbitals [188]. Moreover, theCO adsorption site was found to change upon co-adsorption[120,569].

5.1.2. General considerations on AM co-adsorption withoxygenAM and oxygen co-adsorption on metal surfaces has been

extensively studied for both basic and practical reasons[57,264,570–574]. For example, AM are used for oxidationof metal and semiconductor surfaces. AM oxides are widelyused as additives for obtaining low work-function surfacesfor photocatodes or for improving catalytic reactivity, suchas, e.g., ammonia synthesis [575]. From a fundamental point of

Fig. 68. Dispersion curves of organ-pipe modes in Na films (from 2 to 20 ML) on Cu(001) [317]. The arrows indicate the predicted positions of the organ-pipemodes. The fractions identifying the horizontal dispersion curve are values of (2n�1)/NL where n designates the overtone and NL the number of layers. Thedirections of the Rayleigh wave branches in the film (full line) and in the substrate (broken line) are also shown. For the 2 ML film the longitudinal resonance of thesubstrate is also observed at long wavelengths (dotted line).


view, AM co-adsorption with oxygen is a very challengingsubject because of the great variety of chemical and physicalphenomena involved in the reaction. Despite the remarkableinterest, only few dedicated spectroscopic studies have beenconducted over the years in order to address the electronic,vibrational, and bonding properties of co-adsorbed AM andoxygen and, thus, a clear picture has not emerged yet.

DFT calculations by Liu and Hu [178] revealed that the O-substrate bond length increases in the presence of K. Thisimplies an alkali-induced weakening of the bond betweenoxygen and the substrate. The nature of the K–O interactionwas found to be dependent on their mutual distance. As amatter of fact, their interaction is mainly electrostatic when Kis farther away from the O adatom (4–5 Å). On the contrary,for closer configurations a direct bond occurs. Accordingly, adependence on alkali coverage is expected. The role of thealkali pre-coverage is quite intriguing as the adsorption of AMadatoms can significantly modify the bonds between co-adsorbates and the substrate. Moreover, depending on theAM pre-coverage, different scenarios exist for the structuralevolution of the alkali+O co-adsorption system [576].

5.1.3. General considerations on AM co-adsorption withwater

Water interaction with solid surfaces and interfaces [577–586] plays a key role in many physical phenomena such ascatalysis, electrochemistry, corrosion, and rock efflorescing.Improving the comprehension of the reactivity of surfacestoward water could imply important applications in, e.g.,hydrogen production, fuel cells, and biosensors [587]. Duringthe past two decades, water adsorption on single-crystal metalsurfaces has been intensively investigated by theoreticians[581,588–593] and experimentalists [582] as a prototypesystem for understanding chemical bonds in water-solidinterfaces. Nonetheless, a consistent picture of the water–metalinterface has not been reached yet [594].

In particular, water dissociation on metal surfaces hasattracted a great interest [588,595–601] due to its significantrole in many catalytic reactions in the heterogeneous phase[596]. As an electronegative adsorbate, OH intervenes inseveral electrochemical processes. The chemical reactivity ofthe products of water dissociation is in principle different fromthat of undissociated water [602].

The general effect of co-adsorbed AM is to promote waterdissociation [585]. AM co-adsorption with water has beenalready reviewed (including vibrational measurements) byHenderson in Section 4.1 of Ref. [585]. Thus, in the presentreview we will only update the information in Henderson'sreview [585] with the recent progress. In particular, in recentyears the occurrence of electron quantum confinement and ofan increased density of states at Fermi energy has beenassociated to an enhancement of reactivity [603–610]. Waterreactivity in systems exhibiting electron confinement offers theopportunity to put in relationship the nature of the water-interface bond with the presence of electrons confined into atwo-dimensional space.

5.1.4. General consideration on AM co-adsorption withcarbon dioxideSince the start of the industrial revolution the atmospheric

concentration of carbon dioxide is constantly increasing,causing concern about the global climate change and conse-quent catastrophic economic effects [611,612].Despite its relatively low overall atmospheric concentration,

CO2 has significant effects since it absorbs and emits infraredradiation at wave-lengths of 4.26 mm (asymmetric stretchingvibrational mode) and 14.99 mm (bending vibrational mode),thereby playing a role in the greenhouse effect. Activation ofthe stable CO2 molecule and its conversion into othercompounds represents therefore a grand challenge for cataly-sis, also in consideration of the weak and non-dissociativeadsorption on current transition-metal catalysts [613,614].Since AM promoters are able to negatively charge CO2, theinvestigation AM+CO2 co-adsorbed phases is very promisingfor tailoring new catalysts for CO2 sequestration. In particular,vibrational measurements showed that reactions occur via achemisorbed anionic intermediate CO2

�. This species caneither dissociate into CO and O, or dimerize to oxalateðC2O4

2�Þ, or disproportionate to CO and carbonate ðCO32�Þ,

or gain a second electron so as to form CO22�.

5.2. On copper

5.2.1. AM+CO/Cu(111)The co-adsorption of Na and CO on Cu(111) has been

investigated with HREELS for low and intermediate sub-monolayer pre-coverages of Na [274,283,508]. HREELSspectra at 130 K are characterized by a remarkable increaseof the intensity of the Na-substrate vibration (at 21 meV lossenergy) upon CO exposure (Fig. 69).The CO-derived enhancement of the intensity of the Na loss

at 21 meV may be explained by assuming that electron charge

Fig. 69. Electron energy loss spectra measured for 0.40 ML Na/Cu(111) andsubsequently exposed to different amounts of CO at 130 K. Adapted fromRef. [274].


is transferred from the Na atoms to co-adsorbed CO molecules.At high Na pre-coverage (such as shown in Fig. 69) the COadsorption will then effectively bring the AM overlayer backinto the ionic regime where the loss intensity in electronscattering is high. On the other hand, the increased lossintensity could be also due to a CO-induced loss of metalcharacter for the Na layer, which will reduce the screening ofthe field from the incident electrons and thereby produce theobserved intensity enhancement. The Na–Cu stretch modeshifts from 22 (Na on the clean substrate) down to 19 meV.

On the other hand, HREELS spectra recorded at RT on theNa-modified Cu(111) suggested a fully dissociative COadsorption, as indicated by the existence of a peak at36 meV assigned to the O–Na vibration [281,284,369,615–618]. It should be noticed that the weakening of the Na-substrate bond is more effective whenever molecular CO ispresent on the sample surface, as in the case of Na+CO/Ni(111) [619].

No clear picture has emerged yet to fully explain the AM-induced dissociation on noble-metal surfaces. The adsorptionof CO on low-index copper surfaces modified by sub-monolayers of AM is not dissociative [274,620,621], whileCO dissociation was observed on stepped copper surfaces inthe presence of potassium [180,238,622]. From the lattermeasurements, it was suggested that CO dissociation processprimarily occurs at the steps. In fact, the presence of stepswould cause an electric field with a lateral component whichinduces a quite high occupation of the anti-bonding 2πn

orbitals so as to cause the dissociation of CO molecules inthe close vicinities of step [178,623].

The AM promotion effects on CO dissociation were foundto be related to both the short-range electrostatic interactionand the direct orbital overlap. In particular, the direct CO–AMbond significantly enhances the efficiency of CO dissociation.Accordingly, the dissociation barrier is lowered only for shortAM–CO distances (2–3 Å).

A non-dissociative CO adsorption on AM-doped flat coppersurfaces [274,620,621] has been reported for studies attemperatures ranging from 100 to 180 K. On the other hand,HREELS experiments performed at RT by Politano et al. [283]showed that the dissociation barrier for CO molecules is evenlowered on AM-modified Cu(111) surfaces compared withAM-doped transition-metal substrates [624] (Figs. 70 and 71).

It is worth mentioning that the sticking coefficient for COmolecules on copper surfaces at 300 K is extremely reducedcompared with on transition-metal substrates (Ni, Pt, Ru). Thesaturation coverage for CO molecules on clean copper wasfound to be about zero for temperatures higher than 200 K[625], while for Ni(111) the surface was fully covered by COmolecules at 300 K even for small CO exposures [284].Accordingly, it is quite expected that CO adsorption on AM-modified copper substrates might occur only in the closevicinity of AM adatoms. HREELS spectra acquired for severalNa coverages on Cu(111) exposed to 0.4 L of CO (Fig. 71)indicated that no critical pre-coverage for CO dissociationexists, since the AM–O bond is formed even at the lowest AMpre-coverages.

The reduced reactivity of copper towards CO adsorptionshould imply that CO adsorb only in the close vicinities of AMadatoms, so as to enhance the short-range character of theAM–CO interaction and allow orbital overlap, which plays themain role in CO dissociation [178].By comparison, the CO+K/Cu(111) system at potassium

pre-coverages below 0.18 ML has been investigated by Baoet al. using HREELS [620]. Two molecular adsorption statesof CO were found at low temperature (Fig. 72). One withlower C–O stretch frequency has a stronger interactionbetween the adsorbed CO molecule and co-adsorbed K, whilethe other with higher C–O stretch frequency has a weakerinteraction. The two states are occupied sequentially during the

Fig. 70. HREELS spectra for 0.06 ML Na/Cu(111) exposed to CO moleculesat RT. The S3 acoustic resonance of Cu(111) at 21 meV [384] was not excitedunder these scattering conditions in present HREELS experiments. Theintensity of all peaks was normalized to the intensity of the elastic peak. Allspectra were multiplied by the same factor. From Ref. [508].

Fig. 71. HREELS spectra for different amounts of Na deposited onto the Cu(111) surface and successively exposed to 0.4 L of CO. No critical pre-coverage for CO dissociation exists. The intensity of all peaks was normalizedto the intensity of the elastic peak. All spectra were multiplied by the samefactor. From Ref. [283].


exposure to CO. Vibrational measurements suggest that theeffect of K is not to form K–CO complexes indicated by Yatesand co-workers [295,560] but to form K–O complexes, as alsoconfirmed by TDS results by Solymosi and Berko [626].

5.2.2. AM+CO/Cu(100)For AM co-adsorbed with CO on Cu(100) the formation of a

CO-AM complex was reported [120,566,627]. In particular, Liadsorption on Cu(100) at 300 K induced the (2� 1), (3� 3)and (4� 4) structures with increasing Li coverage [628]. Thestructure of (2� 1)-Li/Cu(100), obtained by Li deposition ontoCu(100) at RT with a Li coverage of 0.2–0.4 ML, is called a‘missing-row structure’, where one of every two [001] copperatomic rows is replaced by a Li atomic row [629]. Thechemical properties of this structure are quite interesting forthe one-dimensional copper atomic rows neighbored andpossibly promoted by Li atomic rows [630].

This system offers the opportunity to study the relationshipbetween the change in surface atomic arrangement and thebehavior of vibrational spectra. In particular, a peak at1200 cm�1 associated with the formation of a Li–CO complexwas not observed on (2� 1)�Li/Cu(100) at 100 K, in whichLi atoms aligned one-dimensionally are separated by Cu-atomrows, whereas its was found after the destruction of the (2� 1)order at 200 K and on c(2� 2)�Li/Cu(100) at 100 K. Theappearance of the 1200 cm�1 peak could be related to theexistence of a two-dimensional ensemble of Li atoms on Cu(100) with different electronic states from those of embeddedLi atoms in the (2� 1) phase.

The existence of features at 228–232 and 251–252 meV(1840–1870 and 2025–2030 cm�1) (Fig. 73) reveals that two

different adsorption sites are occupied by CO molecules:bridge and atop, respectively.

5.2.3. AM+O/Cu(111)Loss spectra for 0.07 ML of K on Cu(111) and for (K+O)

are reported in Fig. 74. For small O2 exposures a new lossfeature at 57 meV arose in the spectrum. For further O2

exposures, such peak shifted from 57 to 61 meV and a newloss peak appeared at 46 meV. The K–Cu vibrational modewas not influenced by O2 exposure. After an annealing of thesurface at 500 K the intensity of the peak at 57 meV notablydecreased.The adsorption of atomic oxygen (in the limit of low-

pressure O2 dosage) on Cu(111) does not occur at RT (inset ofFig. 74(b)), while a peak at 46 meV was recorded inmeasurements with the sample kept at 250 K during O2

exposures (inset of Fig. 74(a)). Such feature was assigned inprevious HREELS studies on the same system [631] to over-surface O vibrating against Cu(111).The feature at 57–61 meV has been assigned to the in-phase

vibration of subsurface and over-surface O against the Aglattice, as occurs for O adsorbed on silver surfaces [632–634].As expected, the presence of O in a higher coordination siteleads to the appearance of a feature with higher vibrationenergy in the loss spectrum. The population of subsurface sitesin oxygenated silver abruptly decreased upon annealing.However, on Ag the subsurface site for O is metastable[632–634], i.e. it is accessible only after the complete

Fig. 72. HREELS spectra for different coverage of K on Cu(111). The K-modified surface was saturated with CO at 150 K. Adapted from Ref. [620].

