Surface Science 159 (1985) L433-L438

North-Holland, Amsterdam L433

SURFACE SCIENCE LETTERS

EVIDENCE, BY METASTABLE QUENCHING SPECTROSCOPY, THAT CO ADSORBS ON K/Ni(lll) NEAR POTASSIUM AND LIES FLAT OR TILTS WHEN HEATED

Jose ARIAS, Jiwha LEE, Joan DUNAWAY, Richard M. MARTIN and Horia METIU

Department of Chemistry, University of California, Santa Barbara, California 93106, USA

Received 26 February 1985; accepted for publication 28 March 1985

By using metastable quenching spectroscopy (MQS) we show that if a K/Ni(lll) surface with

low K coverage (0, = 0.1 and 0.12) is exposed to low doses of CO (0.3 L), the CO molecules arc

adsorbed near the potassium atoms. Heating from 118 to 298 K results in spectral changes that

indicate that the CO molecules closest to the potassium atoms tilt strongly or he flat on the

surface.

It is well known that the presence of small amounts of alkali metal atoms on transition metal surfaces modifies substantially the properties of chemisorbed CO [l-11]. In this letter we report evidence, obtained by metastable quenching spectroscopy (MQS), that at low exposures and temperatures CO adsorbs on K/Ni(lll) sites neighboring the potassium atoms; furthermore, an increase in temperature causes the CO molecules located near potassium to tilt or perhaps lie flat next to the potassium atom. These findings support recent conjectures based on the similarity of the vibrational spectrum of CO coadsorbed with alkali [6,8,9] to that of organo-metallic compounds in which the CO ligand

binds sidewise to the metal atom.

The MQ spectra presented here were taken in an apparatus described in a previous article [12]. Potassium deposition was made by using a SAES getter. Since the getter often generates CO, we have taken precautions to minimize contamination. Thus we prepare potassium sub-monolayers by deposition of excess potassium, which is then thermally desorbed until we achieve the desired coverage; the small amounts of CO produced by the getter and adsorbed on the surface are removed during potassium evaporation. The potassium coverage was calibrated by Auger electron spectroscopy (AES), LEED and thermal desorption in separate experiments [13] in which we determine the coverage achieved by a given deposition-evaporation schedule. The K/Ni(lll) surfaces used for CO adsorption and MQS measurements are then prepared by following the schedule known to yield the desired coverage.

0039-6028/85/$03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

L434 J. Anu., CI cd. / C‘O on K,‘.Vi(lll)

Unfortunately our knowledge of coverage is not as accurate as our use of two

decimals might suggest. Since MQS is very surface sensitive, we consider that the surface is not

contaminated by impurities that might be present in the background gas ( p < 10p’o Torr) or in the metastable beam, if the MQ spectrum of the surface is not changing in time.

To obtain information about the surface, we expose it to a low kinetic energy beam of He atoms prepared so that a small fraction is in the metastable (i.e. long lived) state He*(2lS). Upon contact with the surface He* is quenched to its ground state by processes which cause electron ejection from the surface. The MQ spectrum is a plot of the ejected electron current as a function of the electron kinetic energy.

The conclusions of this paper are reached by making use of the fact that the excited atoms are local probes which cause electron ejection from a surface site only when the Is He* orbital and the surface site orbitals overlap. Further- more, the ejection process is very efficient and very few metastables survive the encounter with a filled orbital; thus each metastable atom is likely to ionize the orbitals of the first molecule or surface atom it meets on its path.

With this in mind let us examine the MQ spectrum presented in fig. 1. The lowest curve shows the spectrum of a K/Ni(lll) surface having a coverage of 0.1 potassium atoms per surface Ni atom. The broad feature at high kinetic energy is typical of surfaces containing low potassium coverages [14]. The peaks (Y and ,f3 are due to the ionization of the K 4s orbital by the singlet He*(2rS) and the triplet He*(23S) atoms, respectively. The triplet species is formed by a surface induced conversion of the singlet [14]. The peak y is the Ni 3d Penning emission originating from Ni sites of low work function located near the K atom (which we call adjacent Ni sites). The peak P, originates from Ni sites having higher local work function which are therefore located further from potassium and which we call remote Ni sites. The P, peak is due to the ionizattion of He* (2’S) followed by the Auger neutralization of the He+ ion by the remote Ni sites.

The middle curve represents the MQ spectrum of the same surface exposed to 0.3 L CO at 118 K. The new peaks Pz and P3 are due to electron emission from the 4a and the la + 5a orbitals of CO [12]. The peaks (Y and ,B (associated with potassium emission) and the peak y (associated with emission from the adjacent Ni sites) disappear, while the peak P, associated with the remote Ni sites is practically unchanged. From this we must conclude that CO is adsorbed near potassium in a configuration that permits the CO orbitals to reach far enough in the vacuum to shield potassium and the adjacent Ni sites from the incoming metastables.

Heating the sample to T = 298 K causes a striking change in the MQ spectrum (the upper curve in fig. 1): the potassium peak (Y reappears while the

J. Arias et al. / CO on K/ Ni(lI1) L435

K+CO/Ni (III)

0 4 8 12 16 KINETIC ENERGY (eV)

Fig. 1 The metastable quenching spectrum of: (a) K/Ni(lll) with 8, = 0.1 potassium atoms per

nickel atom; (b) the surface used at (a) exposed to 0.3 L CO at 118 K; (c) the surface used at (b) at

a temperature of 298 K.

PI peak (typical of remote Ni sites) is unchanged. No CO desorption is detected at this temperature [15]. Therefore we must conclude that the CO

molecules no longer shield the potassium atom even though they shield the adjacent Ni sites (and prevent the singlet to triplet conversion by the surface). Furthermore, the CO molecules have not moved on the remote Ni sites since no shielding of these sites is observed. The only explanation for these changes, that we can imagine, is that the CO orbitals are unable to shield the potassium atoms because a change in the binding configuration of CO prevents them from reaching as far into the vacuum as they did when the temperature was 118 K; therefore we conclude that the CO molecules lie flat on the surface or are very much tilted away from the normal.