Fig. 73. HREELS spectra recorded at 100 K for c(2� 2)�Li/Cu(100) exposedto 1.2 L of CO (0.21 ML) and further annealed to selected temperatures (200,220, and 260 K). Loss features at 32–36 and 42–45 meV (260–290 and 340–360 cm�1) were assigned to the Li–Cu and the CO–Cu stretching,respectively.


occupation of available on-surface sites. Similar conclusionswere reached by a theoretical study of O adsorption on cleanCu(100) [635,636]. On the contrary, for K-doped Cu(111) theenergetic conditions for the oxygen migration underneath theCu(111) surface seem to be completely reversed, as shown inFig. 74. In the first stage of O adsorption, only subsurface sitesare occupied. At the saturation, over-surface sites could bepopulated too. Moreover, spectra in Fig. 74 show that the AM-derived effects on the stability of subsurface O are reducedonce O atoms adsorb also in over-surface sites. Upon anneal-ing at 500 K the intensity of the feature assigned to subsurfaceoxygen (61 meV) drastically decreased. By contrast, only aslight variation in the amplitude of the O–Cu vibration at46 meV was recorded.

The existence of well-defined vibrational frequencies with-out any shift is another decisive fingerprint for the occurrenceof a change in the adsorption site.

Subsurface sites are not big enough to accommodate Oatoms, hence the penetration of O below the surface isfollowed simultaneously by the lattice distortion. This is inprinciple possible for AM/Cu(111) as AM are known to inducea reconstruction of the Cu(111) surface, as evidenced byRAIRS investigation in Ref. [637]. Subsurface oxygen wasrevealed in Na+O/Cu(111) (Fig. 75) but not on Na+O/Ni(111)(inset of Fig. 75).

5.2.4. Na+O/Cu(110)The study by Grider et al. [205] shows the effects of

annealing on the vibrational spectrum in Na+O co-adsorption.Up to an annealing temperature of 390 K, the HREELSspectrum (Fig. 76) is dominated by a loss at 48 meV, which

could be assigned to either a Cu–O stretch (O bonded to Cuatoms with subsurface Na) or to Na–O stretching in Nasuperoxide (NaO2) species. However, upon annealing at645 K sodium peroxide is formed (Na2O2), as evidence bythe appearance of a loss peak at 59 meV.

Fig. 74. HREELS spectra of 0.07 ML of K deposited on Cu(111) at 400 K andsuccessively exposed to O2. Both O2 exposures and measurements were carriedout at 400 K. The intensity of all peaks was normalized to the intensity of theelastic peak. All spectra were multiplied by the same factor. All spectra weremultiplied by the same factor. Inset: HREELS spectra recorded after exposingto 20.0 L of O2 the clean Cu(111) surface: (a) at 250 K; (b) at RT. FromRef. [618].

Fig. 75. HREELS spectra of the Na-doped Cu(111) surface and of Na+O/Cu(111). In Na+O/Cu(111) the presence of subsurface O is argued from theappearance of the 58 meV peak. No on-surface species were revealed for Na-doped oxygenated copper. Inset: spectrum for 0.06 ML Na/Ni(111) exposed to10.0 L of O2. Besides the Na–Ni stretching, only the O–Ni vibration at 65 meVwas observed, due to O adatoms in three-fold over-surface sites. The intensityof all peaks was normalized to the intensity of the elastic peak. All spectra weremultiplied by the same factor. From Ref. [618].

Fig. 76. HREELS spectra for a saturated coverage of pre-adsorbed sodium and0.5 L of oxygen on a well-ordered Cu(110) surface after isochronal annealingat the following temperatures: (a) 310 K (unannealed); (b) 390 K; (c) 645 K;(d) 705 K. Adapted from Ref. [205].


5.2.5. Na+OH/Cu(111)Vibrational studies on the co-adsorption of Na and OH on

Cu(111) have been reported in Refs. [26,40,277,638]. In orderto verify the reactivity of Na layers, the Na/Cu(111) surfacehas been exposed to small amounts of water vapour at RT.Fig. 77 shows HREELS spectra for several thicknesses of Naexposed to a common water exposure (0.20 L) in the region ofthe O–H stretching. The energy of such vibration (450 meV) isthe fingerprint of a dissociative adsorption of water molecules,also confirmed by the absence of vibrational modes of the H2Omolecule [585]. Only OH groups are present on the substrate.The intensity of the O–H peak is maximum in correspondenceof the completion of the first Na layer, i.e. 0.44 ML (Fig. 78).Such measured quantity is proportional to the average adsorp-tion probability and thus it is indicative of the chemicalreactivity of the adlayer.

Recent theoretical findings [599] support these conclusions. Infact, at one monolayer Na coverage, the water molecule shows amaximum ratio between the adsorption energy and dissociationbarrier, favoring the catalytic water splitting reaction [599]. Incorrespondence of this coverage, well-defined Na QWS exist in theoverlayer [106,107,129,137,138,141,599,639,640] which shifted inenergy and disappeared upon increasing Na thickness.

These results demonstrated that the water reactivity isenhanced in systems exhibiting electron confinement. Waterat surfaces forms chemical bond with metal electrons, espe-cially with those whose wave-function describes confinedstates in a two-dimensional space. Hence, electron quantumconfinement could be used for tailoring water reactivity.

Moreover, the minimum of the work function in Na/Cu(111)has been revealed for about one Na physical layer [552]. Thismeans that the largest charge transfer from the interface to

adsorbates could occur at around 0.44 ML, so as to justify theenhancement of the water splitting efficiency.It is worth mentioning that the decomposition of OH to

chemisorbed O and H could be ruled out by the lack of O- andH-derived vibrational peaks.For θNa¼0.44 ML, water exposure also causes the rise of a

strong loss peak at 36 meV (Fig. 79) which is due to thestretching vibration of OH groups against Na atoms. IncreasingNa coverage, the peak at 36 meV shifts towards higher lossenergies and for θNa¼1.00 ML the OH–Na vibration is at53 meV. This is exactly the loss energy of the OH–Navibration in matrix isolated NaOH molecules [641]. The shiftof the Na–OH vibrational frequency is a fingerprint of a chargetransfer between co-adsorbates.

5.2.6. Li+H2O/Cu(100)Vibrational measurements showed that while at small Li

coverages surface hydroxide (LiOH) forms on the surface, athigh Li coverages surface monoxide (Li2O) forms first,followed by the formation of hydroxide on the monoxide[642].In particular, HREELS spectra of 0.15 ML Li+H2O/Cu(100)

showed an intense peak at 74 meV (assigned to the Li–OHstretching) and small features at 136, 161 and 446 meV [270].A linear triatomic molecule of LiOH which sits on the surfaceupright with Li down is formed in the reactions among co-adsorbates. The reaction scheme is expressed as Li(a)+H2O(a)-LiOH(a)+H(a), where (a) denotes ad-species. The intenseloss-peak indicates the LiOH molecule formed on Cu(001) hasan ionic-bond character. In fact the effective dynamic chargeof the Li–OH stretching vibration is estimated to be 0.5e, andthis is larger than that of the Li stretching vibration in the Li/Cu(001) system, i.e. 0.3e.

5.2.7. K+CO2/Cu(110)Fig. 80 shows the HREELS spectrum for the co-adsorption

of K and CO2 and Cu(110) reported in the work by Onsgaardet al. [643]. The frequency and the assignment of the variousvibrational peaks is reported in Table 7. Physisorbed carbondioxide desorbed upon annealing at 173 K. Heating also

Fig. 77. HREELS spectra for various coverages of Na on Cu(111) exposed to0.20 L H2O at RT. The intensity of all peaks was normalized to the intensity ofthe elastic peak. All spectra were multiplied by the same factor. From Ref.[40].

Fig. 78. Intensity of the 450 meV peak (O–H stretching) as a function of filmthickness. The reactivity abruptly increases between 0.20 and 0.44 ML, due tothe increase of the Na-covered area, and drops abruptly above 0.60 ML. FromRef. [40].


induced the appearance of CO species (213 and 256 meV),which desorbed at 233 K.

Whenever H is pre-adsorbed on the sample surface(Fig. 81), HREELS spectra indicate the formation of formatethrough the reaction

Kþ Hþ CO2-Kδþ UHCOOδ� ð33ÞThe observation of the formate is a step in elucidating the

role of alkali promoters for the synthesis of methanol oncopper based supported metal catalysts (Table 8).

5.3. On aluminum

5.3.1. Cs+H/Al(111)HREELS experiments reported in Refs. [648,649] suggested

that hydrogen atoms form alkali hydrides when hydrogen is

co-adsorbed with AM on aluminum surfaces. The TPD resultsclearly indicate that there are three types of hydrogen in the H/Cs/Al(111) system [257,650]: the hydrogen adsorbed at theCs-effect-free Al sites (α-H), the hydrogen adsorbed at the Alsites perturbed by the presence of Cs (β-H), and the hydrogenstrongly interacting with Cs (γ-H).Intense loss peaks ω1 at 99 meV (800 cm�1) and ω2 at

205 meV (1650 cm�1) are present in the HREELS spectrum(Fig. 82) obtained for Cs+H co-adsorption on Al(111) [257].They are H-induced vibrations as indicated by the 1=

ffiffiffi2

pisotopic shift for Cs+D/Al(111). Both the ω1 and ω2 peaks areassociated with γ-H. In fact, ω1 and ω2 are clearly observed forthe H/Cs/Al(111) system with a Cs coverage of 0.36 ML, forwhich only the γ-H exists on the surface. The loss peaks ω1

and ω2 have been assigned to a bending mode and a stretchingmode of a cesium aluminum dihydride (CsAlH2) complex,respectively. This complex is stable on the surface up to about450 K. A strong charge transfer from Cs to AlH2 makes theAl–H bond ionic resulting in the larger loss intensities for theAl–H vibrations of CsAlH2 compared to those for H on cleanAl(111).Furthermore, another weak loss feature observed around

305 meV has been also ascribed to hydrogen-associatedvibration, in particular to a combination mode (ω1+ω2). Atlower Cs coverage (0.04 and 0.07 ML), shoulder features areobserved on the higher frequency side of the ω1 and ω2 peaks.Such shoulders are associated with α-H sites due to thecorrespondence with the frequencies attained for hydrogenon clean Al(111) [649,651].

5.4. On nickel

5.4.1. AM+CO/Ni(111)A remarkable modification of the loss spectrum of Na/Ni

(111) has been obtained upon CO exposure (Fig. 83). The Na–Ni stretching frequency shifted from 22 down to 13 meV. TheC–O stretching energy, initially at 194 meV (0.07 L of CO),shifted upward to 218 meV (for higher CO exposure). As acomparison, the C–O stretching frequency is 235 meV(0.5 ML of CO) for CO on Ni(111) [281,619]. Interestingly,

Fig. 79. HREELS spectra of Cu(111)and of Na-covered Cu(111)for severalcoverages. Each coverage was exposed to 0.12 L of water vapor at RT. Theintensity of all peaks was normalized to the intensity of the elastic peak. Allspectra were multiplied by the same factor. From Ref. [277].

Fig. 80. HREELS spectra acquired for 0.4 ML K/Cu(110) after the exposure to10 L of CO2 at 107 K (bottom spectrum) successively annealed to selectedtemperatures (173, 233, 400, and 500 K). Adapted from Ref. [643].


the co-adsorption process affected both the Na–Ni and the C–O vibrations and their frequencies were found to depend on theNa/CO local ratio. Likewise, the K–Ni stretching energy wasfound to shift from 15 down to 10 meV upon CO exposure.This finding is in excellent agreement with the observedincreasing of the AM-substrate bond length in the K+COco-adsorption on Ni(111) and Ni(100) [652,653].

The Na–CO interaction could be investigated with moredetails by adsorbing Na on the c(4� 2)�CO/Ni(111) surface(Fig. 84). The CO–Ni and the C–O modes were measured at50 and 235 meV, respectively. The CO–Ni mode strengthenedand shifted to higher loss energies up to 72 meV, while a newfeature arose at 220 meV close to the C–O vibration. Bothmodes (at 220 and 235 meV) merge into a single feature at220 meV for a Na coverage of 0.22 ML. Increasing Nacoverage up to 0.37 ML made such peak to shift down to205 meV. The existence of two distinct C–O frequencies forNa coverage between 0.07 and 0.13 ML indicated the presenceof two different species of adsorbed CO molecules. One isessentially unaffected, the C–O stretching energy being at235 meV; the other is instead strongly influenced by Na atoms,as indicated by the C–O stretching at 220 meV. The line-shapeof the CO–Ni loss broadened as a consequence of differentlocal [Na]:[CO] stoichiometries.

As already mentioned, the C–O bond is weakened by AMco-adsorption and it could also be broken. The AM-promotedCO dissociation was found to be strongly dependent on AMcoverage, and therefore on the average Na–CO distance.Depending on temperature an AM critical pre-coverage forCO dissociation was found to exist. At RT such criticalcoverage was found to be 0.40 ML, and 0.10 ML at 400 K.On the contrary, CO adsorption on a Na-modified Ni(111)surface is always dissociative for temperatures higher than430 K [283]. Na vibration was still present in the loss spectrumof Fig. 85 even with the sample kept at 600 K, while the C–Oand the O–Na stretching modes disappeared. Hence, the resultsfound in Ref. [624] are in contrast with old results in literaturebased on the assumption of a concurrent AM and COdesorption [559,560,654].Similar results have been obtained for K+CO/Ni(111) [655]

and are well reproduced by DFT calculations [245]. The latteralso indicates that the energies of both K–Ni and C–Ostretching modes are practically independent of K and COadsorption sites, respectively.

5.4.2. AM+O/Ni(111)Several vibrational studies on Na co-adsorption with O on

Ni(111) have appeared in recent years [279,281,282,616].They have put in evidence a certain dependence of thevibrational properties of the AM adlayer on the sequentialorder of deposition of co-adsorbates.For a p(2� 2)�O pre-covered surface, the loss spectrum

shows (Fig. 86) the O–Ni mode at 70 meV and an O-activatedphonon at 33 meV [616,656] corresponding to the S2 gapphonon at the M-point of the Ni(111) surface [657]. After thedeposition of sodium onto the O-modified surface, the Na–Nistretching appears at 25 meV, while the O–Ni vibration shiftsfrom 70 down to 64 meV. For Na coverages between 0.11 and0.30 ML, a new feature appears at 34 meV and the Na–Ni peakshifts to 21 meV. The feature at 34 meV was assigned to theNa–O stretching mode. Accordingly, a direct interactionbetween O and Na arises with the formation of a bond betweenco-adsorbates. No other relevant changes were observed forhigher Na coverages.The weakening of the O–Ni bond may be interpreted as due

to a charge transfer from the metal surface to the anti-bonding

Table 7Vibrational features of K+CO2/Cu(110) and assignment as a function of annealing temperature. From Ref. [643].

ΔE (cm�1)Assignment

107 K 173 K 233 K 400 K 500 K

362 ν(CO-substrate )627 ν(CO2,phys)

1045 1045 1045 1045 νs(C–O) in carbonate1375 vsðCO2

�Þ, νs(CO2,phys)1440 1512 1512 1512 1512 νs(OCO) in carbonate1704 1704 ν(C–O) in K-modified Cu2050 2050 ν(C–O) in K-free Cu2356 νa(CO2,phys)

Fig. 81. HREELS spectra for CO2/H/0.75 ML K/Cu(110) at 200 K (lowercurve), the CO2/K/Cu(110) surface annealed to 250 K, a H/CO2/K/Cu(110)interface annealed to 350 K before H adsorption, and a difference spectrum(upper curve) between the last two mentioned spectra. Adapted fromRef. [644].


states of adsorbed oxygen atoms, as found by theoreticalcalculations for the K/O/Rh(111) system [178]. In fact, theelectric field by alkali adatoms would affect the O-substratebond by shifting the electronic states. In particular, the anti-bonding orbitals with O 2pz character become partiallyoccupied in the presence of co-adsorbed AM.

Important information could be provided by reversing thesequential order of adsorption with respect to spectra inFig. 87, i.e. sodium before of oxygen. Upon O2 exposures,two different Na–Ni stretching vibrations were revealed at 19and 25 meV. The peak at 25 meV is assigned to Na adatomsinteracting with oxygen. Very likely oxygen adsorptioninduces a notable strengthening of the Na–Ni bond. Increasingoxygen exposure, the number of Na atoms in close contactwith oxygen gradually increased causing a decrease of the

intensity of the component at 19 meV and an increase of thepeak at 25 meV (Fig. 87(b)).An effective charge Q can be then calculated from μ¼Q

(ℏ⧸2Μrω)1⧸2, with Mr the oxygen reduced mass. Since the O

stretching frequency varies with Na coverage θ in a range wellabove the maximum phonon frequency of Ni, the substratecannot vibrate and Mr is approximately constant and equal tothe oxygen mass. The dynamical charge of O–Ni can then bederived from μ which is in turn derived from the experimentalintensities in dipole scattering conditions. The dynamic dipolarcharge Q relative to the value Q0 for θ¼0 was found todecrease exponentially for increasing Na exposure accordingto the best fit curve (Fig. 88)

Q=Q0 ¼ ð1þ ρe�CθÞ=ð1þ ρÞ ð34Þ

where ρ¼0.97 and C¼67.

Table 8Vibrational frequencies for various bonds, expressed in cm�1. Adapted from Ref. [644].

Bond CO2/H/K/Cu(110) [644] HCOOH/Cu(110) [645] HCOOK [646] HCOOH/K/Co(1010) [647]

HCOOδ(OCO) 750 764υs(OCO) 1385 1350 1357 1366υa(OCO) 1600 1597 1614–1637υ(CH) 2795 2950 2808 2780

CO3

υs(OCO) 1050–1480

COυ(CO) 2030a

K–OHυ(OH) 3710

a1710 In the presence of K.

Fig. 82. HREELS spectra for H/Cs/Al(111) for various Cs coverages,successively saturated with hydrogen. The primary energy was 4.1 eV.Adapted from Ref. [257].

Fig. 83. HREELS spectra for 0.05 ML Na/Ni(111) at 400 K for different COexposures at RT. The intensity of all peaks was normalized to the intensity ofthe elastic peak. All spectra were multiplied by the same factor. Adapted fromRef. [278].


Similar results should be obtained when depositing anotherAM, e.g. potassium. HREELS spectra taken for 0.10 ML O/Ni(111), 0.03 ML K/Ni(111), and the co-adsorbed phase0.10 ML O+0.03 ML K on the same substrate are reportedin Fig. 89. The comparison of the last spectrum with the othertwo shows how the K–Ni and O–Ni stretching vibrationschange upon co-adsorption of the two species. It is seen thatthe K stretching energy increases from 15 to 18 meV, whereasthat of the O–Ni vibration decreases from 70 to 66 meV.

The presence of CO on the surface inhibits the AM-inducedpopulation of O 2pz anti-bonding orbitals, as demonstrated inRef. [617]. Hence, measurements taken on very clean alkalioverlayers are needed in order to verify experimentally thepredicted alkali-induced softening of the O–Ni bond. In fact, inthe HREELS study of the K+O co-adsorption on Ni (111)[658], potassium layers were affected by the presence of COcontamination on the surface and no weakening of the O–Nibond was revealed.

5.4.3. K+Ethylene oxide/Ni(111)The stabilization of ethylene oxide is essential for epoxida-

tion chemistry [659–663]. Ethylene oxide (Et–O) is stabilizedin K-modified Ni(111) for sample temperatures up to 450 K[664,665]. In Fig. 88 shows HREELS data for Et–O andacetaldehyde on clean and 0.36 ML K-covered Ni(111) areshown [236]. The loss spectrum for Et–O on the clean Ni(111)surface, reported in Fig. 90, is dominated by the intense Et–O

Fig. 84. HREELS spectra of c(4� 2)�CO prepared at 200 K and afterdeposition of different amounts of Na at the same temperature. FromRef. [281].

Fig. 85. HREELS spectra of 0.08 ML Na/Ni(111) exposed to 0.4 L CO as afunction of the sample temperature. Na deposition and CO exposure weremade at the same temperature. CO adsorption was found to be partlydissociative at 400 K and completely dissociative for temperatures higher than430 K. The intensity of all peaks was normalized to the intensity of the elasticpeak. All spectra were multiplied by the same factor. From Ref. [624].

Fig. 86. Electron energy loss spectra of p(2� 2)�O prepared at 400 K and afterdeposition of different amounts of Na at the same temperature. The p(2� 2)�Ospectrum shows also the weak O-activated 33 meV S2 mode of the Ni(111)surface (↓) [656], which is however quenched by the disordered Na adsorption.The intensity of all peaks was normalized to the intensity of the elastic peak. Allspectra were multiplied by the same factor. Adapted from Ref. [619].


ring deformation mode at 105 meV (850 cm�1). Further lossesare due to CH2 wagging and twisting modes at 158 meV(1275 cm�1), CH2 scissor at 184 meV (1480 cm�1) and C–H

Fig. 87. (a) HREELS spectra of Ni(111) and 0.16 ML Na/Ni(111) exposed toO2 molecules at 400 K. (b) Each curve was obtained by subtracting anexponential background. The resulting curve was fitted by two Lorentzian line-shapes. The intensity of all peaks was normalized to the intensity of the elasticpeak. All spectra were multiplied by the same factor. Adapted from Ref. [279].

Fig. 88. The dynamic dipole charge Q/e (e is the electron charge) of O as afunction of Na exposure for the system Na+O/Ni(111). Adapted fromRef. [616].

Fig. 89. HREELS spectra of: (a) 0.03 ML of K deposited at 400 K onto the Ni(111) surface, (b) 0.10 ML O/Ni(111) at the same temperature, and (c)0.03 ML K co-adsorbed at 400 K with 0.10 ML of O on the Ni(111) surface.The intensity of all peaks was normalized to the intensity of the elastic peak.All spectra were multiplied by the same factor. From Ref. [616].

Fig. 90. HREELS spectra for Et–O and acetaldehyde on the clean Ni(111) andfor the same surface modified by the adsorption of 0.36 ML. (a) 2 L Et–Odosed on 0.36 ML K/Ni(111) at 100 K, (b) the same upon annealing at 500 K,and (c) 0.36 ML K/Ni(111) exposed to 2 L acetaldehyde. The sample has beenheated at 500 K. (d) 2 L acetaldehyde dosed on Ni(111) at 100 K. Adaptedfrom Ref. [236].


stretching at 377 meV (3040 cm�1). A drastic change isobserved upon co-adsorption with K (deposited at 100 K)and annealing to 500 K. This loss spectrum presents losses at67, 105, 141 and 175 meV (544, 850, 1140 and 1410 cm�1)with also a splitting of the C–H stretching at 343 and 379 meV(2770 and 3060 cm�1).

A nearly identical HREELS spectrum has been recordedupon co-adsorption of acetaldehyde with 0.36 ML K on Ni(111). Identical peak positions and intensities demonstrate theformation of the same surface species from both adsorbates.Actually the analysis of HREELS data suggests the formationof aldehyde-like or acetaldehyde polymerization products.

5.5. On platinum

5.5.1. K+CO/Pt(111)No CO dissociation has been found in HREELS study of K

+CO co-adsorption on Pt(111) [218] [271]. A change in theCO adsorption site from atop to bridge has been shown tooccur with increasing K coverage (Fig. 91). The CO stretchingvibrational energy continuously decreases with either increas-ing potassium coverage or decreasing CO coverage [271].Stretching energies as low as 171 meV (1380 cm�1) arereported in Ref. [271]. The analysis of the vibrationalfrequency indicates a considerably weakened C–O bond andit is consistent with an increased adsorption energy.

5.5.2. K+OH/Pt(111)Water dissociates on K films on Pt(111) and Pt (12,12,13)

surfaces at RT, as evidenced by the HREELS study in Ref.

[330]. For K coverages above 0.10 ML, the adsorbed K reactswith residual water molecules to form KOH, with the K atombonding to platinum. Vibrational modes at 120, 230, 770 and3650 cm�1 (Fig. 92) are characteristic of this compound andare assigned to the Pt–K, the K–O stretching, the OH bendingand the O–H stretching vibrations, respectively. The experi-ments on K+D2O co-adsorption further support the validity oftheir conclusions.

5.5.3. K+H2O/Pt(111)By maintaining the sample temperature at 110 K, water

molecules do not dissociate, as indicated by the presence of H–O–H bending. The influence of K on the vibrational propertiesof adsorbed water molecules at 110 K has been investigated inRef. [666]. Co-adsorbed water induced the disappearance ofthe frustrated translation (O–O) stretch and of the Pt–Ostretching (Fig. 93). All frequencies are reported in Table 9together with their vibrational mode assignment.

5.5.4. K+C2H4/Pt(111)Windham et al. [243] showed that addition of potassium

induces a significant charge rearrangement at the ethylene/Pt(111) interface. In the absence of K, the C–H stretching modeat 363 meV (2930 cm�1), which is representative of hydrogen

Fig. 91. HREELS spectra for saturation CO exposures with various potassiumcoverages. From Ref. [218].

Fig. 92. Energy loss spectra at 310 K before and after exposing H2O (a and b)and D2O (c and d) to a K-precovered (θK¼0.23 ML) Pt(12 12 13) surface. Thetime between the deposition of K and the recording of the spectra is given bythe curves. Adapted from Ref. [329].


bonded to an sp3 hybridized carbon atom [667,668], andthe frequency of the C–C stretching mode at 131 meV(1060 cm� l) indicate that ethylene adsorbed on clean Pt(111)is di-s-bonded to the surface. The foremost effect of co-adsorbed potassium on the C2H4 spectrum (Fig. 94) is togenerate weakly interacting π-bonded ethylene species, whichare characterized by new loss peaks. The population of the π-bonded ethylene increases with K coverage.