A very similar behavior is observed (fig. 2) at slightly higher potassium coverage, 0, = 0.12. The intensities of the (Y, p, and y peaks are very different from those shown in fig. 1, because of the change in local work functions with potassium coverage. This is the normal behavior of the K/Ni(lll) surface [l-14].

The suggestion that CO might lie flat next to the potassium atom is consistent with other observations made on this system [15]. CO thermal

L436 J. Arias et al. / CO on K/ Ni(l I I)

L I I I I I 1 I I I

0 4 8 12 16

KINETIC ENERGY (eV)

Fig. 2. Same as fig. 1 but 6’, = 0.12.

desorption of 50% mixture of a 13C’60 and I2 Cl80 shows that the presence of potassium causes isotope mixing to form 12C160 and 13C’*0. This reaction does not take place on defectless Ni(ll1) at UHV pressures. The isotope mixing is not statistical [1,2] and therefore the new isotopic molecules are not formed by the recombination of C and 0 atoms which have wandered on the surface until all memory of their previous pairing was lost. The ratio of “new” to “old” CO isotopes in thermal desorption is very low indicating a preference for the original partnership. The reason for this behavior could be provided by the assumption that two CO molecules lie flat “bracketing” the K atom (fig. 3). While such a picture should not be taken too literally [15b], it is consistent

with the MQ spectra presented above and with the observation that only few new CO isotope molecules are formed by the isotope exchange reaction [2,15]. Of the four “isomers” shown in fig. 3 only (3d) can lead to new isotopes of CO, by cleavage of the old bonds (heavy lines) and formation of new ones (dashed lines). If (3d) undergoes this exchange with a probability LY, then the probability that the surface forms new CO isotopes is a/(3 + (1 - LX)); we have assumed that the four isomers are formed at the surface with equal probability. For (Y = 0.5 this gives 11% new isotopes which is of the order observed experimentally (for the 6k = 0.10).

J. Arias et al. / CO on K/ Ni(l11) I.437

(a) Lb)

84 84

No scrambling No scrambling

(cl (d) ____ 8 z K ____

____

8: K ____

No new Isotopes New isotopes

Fig. 3. A schematic representation of a possible CO potassium configuration and its connection to

isotope exchange yield. The CO drawn with heavy lines is l3 Cl6 0 and the one with lighter lines is

‘2c’*o.

Another encouraging feature of this structural hypothesis is that it proposes a concerted mechanism for isotope scrambling which does not require dissocia- tion with subsequent recombination. Our previous MQS measurements [1,2] suggested the possibility of such a concerted process to interpret the fact that the MQ spectrum of a (CO + K)/Ni(lll) surface on which isotope scrambling reaction was taking place had no features attributable to CO decomposition.

This work was supported by the National Science Foundation through Grant No. CHE83-10106. We are grateful to David Kind for information concerning his unpublished work.

References

[l] J. Lee, J. Arias, C. Hanrahan, R.M. Martin and H. Metiu, Phys. Rev. Letters 51 (1983) 1803,

1991.

[2] J. Lee, J. Arias, C. Hanrahan, R.M. Martin and H. Metiu, J. Chem. Phys. 82 (1985) 485.

[3] J. Benziger and R.J. Madix, Surface Sci. 94 (1980) 119.

[4] G. Broden, G. Gafner and H.P. Bonzel, Surface Sci. 84 (1979) 295;

M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 133 (1983) 321.

[5] M.P. Kiskinova, Surface Sci. 111 (1981) 584.

[6] J.E. Crowell, E.L. Garfunkel and G.A. Somojai, Surface Sci. 121 (1982) 303;

E.L. Garfunkel, J.E. Crowell and G.A. Somojai, J. Phys. Chem. 86 (1982) 310.

[7] H.S. Luftman, Y.M. Sun and J.M. White, Surface Sci. 141 (1984) 82;

H.S. Luftman, Y.M. Sun and J.M. White, Surface Sci. 140 (1984) L259.

[8] (a) F.M. Hoffmann, J. Hrbek and R.A. de Paola, Chem. Phys. Letters 106 (1984) 83;

(b) F.M. Hoffmann and R.A. de Paola, Phys. Rev. Letters 52 (1984) 1697.

[9] L. Wallden, Surface Sci. 134 (1983) L513. [lo] F.P. Netzer, D.L. Doering and T.E. Madey, Surface Sci. 143 (1984) L363.

[ll] C.T. Campbell and D.W. Goodman, Surface Sci. 123, 413 (1982);

D.W. Goodman, Appl. Surface Sci. 19 (1984) 1; Act. Chem. Res. 17 (1984) 194; D.E. Peebles, D.W. Goodman and J.M. White, J. Phys. Chem. 83 (1983) 4378.

L438 J. Arias et al. / Coon K/Ni(III)

[12] F. Bozso. J.T. Yates. Jr.. J. Arias, H. Menu and R.M. Martin, J. Chem. Phys. 78 (1983) 4256.

[13] J. Lee, J. Arias, C. Hanrahan. R.M. Martin and H. Metiu, in preparation.

[14] J. Lee, C. Hanrahan, J. Arias, F. Bozso, R.M. Martin and H. Metiu, Phys. Rev. Letters 54

(1985) 1440.

[15] (a) J. Arias, J. Lee, J. Dunaway, R.M. Martin and H. Metiu. in preparation:

(b) D.A. King (private communication) has also proposed the existence of a CO potassium

complex, which differs from ours.