For weakly chemisorbed species, the strongest modes arethose with a dipole moment perpendicular to the molecularplane [345]. The loss peak at 108 meV (870 cm�1) has beenassigned to the CH2 wag mode of weakly bonded ethylenespecies with a double bond CQC parallel to the Pt surface.The other features are the CQC stretching at 201 meV(1620 cm�1), the CH bending at 169 meV (1360 cm�1) andthe C–H symmetric stretching at 379 meV (3060 cm�1).

5.6. On ruthenium

5.6.1. K+CO/Ru(0001)The adsorption of CO on a K-pre-covered surface Ru(0001)

surface is non-dissociative [669–671]. The observation of a�22 meV (�180 cm�1) shift of the first overtone of the K-perturbed C–O stretching with respect to twice the funda-mental frequency (Fig. 95) indicated a strong anharmonicity ofthe C–O potential well as an effect of K pre-adsorption [670].Both strength and range of the interaction are found to varywith the ratio of CO:K coverages. At low CO coverage, astrong, short-range interaction between K and CO occurs andCO molecules adsorb in a side-on bonding mode. Withincreasing CO coverage (at constant K pre-coverage) COadsorb in bridge sites, with their molecular axis orientedperpendicular to the surface.

Fig. 93. Vibrational spectra for (a) H2O/Pt(111) and (b) K+H2O/Pt(111). Adapted from Ref. [666] for several coverages of H2O. Sample was kept at 110 K duringmeasurements.

Table 9Vibrational frequencies of H20 and D20 adsorbed on clean and K-doped Pt(111). Adapted from Ref. [666].

Frequency H2O (cm�1) Frequency D2O (cm�1) Mode assignment

240 240 Frustrated translation of H20, O–O stretch in solid ice560 Pt–OH2 stretch, (frustrated z-translation)695 510–580 Frustrated rotation1630 1195 H–O–H symmetric bending (“scissor”)3430 2560 O–H stretch, due to H-bridge bonding3680 2700 O–H stretch, due to free –OH groupsCo-adsorption with 0.06 ML of K110 110 Pt–K stretch330 260 Frustrated rotation (Ry “rocking”), low coverage, supports model (a)485 375 Frustrated rotation (Rx “wagging”), higher coverage1525 1525 Impurity CO (stretch)1590–1630 1200 Symmetric bending3225 2350 O–H stretch, with hydrogen bonding to Pt, only at very low H2O coverage3395 2550 O–H stretch, H bridge bonding to H2O, only at high coverage3620 2685 O–H stretch, due to free –OH groups, only at very low coverage


5.6.2. Cs+CO/Ru(0001)Recently, the occurrence of a predominant long-range

character of the alkali-induced effects has been invoked bydifferent researchers [2,3,212,560,669,672–677]. He andJacobi [336,678] found a significant weakening of the C–Ostretching energy even if CO is adsorbed on the Ru(0001)surface far from the Cs atoms. They discussed the long-rangeor nonlocal effects in the following picture: the CO outside the(Cs+2 CO)�2� 2 islands reduces the amount of charge back-donated to the CO within the island. Moreover, in the co-adsorption of K and CO on Ru(0001) an upward shift of theC–O mode for increasing CO exposures was revealed [669]and attributed to a long-range substrate-mediated interactionbetween co-adsorbates.

Co-adsorption of a small amount of Cs (0.06 ML) gives riseto a new C–O stretching mode at 223 meV (Fig. 96) and anextra LEED pattern of p(2� 2) in addition to the "compres-sion" pattern arising from the incommensurate hexagonal COlattice with a compressed unit cell [679]. The appearance of thep(2� 2) pattern at this low Cs pre-coverage indicates forma-tion of Cs+CO islands with the p(2� 2) lattice. Uponincreasing Cs coverage the low-frequency peak increases inintensity, though it shifts to lower frequency. Furthermore, thep(2� 2) spots become sharp and intense with Cs coverage.These observations indicate two-dimensional growth of the Cs+CO islands. On the other hand, the C–O stretching mode themetal-CO stretching mode (at around 55 meV) due to the COspecies adsorbing outside the islands reduce in intensity andfinally disappear at around a Cs coverage of 0.25 ML, wherethe islands cover the whole surface and the sharpest p(2� 2)LEED pattern is observed. In particular, the disappearance ofmetal-CO stretching is due to the screening by the nearbyAM atoms.

5.6.3. Cs+O/Ru(0001)The interaction between Cs and O on Ru(0001) is char-

acterized by a strong interaction among co-adsorbates with theformation of an AM–O bond [262]. The peak at 24 meV andthe shoulders at 35 and 42 meV were assigned to Cs–Ovibrations. In agreement with matrix isolation spectroscopy[680,681], the latter two frequencies are typical of Cs–O

Fig. 94. HREELS spectra of saturation coverages of ethylene adsorbed at100 K for various potassium pre-coverages on Pt(111). Adapted fromRef. [243].

Fig. 96. HREELS spectra for saturated CO layers on the Cs-pre-adsorbed Ru(0001) surface at 85 K as a function of the Cs coverage. The spectra weremeasured after annealing the surface at 210 K and recorded in the speculargeometry with an incidence angle of 601 from the surface normal. LEEDpattern changes are also shown at the right side of the figure. Adapted fromRef. [558].

Fig. 95. HREELS spectrum for (a) 0.25 L CO adsorbed at 80 K on 0.10 MLK/Ru(0001) (b) the same after annealing to 950 K. Adapted from Ref. [670].


stretching in CsO and CsO2Cs. On the other hand, the peak at24 meV has been ascribed to a Cs–O bending rather than to astretch mode.

Concerning the O–Ru stretch, it has been found to shift from55 to 75 meV upon increasing O coverage (Fig. 97). At higherO coverage and low sample temperature (95 K), also di-oxygen species are revealed. The peak at 98 meV is assignedto O–O stretching in O2

2�. For sake of comparison, its energyis 133–148 meV in O2

�. The low intensity of the O–Ovibration is typical of homonuclear molecules, in which nodipole exists. However, under impact or resonance scatteringconditions, also O–O stretching is observable. On the otherhand, for 0.25 and 0.33 ML of Cs a superoxo ðO2

�Þ speciescan be stabilized at 90 K [682], characterized by its overtonesat 278.4, 414.0 and 546 meV and the O2

�–Ru vibration at15 meV. From the overtone energies the dissociation energy ofO2

� is derived as D¼3.970.3 eV close to the gas-phasevalue (Table 10).

5.6.4. AM+H2O/Ru(0001)HREELS measurements showed that potassium hydroxide is

formed when water is adsorbed at 80 K on a Ru(0001) surfacepre-covered with potassium. Heating the surface to 500–600 Kinduces KOH decomposition and desorption of the dissocia-tion products. The decomposition pathway has been found todepend strongly on the potassium pre-coverage [683]. On theother hand, vibrational measurements on H2O adsorption onCs-pre-dosed Ru(0001) surfaces [684] showed that forθCs¼0.08 ML undissociated H2O adsorbs at 90 K. H2O reactswith the metallic Cs layer for θCs¼0.25 and 0.33 ML. Whilefor θCs¼0.25 ML dissociation into adsorbed H and OHspecies is observed, complete dissociation into 2H(ad)+O(ad)occurs for θCs¼0.33 ML.

5.6.5. K+CH3OH/Ru(0001)Vibrational measurements showed that the presence of

potassium inhibits O–H bond breaking and thus methoxyformation. The methanol molecule does not interact directlywith the potassium atom (Fig. 98). No evidence is found forK–O related vibrations at 25–37 meV (200–300 cm�1). Theonly low frequency vibration observed at temperatures below

Fig. 97. HREELS spectra for Cs co-adsorption with O. Spectrum a is related tothe clean Cs monolayer. Spectra b, c, and d are related to O2 exposure at 220 Kwith a dose of 0.8, 1.0, and 3.2 L, respectively, on the Cs-saturated Ru(0001)surface. Spectrum e is related to O2 exposure of 1.3 L at 95 K on the Cs-saturated Ru(0001) surface. Adapted from Ref. [262].

Table 10Vibrational energies from HREELS and mode assignments for a monolayer of Cs (θCs¼0.33) at Ru(0001) as a function of O2 dose (�) indicates weak intensity.From Ref. [682].

O2 dose (L) Cs–O2� (meV) Cs–O (meV) Ru–O (meV) (O–O)� (meV) 2(O–O)� (meV) Cs–O+ Ru–O (meV)

0.08 250.23 28 56 84, 960.38 28 64 940.53 15 28 66 140 940.75 15 26 71 140 278 951.13 15 25 74 140 278 972.30 15 25 75 140 278 98

Fig. 98. Vibrational spectra of methanol adsorbed on a 0.10 ML K/Ru(0001)surface after annealing to indicated temperatures. Adapted from Ref. [686].


500 K (i.e. prior to methanol desorption) is the Ru–K mode at17 meV (140 cm�1) [669]. The red-shift of the ν(O–D) modefrom 305 meV (2460 cm�1) on clean Ru(0001) to 287 meV(2315 cm�1) (Fig. 96) [685] indicates bond weakening due tointeraction with the potassium-modified surface. The observa-tion of O–D stretch and the out-of-plane O–D bend at 65 meV(525 cm�1) is a fingerprint of the presence of undissociatedmethanol.

5.7. On iron

5.7.1. K+CO/Fe(110)The co-adsorption of CO and K on Fe(110) has been studied

by Zhu et al. [687]. Three adsorption states with distinct C–Ostretch frequencies have been found. Such states, sequentiallyoccupied upon CO exposure and named as α1, α2 and α3, havestrong (α1), intermediate (α2) and weak (α3) interaction withco-adsorbed K, respectively (Fig. 99). The distance betweenthe CO molecule and the nearest K neighbor is the shortest forthe α1 state and the longest for α3 (6 Å).

5.7.2. K+N2/Fe(111)The interaction of N2 with K on Fe(111) surface was

investigated by HREELS [688,689]. Co-adsorbed K atomsenhance the efficiency of the back-donation process from metald-orbitals to the N2 πn-anti-bonding orbital (Fig. 100), due tothe K-promoted decrease of the work function which alters thelocal electrostatic potential.

The HREELS spectra recorded after the saturation exposureof N2 on K/Fe(111) are shown in Fig. 101. Two components at174 and 180 meV (1405 and 1450 cm�1) have been observedand assigned to the N–N stretch vibration of K-coordinatedand Fe-coordinated N atoms, respectively. The K-coordinatedN atoms are characterized by a higher desorption energy and alower stretch frequency with respect to those ones of N atomsfar from AM. This implies a strengthening of the N2–Feinteraction and a weakening of the N–N bond strength.

5.8. On cobalt

5.8.1. K+CO2/Coð1010ÞToomes and King studied by infrared spectroscopy the

reactions of carbon dioxide at a saturated K monolayer onCoð1010Þ [690]. Vibrational measurements (Fig. 102) shed thelight on the pivotal role of the chemisorbed CO2

� asintermediate in the surface chemical reactions. On a Kmonolayer at RT the favoured reaction products are chemi-sorbed CO and O. At 160 K, adsorbed CO and CO2

� havebeen observed. The interaction of CO2 with multilayer K at160 K leads to CO2

� and surface oxalate formation. Inaddition, disproportionation of CO2

� yields adsorbed COand carbonate, CO3

2�. The amount of CO32� formation can

be substantially increased by pre-adsorbing oxygen, whichinhibits CO and oxalate formation. A well ordered CO3

2�

overlayer can be formed by annealing to 600 K. AdsorbedCO3

2� can also be formed on the saturated monolayer of K onthe Co(1010) surface, but with a perpendicular orientation.Vibrational modes in Fig. 102 are reported with their assign-ment in Table 11. For the sake of comparison, in the sameTable 11 the normal modes of CO2 and CO2

� are reported.

5.9. On rhodium

5.9.1. K+NO/Rh(100)In the gas phase, NO has one unpaired electron in the 2πn

anti-bonding orbital, unlike CO whose 2πn orbital is

Fig. 99. HREELS spectra for 0.04 ML K/Fe(110) exposed to CO. Adaptedfrom Ref. [687].

Fig. 100. Left: the notable softening of the N–N bond can be explained by thesignificant electronic charge transfer from metal d-orbitals to the N2 π*-orbital.Right: In the vicinity of K, a strong electron donor, the d–π* back-donation iseven more enhanced. This results in further weakening of the N–N bond.Adapted from Ref. [688].

Fig. 101. HREELS spectra for the saturation exposure of 15N2 on a Fe(111)surface pre-covered with K. Adapted from Ref. [688].


unoccupied. Hence, co-adsorption of NO with AM could shedlight on the role of 2πn states in AM co-adsorption. Theadsorption of NO on Rh(100) is characterized by twomolecular species, the side-on bonded (or highly inclined)α1-NO species, whose N–O stretch is revealed at 115 meV(panel (a) of Fig. 103) and the vertically bonded α2-NO species(N–O stretch at 200 meV). On the clean Rh(100) surface NOinitially adsorbs as α1-NO but with increasing NO coverageα2-NO starts to adsorb and α1-NO reorients to α2-NO [549].The NO dissociation reaction is strongly dependent on cover-age on both K-free and pre-covered surfaces. The promotion ofNO dissociation on Rh(100) by K can be accounted for by thestabilization of the α1-NO species, which is a precursor for NOdissociation.

The analysis of the vibrational spectrum of NO/0.30 ML K/Rh(100) (panel b of Fig. 103) indicates that the stretchfrequency of the α2-NO species depends on the [K]:[NO]stoichiometry. In particular, the N–O stretch of α2-NO speciesis strongly red-shifted in the K-modified surface for low NOcoverage. However, by increasing the content of NO on thesurface, the frequency of the N–O stretch undergoes a blue-shift up to reach the values observed for the clean substrate.

It should be also mentioned that the saturation NO coverageincreases monotonically with K coverage from 0.4 ML with aK-free surface to 0.8 ML for 0.41 ML K.

5.9.2. K+dimethyl ether on Rh(111)There is a considerable interest in the production of DME

which is a key compound in the conversion of methanol togasoline [688–702]. For surface science studies DME repre-sents a challenging compound since its activation and dis-sociation on metal surfaces are very difficult. Therefore onlyfew studies have been devoted to its bonding, structure andreactivity on metals.The influence of potassium on the interaction of DME with

Rh(111) could be put in evidence by the analysis of HREELSexperiments [703]. DME adsorbs molecularly on K-dosed Rh(111) at 90 K as indicated by the spectral features character-ized for DME. Depending on K coverage, a fraction ofadsorbed DME decomposes to adsorbed H and CO duringannealing. Analysis of HREELS spectra suggested the

Fig. 102. RAIRS spectra acquired for CO2 dosed on multilayers of potassium,grown at 160 K and successively annealed at selected temperatures (200, 250,550, and 660 K). Adapted from Ref. [691].

Table 11Vibrational frequencies of CO2 and CO2

� recorded on several K-promoted surfaces.

Sample δ(CO2) δðCO2�Þ vsðCO2

�Þ vaðCO2�Þ νs(OCO) monodentate CO3 νa(CO2) Ref.

Rh(111) 640 840 1340 1630 1440 2350 [692]Pt(111) 640 660 835 1380 1645 1440 2350 [693]Pd(111) 645 744 1210 1530 1452 2350 [540,694]Ru(0001) 666 884 1240 1625�1708 1467 2357 [695]Mo2C/Mo(100) 660 750 1250 1628�1670 1445 2355 [696]Co(1010) – 883 1133 1600 1438 – [690]Au(111) – 957 1350 1620 – – [697]

Fig. 103. (a) HREELS spectra upon adsorption of NO/Rh(100) at 100 K.Panel (b) shows the recorded HREELS spectra for several coverages of NOadsorbed onto a Rh(100) surface modified by the pre-coverage of 0.30 ML K.Adapted from Ref. [549].


transitory formation of methoxy species which is stabilized bypotassium. Strong CO vibration losses at 206 meV(1660 cm�1) for ΘK¼0.03 and 185 meV (1490 cm�1) forΘK¼0.36 are observed as a signature of K-promoted dissocia-tion of DME (Table 12).

5.10. On palladium

5.10.1. Na+CO2/Pd(111)The co-adsorption of Na and CO2 on Pd(111) has been

studied in Ref. [704]. For low Na coverage CO2 moleculesdissociate into CO and O even at 90 K. At intermediate Nacoverage the reaction proceeds via a bent anionic CO2

δ� specieswhich dissociates at higher temperatures into CO and O. Thechemisorbed bent CO2

δ� is characterized (Fig. 104) by vibra-tional losses at 35 (metal–CO2 stretch), 92 (bending mode) and

150 meV (symmetric stretch). The peak around 190 meV(1530 cm�1) might be either attributed to the asymmetricstretch of CO2

δ� or to an AM-influenced C–O stretch.At high Na coverage (2 ML) losses at 83 and 293 meV (670

and 2363 cm�1) were recorded and ascribed to physisorbedlinear CO2. In the region between 105 and 220 meV severallosses were recorded, which were attributed to the vibrationalmodes of mono- and bidentate carbonate [705,706].

5.10.2. K+NO/Pd(111)RAIRS has been used by Nakamura et al. [293] to

investigate NO adsorption on the K/Pd(111) surface. Theadsorption of NO on the clean Pd(111) surface was purelymolecular and reversible, whereas the decomposition of NOproceeded only on the K/Pd(111) surface. Fig. 105 shows the

Table 12Characteristic desorption and HREELS data for the DME+K/Rh(111) system. From Ref. [703].

Products from DME+K/Rh(111)TPD peaks (K) HREELS losses (cm�1)

θK¼0.09 θK¼0.36 θK¼0.09 θK¼0.36

DME+K/Rh(11) 180 120 890 γs(CO) 890 γs(CO)1180 γ(CH3) 1150 γ(CH3)1450 δ(CH3) 1470 δ(CH3)2910 ν(CH3) 2930 ν(CH3)

H2/DME+K/Rh(111) 410 480 660 –

H2/K/Rh(111) 450 520 – –

CO/DME+K/Rh(111) 640 690 1480 ν(CO) 1490 ν(CO)CO/K/Rh(111) 640 690 500 ν(metal–C) 1410 ν(CO)

1500–1770CH3O/DME+K/Rh(111) – – 1010 1030CH3O/CH3OH+K/Rh(111) – – – 180 ν(metal–C)

1050 ν(CO)2780–2900 ν(CH3)

Fig. 104. Series of HREELS spectra for several coverages of Na on Pd(111)exposed to 2 L of CO2 at 90 K. Adapted from Ref. [704].

Fig. 105. IRAS spectra recorded for the K/Pd(111) surface after a saturationexposure to NO at 320 K. Spectra have been collected for K coverage rangingfrom 0 to 0.54 ML. Adapted from Ref. [293].


RAIR spectra measured after the saturated adsorption of NOon the K/Pd(111) surface (θK¼0–0.54) at 320 K. The vibra-tional feature due to NO adsorbed on the threefold hollow siteof Pd was observed at 192 meV for the clean Pd(111) surface[707]. For the K/Pd(111) surface, three N–O stretchingfeatures were observed at 167, 174, 186 meV. The existenceof three N–O spectral components is a clear fingerprint ofdifferent local chemical environments for NO molecules, thussuggesting short-range interactions among co-adsorbates. Inparticular, the latter peak at 167 meV has been assigned to NOadsorbed on K islands on Pd(111) [293].

5.10.3. K+C2N2/Pd(100)HREELS data in Ref. [708] show that ionic KCN forms

from the dissociative co-adsorption of C2N2 and K depositedon the Pd(100) surface. HREELS of this phase shows a surfacemode due to bulk KCN and a C–N stretching frequency of256 meV. Upon heating to 800 K, a new species of KCN, withvibrational losses at 17 and 232 meV, is observed. This speciesis identified as covalently bonded species of KCN, where thereduced ionicity of KCN is due to its interaction with theunderlying Pd substrate.

5.10.4. Cs+C2H4/Pd(110)Vibrational investigations [709] showed that ethylene on the

Pd(110)(l� 2)�Cs surface is more weakly bonded to thesurface as compared with ethylene on the clean Pd(110)surface. Moreover, the de-hydrogenation and CQC bondscission processes are promoted by the pre-adsorbed Cs atoms.

5.11. On gold

5.11.1. K+CO2/Au(111)Potassium adsorption could convert the neutrally adsorbed

CO2 into a more reactive negatively charged CO2 due to acharge transfer from K-dosed Au(111) to an empty π orbital ofCO2. CO2 adsorbs only weakly on a clean Au(111) surface,

with a desorption temperature of Td¼124 K. Adsorption ofCO2 on K-dosed Au(111) leads to its stabilization and to theformation of the CO2

� anion radical. The reaction pathway ofCO2

� species depends on K coverage. It undergoes dissocia-tion at low coverage, while a stable carbonate is formed at highK coverages.HREELS spectra of the CO2+K co-adsorbed layer on Au

(111) at different potassium coverages are shown in Fig. 106.The presence of potassium induces the appearance of vibra-tional modes at 119, 167, and 201 meV (957, 1350, and1620 cm�1), which were assigned to the bending, symmetric,and asymmetric stretching modes of CO2

� [614], respectively.

5.11.2. K+nitriles/Au(100)The interaction of two different nitrile molecules, acetoni-

trile (CH3CN; ACN) and benzonitrile (C6H5CN; BCN), with aK-modified (5� 20)-reconstructed Au(100) surface was stu-died in Ref. [710]. HREELS measurements on K+BCN(Fig. 107) revealed characteristic blue shifts of ν(C�N) inthe presence of co-adsorbed K. This is attributed to a removalof the slightly anti-bonding charge density at the nitrogen endof the C�N group.

5.12. On molybdenum carbide

5.12.1. K+CO2/Mo2CThe adsorption of CO2 on K-dosed Mo2C [696] for K

coverages above 0.18 ML induced the appearance of peakscharacteristic of negatively charged CO2 at 93, 120, 182, and201 meV (750, 965, 1470 and 1625 cm�1) (Fig. 108). Forlower K coverages, the vibrational modes of CO2 are notinfluenced by the presence of AM atoms. Very likely, CO2

� isformed in a direct interaction between potassium and CO2

KðadÞ þ CO2ðgÞ ¼Kδþ�CO2ðadÞδ� ð35Þthrough a charge transfer from the K-dosed Mo2C into theempty πn orbital of CO2.

Fig. 106. (a) Effects of potassium coverage on the HREELS spectra of adsorbed CO2 on Au(111) at 95 K. The CO2 exposure was 8.0 L. (b) Effects of CO2

exposure on HREELS spectra of Au(111) at θK¼1.0 ML. Adapted from Ref. [697].


5.13. On graphite

5.13.1. K+O2/graphiteThe system K+O2/graphite was investigated by HREELS

experiments in Ref. [32]. Besides the presence of the vibra-tional transition of O2 at 190 (v¼0–1) and 380 (v¼0–2) meV(Fig. 109), also another mode at 140 meV with its overtone at280 meV has been revealed. It has been ascribed to O2

�, as thevibrational frequency of the 2Πg state of the O2 molecule inthe gas phase is 133 meV [711]. The coexistence of ionic andneutral oxygen species is a fingerprint of a local charge transferbetween co-adsorbates. Moreover, the analysis of vibrationalpeaks in the work by Chakarov et al. [712] suggests theoccurrence of continuous transformations between different[K]:[O] stoichiometries, with also the coexistence of differentKOx complexes on the surface.

Fig. 107. HREELS spectra recorded in specular geometry (451) with animpinging energy of 3 eV for: (a) BCN multilayers on the clean Au(100)surface; (b) a nearly complete monolayer of BCN on Au(100). The spectrum(c) was recorded after a (i) pre-dosing of 0.06 ML K, (ii) exposure to BCN upto saturation, and (iii) selective desorption of the molecular BCN species whichare not in direct contact with K (as indicated by their known desorptiontemperatures). Adapted from Ref. [710].

Fig. 108. (a) Effects of K coverage on the HREELS spectra of adsorbed CO2 at 100 K. Effects of CO2 exposure on the HREELS spectra of K/Mo2C system: (b)ΘK¼0.1 ML; (c) ΘK¼1.0 ML. Adapted from Ref. [696]. For the assignment of the vibrational modes, the reader is refereed to Table 11 of this review.

Fig. 109. HREELS spectrum from co-adsorbed K and O2 on graphite atTo30 K, adapted from Ref. [32]. The scattering conditions are shown inthe inset.


5.13.2. AM+H2O/graphiteThe HREELS spectra in Fig. 110 show the effect of Na co-

adsorption with H2O on graphite [713] for two typical valuesof θNa. When co-adsorbed with Na, the H2O molecules areonly partially dissociated as it is evident from the OH stretchvibration at 416 meV and the HOH bending vibration at�210 meV. The partial dissociation and probable formationof NaOH is further indicated by the O–H stretch in OH that isseen around 446 meV. The appearance of the collective modesbetween 30 and 105 meV is due to hydroxide and the un-dissociated water being incorporated in an H-bonded networkat the graphite surface. In fact, it was found that co-adsorptionwith NaOH results in increased clustering of the H2Omolecules at the graphite surface [585].

Similar results have been obtained for K co-adsorption withwater [217]. The transition from non-dissociative to dissocia-tive co-adsorption as the K coverage is increased as evidencedby several changes in the vibrational spectra (reported inFig. 111): (i) the disappearance of the H2O collective modes;(ii) the frequency shift of the O–H stretch (from 415 to454 meV); (iii) the intensity variation and the frequency shiftof the frustrated rotation mode νR (88�78 meV), and (iv) theappearance of K–O and K–H losses at 33–55 and 106–126 meV, respectively. The development of these featureswith increasing K is shown in Fig. 112, for a constant watercoverage of 0.25 ML. Water dissociation does not occur forpotassium coverages below 0.3 ML. For 0.4 ML of K waterdissociation is complete.

5.14. On diamond

5.14.1. K+O/C(100)Chemisorption of the O2 and CO molecules on the K-

modified C(100) surface has been studied by HREELS at300 K [714]. Although O2 does not react with the clean C(100)surface, it readily reacts with the K-modified surface at 300 K.Two different chemisorbed species are observed which arecharacterized by the loss peaks at 150 and 214 meV (Fig. 113).The loss peaks at 150 and 214 meV are ascribed to the C–O

Fig. 110. HREELS spectra of H2O and H2O co-adsorbed with Na on graphite(0001) surface at 95 K: (a) 0.65 ML H2O; (b) 0.1 ML Na+0.65 ML H2O; (c)0.22 ML Na+0.65 ML H2O. Adapted from Ref. [713].

Fig. 111. HREELS spectra for 0.25 ML H2O on graphite for differentpotassium coverages. The spectra are taken at 85 K, which was also thedeposition temperature. In all cases the first co-adsorbate deposited on thegraphite surface was water. Adapted from Ref. [217].

Fig. 112. A summary of the HREELSobservations for potassium-inducedwater dissociation. Water coverages vary between 0.25 and 0.4 ML. Σ(diss.products) shows the appearance and growth of K–O and K–H vibrations at 33–55 and 106–126 meV, respectively. Adapted from Ref. [217].


stretch of C–O–C (ether) and CQO (carbonyl) species, whichare formed by breaking both s and π bonds of the surfacedimer, respectively.

The large decrease of the work function of C(100) due to theK adsorption (3.2 eV at 1 ML K) facilitates the electrontransfer to the 2πn anti-bonding orbitals of O2 resulting inthe dissociative adsorption of O2 molecules. In contrast to whathappens in Si(100) (see below), the K-induced oxidationoccurs mainly at the top layer of C(100).

5.14.2. K+CO/C(100)While the CO molecule is known not to adsorb on the clean

C(100) 2� 1 surface, the adsorption of CO occurs on the K-modified C(100) surface. The adsorbed species are character-ized by the loss peak at 154 meV with a shoulder at 192 meV(Fig. 114), assigned to C–O stretch in (C2O2)

2�2K+ and(C4O4)

2�2K+ complexes, respectively. The possible productsformed upon CO exposure on the K-modified surface areshown in Fig. 115.

5.15. On silicon

5.15.1. AM co-adsorption with organic molecules on Si(100)Vibrational spectroscopy is also a powerful tool for obtaining

information on structural properties. An example is representedby HREELS experiments on AM co-adsorption with organicmolecules on Si(100). The deposition of a sodium template onthe Si(100)�2� 1 surface significantly modifies the electronicproperties of the substrate [715,716]. In order to understand andthereby to control the interface between organic molecules andinorganic substrates, it is important to study model systems indetail. In particular, the interaction of organic molecules withsilicon surfaces is attracting great attention [717–729] due to theenormous possibilities of Si-surface functionalization for mole-cular devices, primarily hybrid structures made from organicmolecules on silicon surfaces.

The co-adsorption of Na and 4-aminobenzoic acid [716],benzoic acid [276,730] and glycine [731] have been studied onSi(100) by Richardson's group. The analysis of the HREELS

spectrum could provide important information on surfacechemical bonds at the interface between organic moleculesand the Na-modified silicon surface.It is worth remembering that molecules, whose primary

functionality is a carboxylic acid, such as benzoic acid, adsorbby cleavage of the O–H bond and its replacement by a singlecovalent O-Si bond [732]. In the case of 4-aminobenzoic acidon Na/Si(100)�2� 1 [716] the species were bonded to thesubstrate through the carboxylate group in bi-dentate coordina-tion leaving the amino group unperturbed leading to an aminoterminated surface.Fig. 116 shows the vibrational spectra of 4-aminobenzoic

acid adsorbed at RT on (a) Si(100)�2� 1 and (b) Na/Si(100)�2� 1. Table 13 provides the frequency assignment ofthe observed features by comparison with ab initio calcula-tions. Vibrational data indicate that 4-aminobenzoic acidadsorbs on clean Si(100)�2� 1 through the carboxylategroup. The observation of the ν(CQO) stretch indicatesspecies bonded to Si(100)�2� 1 in mono-dentate coordina-tion. The bonding geometry of 4-aminobenzoic acid to Si(100)�2� 1 is dramatically modified by co-adsorbed Na

Fig. 113. HREELS spectra in the specular mode of: (a) the clean C(100) 2� 1surface characterized by its own phonon modes; (b) the clean surface exposedto 1 ML of K; and (c–g) K-modified surface exposed to O2 with increasingexposure at 300 K. All spectra are normalized by the elastic peak intensities.The primary energy is 6.7 eV. Adapted from [714].

Fig. 114. HREELS spectra recorded for: (a) the clean C(100) 2� 1 surface; (b)the same surface covered by 1 ML of K; and (c,d) K-modified surface exposedto 0.3 and 15.0 L of at RT. The primary energy is 6.7 eV. Adapted from Ref.[714].

Fig. 115. Possible products formed by the CO exposure on the K-modified C(100) surface. Adapted from Ref. [714].


atoms. The sharp feature observed at 176 meV (1420 cm�1) inthe HREELS spectrum of 4-aminobenzoic acid on Na–Si(100)�2� 1, shows clearly that bonding occurs exclusivelyvia the carboxylate group, leading to 4-aminobenzoate speciesin bi-dentate coordination. The amino group related bandsobserved at 46, 201 and 429 meV (370, 1620 and 3460 cm�1)indicate that this adsorption geometry leads to an aminoterminated surface with NH2 groups pointing out into thevacuum.

HREELS measurements support a structural model in whichthe 4-aminobenzoate species adsorb on Na/Si(100)�2� 1through the formation of a delocalized bond between the de-protonated carboxylate group and the adsorbed Na atom.

Upon adsorption of benzoic acid on Na/Si(100)�2� 1,benzoate species are found in a bi-dentate coordination andtilted with respect to the surface normal [730]. HREELSspectra in Fig. 117 show an intense νs(OCO) stretchingvibration, which is characteristic for benzoate aligned perpen-dicular to the substrate surface. In contrast, benzoate bonds toSi(100)�2� 1 through a Si–O–(CO)–C connection. Theobservation of SiH species on both surfaces shows that thechemisorption of benzoic acid occurs through a de-protonationof the acid.

Fig. 116. HREELS spectra of 4-aminobenzoic acid adsorbed on (a) clean Si(100)�2� 1 and (b) on Na/Si(100)�2� 1. The spectra were recorded inspecular scattering geometry (θi¼θf¼451) and an electron energy of Ei¼8 eV.Adapted from Ref. [716].

Table 13Comparison of the experimental HREELS results with the predicted vibrational spectrum of 4-aminobenzoic acid (ABA) on the clean and Na-precovered Si(100)�2� 1 surfaces. Adapted from Ref. [716].

Gaussian 98 (cm�1) Observed in HREELS (cm�1) Assignment

ABA/Na/Si ABA/Si ABA/Na/Si ABA/Si

3512 3540 3513 3513 νas(NH2)3408 3490 3460 3460 νs(NH2)3180 3110 3120 3120 νas(CH)3050 3000 3060 3060 νs(CH)– 2090 2075 – ν(SiH)– 1780 – 1705 ν(C¼O)1685 1690 1620 1620 δ(NH2)1495 – – – νas(OCO)1404 – 1420 – νs(OCO)1213 1300 1290 1290 ν(CO)1156 1190 1150 1180 β(CH)854 860 825 830 δ(CO2)– 800 – 750 ν(SiO)– 620 – 630 Ring643 – 610 – ν(CN)429 480 495 490 Amin/carbox364 – 370 – β(NH2)

Fig. 117. HREELS spectra of benzoic acid adsorbed on Na–Si(100)�2� 1(spectra (a) and (b)) and on Si(100)�2� 1 (spectra (c) and (d)). Spectra (a)and (c) were recorded in specular (θi¼θs¼451) geometry while spectra (b) and(d) in off-specular (θi¼651, θs¼451) scattering conditions. The primaryelectron energy was 8 eV. A schematic representation of the benzoate surfacespecies is also given. Adapted from Ref. [730].


As regards glycine and deuterated glycine co-adsorptionwith Na [731] (Fig. 118), HREELS measurement showed thatglycine bonds to Na/Si(100)�2� 1 through the carboxylategroup in bi-dentate coordination. In contrast, mono-dentatespecies were found after the exposure of Si(100)�2� 1 toglycine.

6. Ternary co-adsorption systems and multilayeredsubstrates

6.1. K+CO2+H2O/Cu(111)

Hoffmann et al. [733] have presented a time-evolved IRASand TPD study of the reaction of CO2 and H2O in thinpotassium layers on Cu(111). Potassium promotes the forma-tion of formate species by reduction of carbon dioxide and bydissociation of water, as sketched in Fig. 119. Water splittingserves as a source of hydrogen for CO2 hydrogenation.Conversion of carbon dioxide utilizing protons from waterdecomposition is likely to provide a sustainable source of fuelsand chemicals in the future. Thermal stability and decomposi-tion pathways studies by TPD and IRAS point to a relatively

high stability of formate and its sequential and selectiveconversion to oxalate and carbonate.The IR spectrum (a) of Fig. 120 is characterized by several

vibrational bands, which are due to potassium formate. Theband at 345 meV (2785 cm�1) is assigned to the C–H stretchand a weak band at 1388 cm�1 to the CH bending mode. Thebands at 201 and 170 meV (1619 and 1367 cm�1) correspondto the asymmetric and symmetric OCO stretch, respectively,while the band at 95 meV (766 cm�1) is that of OCO bendingmode. The remaining band at 2695 cm�1 is the overtone of theCH bend 2δ(CH), whose intensity is enhanced by a Fermiresonance with the C–H stretch.Annealing from 200 (spectrum (a)) to 250 K(spectrum (b))

produces small shifts and sharpening of the vibrational modesof formate.C–H bands due to formate survive up to 425 K and

disappear at higher temperature. The shift in the C–O stretchmodes is a fingerprint for oxalate formation. Upon heatingabove 500 K, asymmetric and symmetric C–O stretches are nolonger seen. The occurrence of an intense band at 181 meV(1460 cm�1) and a weaker line at 102 meV (822 cm�1)indicates the transformation into carbonate, which is stableon the surface up to 675 K.

6.2. AM+CO+O/Ni(111)

Studying the co-adsorption of CO and O in the presence ofAM allows to follow step-by-step the reaction pathways for theformation of carbonates [704] or carbon dioxide [245]. Thepresence of CO molecules has a strong influence on theoxidation process of AM atoms exposed to oxygen [617]. Inorder to investigate the effects induced by CO adsorption onthe surface chemical bonds of Na and O, oxygen was co-adsorbed with Na onto a CO-modified Ni(111) surface [617](Fig. 121). The co-adsorption of Na atoms (third spectrum ofFig. 119) caused the disappearance of the CO–Ni stretching at43 meV (as common in co-adsorption of CO and AM [619])and a softening of the C–O stretching energy down from 226down to 197 meV. An oxygen exposure of 8.0 L causedremarkable changes in the vibrational spectrum, thus suggest-ing the occurrence of striking modifications of surface chemi-cal bonds. The appeareance in the loss spectrum of a feature at

Fig. 118. HREELS spectra recorded for (a) glycine and (c) d5-glycineadsorbed on clean Si(100)�2� 1 and (b) and (d) on Na/Si(100)�2� 1.Spectra were acquired in specular scattering geometry (θi¼θf¼451) and for animpinging energy of 8 eV. Adapted from Ref. [731].

Fig. 119. At temperatures below 200 K potassium pre-covered Cu(111)promotes the hydrogenation of carbon dioxide to potassium formate. Atincreasing temperature a sequential transformation of formate into oxalate andcarbonate is observed. The processes can be monitored spectroscopically asshown in Fig. 119. Adapted from Ref. [733].

Fig. 120. Vibrational spectra of different alkali compounds obtained duringannealing of the surface: (a–c) formate at 200, 250, and 375 K; (d) oxalate at475 K; and (e) carbonate at 600 K. Adapted from Ref. [733].


34–36 meV indicated the formation of the Na–O bond [279–282,284,508,617,618]. In the meantime, the energy of the C–Ostretching shifted from 197 to 209 meV. Further O2 exposurescause a shift of the C–O peak up to 214 meV.

For a CO-modified Ni(111) surface (by predosing thesubstrate with the same CO exposure of 1.0 L), the C–Ostretching energy as a function of O2 exposure was found tofollow a power law (Fig. 122) depending on the initial Na pre-coverage:

vC�O ¼ E0 þ ALOB ð36Þ

where LO is the exposure to oxygen, E0 the C–O stretch energyat vanishing oxygen exposure, A, and the exponent B are thefitting parameters, whose values are reported in Table 14. Onlysmall variations of A and B with respect to the Na coveragewere found for the AM pre-coverage varying over an order ofmagnitude, so as to suggest that only E0 is affected by theinitial amount of adsorbed AM. This supports the conclusionthat the C–O stretch frequency versus O2 exposure obeys thesame power law over a wide range of AM pre-coverage, whoseeffect only consists in a shift of E0 [734].As expected, the energy of the C–O loss peak is strictly

related to the Na/CO ratio. Exposing to O2 a CO-modified Ni(111) surface with a Na coverage beyond 0.20 ML, theformation of carbonates CO3

2� occurs, as inferred by theappearance of loss peaks at 92 and 108 meV. This is a directconsequence of the CO-induced change of the electroniccharge of AM adatoms. Such results indicate that oxygeninteracts strongly with charged AM species and less withneutral adatoms. Furthermore, it should be noticed that also theCO-induced blocking of the population of O 2pz anti-bondingorbitals in the AM+O co-adsorbed phase may play a key rolein the microscopic mechanisms leading to AM oxidation.For Ni(111), the efficiency of the AM oxidation via CO

dissociation [282] or via the CO-promoted ionization of AMadatoms [617] depends on the local stoichiometries betweenAM and CO, on sample temperature and on CO and AM pre-coverages. We want to point out that the method reported inRef. [282] implies a dissociative adsorption for CO, while AMoxidation obtained by the CO-induced ionization of AMnecessitates pre-adsorbed CO molecules. CO exposure wasfound to cause a remarkable increase of the intensity of theNa–Ni peak [278]. Such finding is a clear fingerprint of theoccurrence of a local charge transfer between co-adsorbedspecies, i.e. a CO-induced partial ionization of AM adatoms. Infact, the intensity of vibrational modes is expected to be higherfor ionized species rather than for strongly polarized butessentially neutral adatoms. In Fig. 123 the behavior of thedynamical charge of Na as a function of CO exposure isshown. As the amount of CO deposited on the surfaceincreased, the dynamical charge Q of Na decreased almostlinearly, giving hints on the charge status of AM atoms. Hence,

Fig. 121. HREELS spectra of a CO-modified 0.06 ML Na/Ni(111) exposed tooxygen at RT. The intensity of all peaks was normalized to the intensity of theelastic peak. All spectra were multiplied by the same factor. From Ref. [284].

Fig. 122. Behavior of the C–O stretching energy as a function of the O2

exposure for different Na pre-coverages. The inset shows an approximate lineardependence of E0 on the Na pre-coverage.

Table 14Values of the coefficients of the fit procedure using Eq. (36) of curves presented in Fig. 120. From Ref. [617].

Na coverage (ML) E0 (meV) A B

0.02 205.070.5 5.270.7 0.3770.050.06 197.570.3 5.370.5 0.3670.040.20 179.970.3 5.470.4 0.3670.02


the occurrence of a charge transfer between co-adsorbates is atthe basis of the formation of the AM–O bond.

Measurements were also performed (in the absence of CO)by reversing the sequential order of the adsorption, i.e. oxygenadsorbed before the AM. In these conditions, AM oxidation isreadily observed, as suggested by the appearance of the O–K(28 meV) and O–Na (34 meV) vibrational modes in HREELSspectra (Fig. 124). The direct bond of atomic oxygen with thesubstrate modifies the electronic properties of the system and itshould be responsible of the observed behavior. Very likely,the charge state of O atoms plays a decisive role.

6.3. K+perylene-tetracarboxylic-dianhydride films on Ag(110)

Doped molecular organic semiconductors are of increasinginterest as promising materials for novel molecular electronicand photonic devices, such as light-emitting diodes, lasers andthin-film transistors [735,736]. The vibrational properties of K-doped ultrathin films of the organic semiconductor 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) (for areview, see Ref. [737]) deposited on Ag(110) have beenstudied in Ref. [738]. HREELS indicated that K-doping leadsto a semiconductor–metal phase transition for low K dopingand to a metal–semiconductor one for higher K doping, as alsoobserved for AM doping of fullerenes [739].

For small amounts of K (up to approximately 0.5 K/PTCDA,i.e. 0.5 K atoms for PTCDA molecules) the vibrational spectrum

is nearly identical to that from undoped films (see Fig. 125 andRefs. [740,741]). Above a concentration of 0.7 K/PTCDA fivemodes at 48, 66, 129, 150, and 191 meV (marked in Fig. 125 asA–E) appear and their intensity increased with K coverage. TheC–H out-of-plane mode at 108 meV, indicative of PTCDAmultilayers [742], disappears at K concentrations above 1 K/PTCDA. This mode is known to be very sensitive to changes inthe environment of the PTCDA molecule. The suppression withincreasing K-dose in the present case is therefore attributed to thestructural and chemical changes occurring in the presence of K.In agreement with Ref. [740] the first two modes (A and B

in Fig. 125) are assigned as C–CQO and C–O–C bendingmodes in the anhydride group. The next two (C and D) involveC–O–C stretching and C–H bending vibrations, accompaniedby ring deformations. The fifth marked mode (E) is attributedto C–H bending and ring deformation vibrations. The K–PTCDA interaction leads to a remarkable charge transfer andhence to the formation of new inter-molecular chemical bonds,accompanied by structural changes. At about 1 K/PTCDA theeffect is most pronounced leading to a partially filled (“metal-lic”) molecular valence band. For high K-coverages thevalence band is filled and the molecular layer becomes againa semiconductor.

6.4. K+CO2 on copper films

SERS has been used for investigating the co-adsorption ofCO2 on AM-doped copper surfaces [742]. On potassium-doped copper films, monodentate carbonate as well as carbonmonoxide has been observed in addition to the two carbon

Fig. 123. The dynamic dipole charge Q/e (e is the electron charge) of Na as afunction of CO exposure. From Ref. [328].

Fig. 124. HREELS spectrum acquired for 0.10 ML K deposited onto a0.10 ML O-modified Ni(111) surface (spectrum a). Reversing the order ofadsorption, the alkali–O bond is formed. Similar results were obtained bysubstituting K with Na (spectrum b). The intensity of all peaks was normalizedto the intensity of the elastic peak. All spectra were multiplied by the samefactor. From Ref. [617].

Fig. 125. HREELS spectra as a function of K coverage for PTCDA/Ag(110).Spectra were recorded with a primary electron beam energy of 2.5 eV. Adaptedfrom Ref. [738].


dioxide species. When co-adsorbing CO2 and atomic H onsuch a substrate, the formation of a surface-bound formatespecies seems to occur when hydrogen is adsorbed beforecarbon dioxide (Fig. 126). Formate is stable on clean coppersurfaces at 200 K, but it completely dissociates when co-adsorbed with AM.

6.5. K+CO2 and CO on silver films

SERS experiments have been carried out by Maynard andMoskovits [297] on silver films dosed with submonolayeramounts of potassium. They found that CO2 reacts to form theC2v isomer of a KþCO2

� surface complex. Bands at 94 and151 meV (755 and 1220 cm�1) observed with K-modifiedsurfaces (Fig. 127) were assigned to the stretching and bendingvibrational modes of CO2

�. Very likely, the orientation of

CO2� is with its molecular plane perpendicular to the surface

and bonded to the AM through the O atoms.Experiments with varying potassium coverage imply that the

KþCO2� pair is not randomly distributed on the surface but

aggregated into islands, as supported by a Monte-Carlosimulation.Experiments on Li- and Cs-dosed Ag surfaces led to similar

conclusions. The same authors studied CO adsorption on K-modified silver [298]. Therein the formation of a KOC surfacecomplex was proposed.

6.6. AM co-adsorption on Mo2C/Mo(100)

6.6.1. K co-adsorption with alcohols on Mo2C/Mo(100)As noted in Section 5, metal carbides have been demon-

strated to be efficient catalysts [743–750]. Mo2C is a particu-larly active material for converting methanol and ethanol intomore valuable compounds [751,752]. An important feature ofMo2C catalyst in these reactions is its high thermal stabilityand long life. The catalytic activity of Mo2C is furtherimproved by adding potassium to Mo2C, which considerablyenhanced the rate and the yield of the hydrogen formation[751,753]. The influence of potassium on the nature of theadsorption, on the dissociation pathways, and on the decom-position of adsorbed alcohols (methanol and ethanol) onMo2C/Mo(100) surface has been studied in Ref. [753].Potassium stabilizes the adsorbed alcohols on the Mo2C/Mo(100) surface, which decompose only at higher temperatureyielding H2, CO and CH4 (for methanol) and H2, CO, C2H4,CH3CHO and CH4 (for ethanol). Vibrational measurementsprovided evidence for the ruptures of both O–H and C–Obonds in the adsorbed alcohols and the possible formation ofCH3O–K, CH3 (for methanol) and C2H5O–K, C2H5, species(for ethanol), respectively.The relationship between adsorption sites and the experi-

mental vibrational spectrum has been also studied by theory.For methanol adsorbed on a K top site, Pistonesi et al. [273]found that the C–O distance increases and C–O frequencypresents the maximum decrease when compared with themethanol adsorption in on-top site of Mo. The C–O bond issoftened and O–H bond is strengthened. This suggests ahypothetical dissociating mechanism in which a C–O bondbreakage on K-covered surfaces could be more favorable.

6.6.2. K co-adsorption with C3H7 on Mo2C/Mo(100)HREELS spectra obtained after an exposure of 8.0 L of

C3H7I on clean and K-dosed Mo2C at 100 K showed nosubstantial difference [754] with respect to those recorded formolecularly adsorbed C3H7I on different solids [755–757]. Acompletely different picture was, however, obtained by lowC3H7I exposure (0.4 L), for which new features at 170, 336,365 meV appeared. Based on the XPS measurements, thesenew spectral features are very likely due to the vibration ofpropyl fragment formed in the dissociation of C3H7I. Thisresult suggests that the K-promoted effects are short-rangedand they are evident only for a reasonably high [K]:[C3H7I]stoichiometry.

Fig. 126. SERS spectra of 20 L CO2 adsorbed at 86 K onto a copper film anddoped with 3.6 ML K at the same temperature. Spectra have been recorded as afunction of temperature. For the assignment of the various peaks the reader isrefereed to Table 1 of Ref. [742].

Fig. 127. SERS spectra of CO2 on a K-predosed surface at 50 K with (a)0.15 ML of K and (b) 0.40 ML of K, respectively. Adapted from Ref. [297].


The analysis of the HREELS spectra also provided evi-dences for the formation of KI (formed from the reagents Kand C3H7I present on the sample surface) upon annealing at207 K, as indicated by the appearance of FK modes ofadsorbed KI at 46 and 81 meV [758].

6.7. AM co-adsorption on Cr2O3(0001)/Cr(110)

6.7.1. Na+CO2/Cr2O3(0001)/Cr(110)CO2 adsorption and reaction on Cr2O3(0001) is considerably

modified by the presence of Na on the surface. RAIRS hasbeen used in [759] for investigating the dependence of thecatalytic properties on Na coverage (Fig. 128). At low cover-age Na adsorbs by transferring an electron to the Cr2O3(0001)surface favoring the formation of bent CO2

� (carboxylate)upon CO2 exposition, which has different vibrational features(see Table 11) with respect to physisorbed CO2 (characterizedby an intense peak due to asymmetric stretch at 292 meV,2357–2359 cm�1). NaCO2 salts form and also Na2CO3 can beobserved (asymmetric stretch at 179 meV, 1440 cm�1). Car-bonate forms via disproportionation of two CO2 molecules intocarbonate and CO, with the latter being released into the gasphase:

CO2� UCO2-e�-2CO2

�-CO32� þ CO↑ ð37Þ

6.7.2. Na+NO/Cr2O3(111)/Cr(110)For NO adsorption on Na-modified Cr2O3(0001)/Cr(110),

vibrational spectra indicate the formation of the alkali nitrosylsalt Na2N2O2 at lower temperatures and the formation of analkali salt of the (NO)2 dimer, i.e. sodiumhyponitrite Na2N2Oat higher temperatures. N2O is released via decomposition ofthe latter. NO dissociation occurs even at low temperature and

it leads to the formation of N2O, which desorbs just above100 K.Fig. 129 shows RAIR spectra recorded after a saturation

exposure of NO at 90 K of the Na-modified surface. Theintense N–O stretching mode recorded at 218 meV(1760 cm�1) on clean Cr2O3(0001) is completely absent.Instead, for the lower Na coverages two weak features near161 meV (1300 cm�1) and another weak band at 278 meV(2243 cm�1) were observed. For higher Na coverages threeintense bands with maxima at 107, 132, and 163 meV (864,1064 and 1315 cm�1) were recorded.The band at 278 meV can be identified as the N–N-

stretching vibration of weakly bound N2O molecules which,therefore, already forms at very low temperature on thesurface. Bands near 160 meV are indicative of the existenceof Na+NO� salts in different chemical environments. Thedisappearance of the signal at 500 K is compatible with thethermal decomposition of Na2N2O2 with the release of N2Oaccording to:

Na2N2O2-Na2Oþ N2O↑ ð38Þ

6.1. AM co-adsorption with CO on bimetallic surfaces

Recently, chemical reactions at bimetallic surfaces haveattracted a notable interest [761–770] for both fundamentalinterest and technological applications (fuel cells, heterogeneouscatalysis, chemical sensors, pollution). AM co-adsorption onbimetallic surfaces has been scarcely investigated. In the recentwork by Politano et al [1]. the co-adsorption of AM atoms (Na,K) with CO on Ag/Cu(111) and Ag/Ni(111) has been studied.The existence of QWS within the Ag adlayer is proven for bothAg/Cu(111) and Ag/Ni(111) [61] systems. However, due to theabsence of a gap in Ni(111), the character of the QWS in such

Fig. 128. Series of IRAS spectra of CO2 (30 L at 90 K) adsorbed on clean andNa pre-covered Cr2O3(0001)/Cr(110) surfaces. Na coverage was prepared withthe sample at 90 K. Adapted from [759].

Fig. 129. IR spectra of NO adsorbed (30 L at 90 K) on a sodium coveredCr2O3(0001) surface as a function of sodium coverage. The sodium layer wasprepared at 90 K. Adapted from Ref. [760].


two systems is substantially different [771]. QWS on Ag/Cu(111) are characterized by standing wave patterns, which on thecontrary are not supported on Ag/Ni(111). Such significantdissimilarities in the electronic properties between these twobimetallic surfaces could in principle lead to quite differentcatalytic properties. Accordingly, they may be used as modelsystems for a comparative study on the chemical reactivity ofbimetallic surfaces.

To verify the effects of the adsorption of submonolayers of AMon the chemical reactivity of the above bimetallic surfaces,0.06 ML of Na deposited on a single layer of Ag on Ni(111)were exposed to 0.4 L of CO (Fig. 130, spectrum d). For the sakeof a comparison, the same experiment was performed for 0.06 MLof Na deposited onto the clean Ni(111) and Cu(111) substrates(Fig. 130, spectra a and c).

The occurrence of CO dissociation was found for 0.06 MLNa/1 ML Ag/Ni(111) (Fig. 130, spectrum d) while onlymolecular CO adsorption was observed on 0.06 ML Na/Ni(111) (spectrum c). The partial dissociation of CO moleculeswas inferred from the appearance of the stretching vibration ofNa against O atoms at 36 meV coming from dissociating COmolecules. As concern the C–O internal vibration, its energywas recorded at 215 and 232 meV for 0.06 ML Na/Ni(111)(spectrum c) and at 186 meV in the 0.06 Na/1 ML Ag/Ni(111)(spectrum d), respectively. Interestingly, at a fixed Na pre-coverage (0.06 ML) the C–O bond is much more softened in thebimetallic surface with respect to the clean Ni(111) substrate.The weakening of the C–O bond leads to CO dissociation and tochemical reactions of the derived species, i.e. the Fischer-Tropsch process, CO oxidation, carbonate formation and so on.

On the other hand, for Na+CO co-adsorption on clean Ni(111), CO dissociation was not revealed for AM pre-coverages

below 0.40 ML Na. For such coverage two loss features at 36and 55 meV appeared (Fig. 130, spectrum b), thus indicatingthe occurrence of CO dissociation. Na doping determines theactivation of a local charge transfer from the Ag/Ni(111)interface to the 2πn anti-bonding orbitals of CO leading to asoftening of the C–O bond up to the dissociation of COmolecules. Such charge transfer is very likely due to thehybridization between the CO 2πn states and the QWS of Na-promoted Ag/Ni(111).A large hybridization, and probably an efficient charge

transfer, is possible only whenever the Na–CO distance isshort, as also found in theoretical calculations performed forCO adsorption on Na quantum dots on Cu(111) [772]. More-over, recent theoretical results performed for CO adsorption onAl/Fe(100) [773] have demonstrated that the activation barrierfor CO dissociation is strongly reduced in the bimetallicsurface with respect to the clean Fe(100) surface. On the basisof the latter result, it can be suggested that the joint action ofAM doping and the existence of a bimetallic Ag/Ni(111)interface should further reduce the activation barrier so as toproduce an enhanced CO dissociation rate. On the other hand,the ratio between the intensity of the O–K vibration (recordedat 27–29 meV) with respect to the C–O stretching indicatesthat in the K-doped Ag/Cu(111) surface the activation barrierfor CO dissociation is higher than on K/Cu(111).The remarkable differences towards the promotion of CO

dissociation in AM-modified Ag/Cu(111) with respect to thecase of AM-doped Ag/Ni(111) could be ascribed to thedifferent electronic properties of these two systems. Due tothe absence of a gap on Ni(111), hybridization effects betweenQWS and substrate states are enhanced on Ni(111). This effectis very important for a single layer of Ag [774]. Suchhybridization enhances the efficiency of the charge transferto 2πn anti-bonding orbitals of CO, so as to soften the C–Obond and decrease the activation barrier for CO dissociation.On the other hand, the surface obtained by depositing

0.15 ML of potassium on 10 ML Ag/Ni(111) was found tobe strongly reactive towards water molecules arising fromresidual gas in the UHV chamber. Vibrational spectra(Fig. 131) revealed the adsorption of partially dissociatedwater, as suggested by the presence of O–H stretching peaksat 435 meV (water molecules) and 450 meV (OH groups)

Fig. 130. HREELS spectra acquired after having exposed 0.4 L of CO at RTon (a) 0.06 ML Na/Cu(111), (b) 0.40 ML Na/Ni(111), (c) 0.06 ML Na/Ni(111), and (d) 0.06 ML Na/1 ML Ag/Ni(111). From Ref. [1].

Fig. 131. HREELS spectra acquired for 0.15 ML of K deposited onto 10 MLAg/Ni(111) at RT.


[40,585]. Another clear fingerprint of adsorbed molecularwater is the appearance of HOH bending at 199 meV [585].Interestingly, water adsorption was not allowed for AM-modified 1 ML Ag on both Cu(111) and Ni(111) substrates.The enhanced reactivity of the AM/bimetallic surfaces towardswater adsorption could be ascribed to the presence of well-defined Ag 5sp-derived QWS within the adlayer for 10 MLcompared to 1 ML [775]. In fact, water reactivity was found tobe enhanced in systems exhibiting electron quantum confine-ment [40,277,588]. Such finding may be of great interest aswater interaction with solid surfaces is useful for understand-ing a wealth of physical and chemical phenomena such aspollution, corrosion, energy production, and heterogeneouscatalysis.

6.2. AM co-adsorption on epitaxial graphene

The vibrational spectrum of the co-adsorbed phase ofpotassium with water on graphene/Pt(111) has been studiedby Politano and Chiarello [776] for a sample temperature of100 K.

In the absence of alkali dopants, water on graphene/Pt(111)adsorbs undissociatively at 100 K, as also reported in Ref.[370]. Water molecules on graphene/Pt(111) at 100 K arephysisorbed and they desorb from graphene by heating thesurface at 130 K. To investigate the effects of K on wateradsorption, the MLG/Pt(111) surface precovered by 0.08(bottom spectrum of Fig. 130) and 0.25 ML (top spectrum ofFig. 130) of K, respectively, was exposed to 1 L of water at100 K. Loss spectra (Fig. 130) indicate that pre-adsorbed Khas dramatic effects on the adsorption of water. Contrary to theresults for pristine graphene in Ref. [370], there is no evidenceof adsorbed molecular water. Vibrational bands centered at177, 364, and 445 meV were observed in both loss spectra.Interestingly, the bands at 177 and 364 meV indicate theoccurrence of water dissociation as they are a fingerprint of theformation of C–H bonds [777]. Their energy corresponds tothe bending (177 meV) and stretching vibrations of C–H(364 meV). The energy of the C–H stretching indicates theformation of a sp3 bond. In fact, such mode is expected around375–380 and 360–365 meV for the sp2 and sp3 hybridization,respectively [667,778]. The band centered around 445 meV isassigned to the O–H stretching of residual adsorbed hydroxylgroups [40,585] while the peak at 42 meV in the 0.25 ML K/H2O/MLG/Pt(111) is due to a vibration of K atoms against OHgroups in K(OH)x complexes.

No K-graphite vibrational features were found in the range10–20 meV and this indicates that no agglomeration ofpotassium in clusters occurs, in agreement with theoreticalresults [601,779].

Results in Fig. 132 provide a novel mechanism for attaininga partial graphene hydrogenation by water dissociation as aconsequence of the promotional role of K dopants in thedissociation process [178,283,624].

The partial hydrogenation of graphene induced by thepresence of K could be driven by the spillover phenomenon.The decoration of graphene with alkali atoms can improve the

capability of storing hydrogen. In particular, the presence ofmetal catalysts on graphitic systems enhances the hydrogenuptake of the system via spillover [780].Reversing the order of adsorption, i.e. water before of

potassium, could reveal other aspects of the water-alkaliinteraction at epitaxial graphene. HREELS spectra for0.01 ML of potassium adsorbed on 1.5 L H2O/MLG/Pt(111)surface are shown in Fig. 131a. The analysis of HREELSspectrum indicates that low K coverage has negligible effectson the water adsorption and no dissociation occurs. On thecontrary, significant changes in the loss spectrum have beenobserved at 0.20 ML of K. Vibrational features related tomolecular water disappear and the loss spectrum is dominatedby the K–OH vibration at 42 meV. Weak modes due to C–Hbending and stretching and to C–OH stretching are observed(Fig. 133b). Again, water dissociation occurs provided that the[K]:[H2O] stoichiometry is enough. C–H and C–OH areweakly bonded to the graphene surface. Heating the surfaceto 160 K induces a reduction of the K–OH peak by three times(Fig. 133a) and finally coadsorbates completely desorb byannealing at 200 K.

7. Conclusions and outlook

We have presented an extended survey on the adsorbedphases of alkali atoms on metal surfaces and their role in theheterogeneous catalysis of several fundamental chemicalprocesses, which have relevance in the area of renewableenergy. We specifically addressed the characterization ofstructures and processes by means of vibrational spectroscopy.The wide arsenal of spectroscopic methods available today,

Fig. 132. HREELS spectra for 0.08 and 0.25 ML of K adsorbed on MLG/Pt(111) and successively exposed to 1 L of H2O at 100 K. The inset shows amagnification of the outermost spectrum (0.25 ML K).


ranging from the inelastic scattering of electrons (EELS,HREELS) and atoms (HAS, QHAS, 3HeSE), to opticalmethods like surface-sensitive infrared (RAIRS), Raman(SERS) and sum frequency generation (SFG) spectroscopies,allows to determine the bonding nature and strength ofadsorbed atoms and molecules with the substrate and theirmutual interactions. The latter are of great relevance for co-adsorbed phases.

Besides being a vehicle of information on the bondingstructure, vibrations play an important role in surface chem-istry not only collectively in thermal activation, but alsoindividually through the modulation of the distances amongreactants and catalysts. As shown in recent years, surfacephonons may selectively induce chemical reactions viaphonon-induced surface charge density oscillations [533], thelatter being governed by the mode-selected electron–phononinteraction [401,781]. As discussed in Section 2.2, suchinteraction can be directly extracted for conducting surfacesfrom inelastic HAS amplitudes. Moreover the high sensitivityof helium atom scattering to surface charge oscillations,especially those originating from the surface electronic stateswhich are the most peripheral and the first being probed by theimpinging atoms, bears great promises for the spectroscopy ofASP [364,494,496–498,782–785] in the low meV regime,presently inaccessible to electron energy loss spectroscopiesdiscussed in Section 2.1. Alkali atoms work in adsorbed and

co-adsorbed phases, as well as in surface catalytic processes, asfree charge reservoirs. Thus low-energy plasmon and ASPspectroscopy would represent a further characterization tool ofgreat potential in surface chemistry as well as in appliedplasmonics.On the optical side, TRSHG spectroscopy is proving to be a

very versatile tool in the investigation of coherent phonondynamics at surfaces. This method appears to be ideally suitedfor alkali overlayers because of the marked enhancement ofSH intensity. So far applications of TRSHG to other adsorp-tion systems or to co-adsorbed phases have been very limited,mainly because species other than AM lack such a largeenhancement in SH intensity. This obstacle can be removed ifSH intensity is enhanced by tuning the photon energy of probepulses to an electronic resonance of adsorption systems. Inaddition, using much shorter pump pulses extends the applic-ability of TRSHG to coherent surface phonons and adsorbatevibrations at higher frequencies. The recent progress made inthese respects opens a further window on the investigation ofalkali metals on metal surfaces and their applications in surfacecatalytical processes of relevance in materials science forrenewable energy.

Acknowledgments

One of us (AP) acknowledges the hospitality at the DonostiaInternational Physics Center (DIPC) for the finalization of thepresent review. GB acknowledges IKERBASQUE for support(project ABSIDES). We are indebted to Professor Renée D.Diehl (Penn State University) for the complete information onher and her coworkers' work on AM's on metal surfaces andgraphite and for very useful discussions. We also thank Prof. J.P. Toennies (Max-Planck-Institut für Dynamik und Selbstor-ganisation (MPI-DS) Göttingen) for useful discussions onHAS experiments on AM/metal surfaces, especially during avisit of one of us (GB) at MPI-DS in the framework of the Re-invitation Program of the Alexander-von-Humboldt Founda-tion, which is gratefully acknowledged.

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Vibrational spectroscopy and theory of alkali metal...

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