Vibrational spectroscopy of clusters with a Vibrational spectroscopy of clusters with a Free Electron LaserFree Electron Laser

André FielickeFritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin

Abteilung Molekülphysik

http://www.fhi-berlin.mpg.de/mp/fielicke/

Now: Free Electron Laser for Infrared eXperiments (F

Nieuwegein, NL

FHI-FEL (under construction)

far-IRFEL

mid-IRFEL

to user lab

linearaccelerator

…first light end of 2011

F

D

C

A

B

Vibrational spectroscopy of clusters with a Free Electron Laser

• “Action spectroscopy”

• Multiple photon absorption

• Free Electron Lasers (how it works)

• Examples of cluster spectroscopy

Infrared spectroscopy of metal cluster complexes

ligand modes500-3500 cm-1 (0.06-0.43 eV)

internal cluster modes< 500 cm-1 (0.06 eV)

Structure of “bare”metal clusters

Exploring the cluster’s surface chemistry

IR spectroscopy of clusters in molecular beams

Not sensitive enough (low particle density)Not species specific (cluster distribution)

Direct measurement of absorption

More sensitive and selective:Mass spectrometric detectionof absorption via “Action Spectroscopy”

Changes of the charge state (ionization)Changes of particle mass (dissociation)

An intense and tunable IR source is needed for the excitation

Cluster dissociation; ”Depletion” spectroscopy

Xn+

hνXn

+* fragments

H. Haberland, Clusters of Atoms and Molecules, Springer-Verlag, Berlin, 1995.

Xn+

hν

MS

i) Ions

geometry

)exp(-σφ=oII

Lambert Beer “absorption law”

IR spectra of clusters via messenger technique

clusterdistribution

massselection

Laser

2nd mass filter forselecting fragments

Example: IR photodissociation of mass selected VxOy+·He

Absorption spectrum is obtained from the depletion of the He complexintensity vs. Laser frequency

pare

nt io

n de

plet

ion

K.R. Asmis, et al., J. Chem. Phys. 2004, 120, 6461.

AB+·M (AB+·M)* AB+ + M

chromophore spectator

14 Kion trap

“fingerprint” region

ν(M-M)

IR photon energy vs. bond dissociation energies

Chemisorption energies: 1-3 eVBinding energies in transition metal clusters 3-6 eVPhysisorption energies <0.2 eV

Photodissociation of most systems requires absorption of multiple IR photons

ν(X-H)ν(C=O)ν(MO)

1 eV = 96.5 kJ mol-1

Resonant multiple photon absorption

resonant absorption fast intramolecular vibrational redistribution (IVR), tIVR << 1 nsabsorption of the next photonetc.

harmonicoscillator

)(h 21+ν= vEvib

hνhνhνhνhνE

d

anharmonicoscillator

L++ν−

+ν=2

21

21

)(h)(h

vxvEee

evib

hν

E

d

hνd

hνhν

E

IVR

Cluster with manyvibrational modes

“fingerprint” region

ν(M-M)

IR sources for Multiple photon excitation

ν(X-H)ν(C=O)ν(MO)

Free Electron Laser

ns pulsed DFM, OPO/OPACO2 laser

Infrared multiple photon dissociation spectroscopy

CO2 laser: line tunable around 10 µm (1000±100 cm-1 )

Later: Knickelbein, Rayner, Walther

TodayIR-OPO, DFM: down to ~16 µm

(600 cm-1)but: rel. low power

General properties of Free Electron Lasers

accelerator based light source

intense, high brightness

(usually) wavelength tunable

monochromatic

pulsed or continuous (less common)

IR-FELs worldwide (not complete)

n.c.

n.c.

n.c.

n.c.

n.c.

s.c. cw

El.stat.

n.c.

n.c.

s.c. cw

n.c.

Surgery2 – 9 μm25-45 MeVVanderbilt (US)High power1.5 – 14 μm80-200MeVJLAB (US)Solid state 30 – 2500 μm2-6 MeVUCSB (US)High magn. field100–1500 μm10-15 MeVNijFEL (NL)

Solid state1 – 100 μm20-75 MeViFEL (JP)Solid state, biochem.4 – 13 μm25 MeVKU-FEL (JP)

Mat. science5 – 25 μm15-30 MeVBFEL (China)

Clusters, isotope separation

5 – 16 μm300–1000 μm

30-40 MeV10-20 MeV

FEL-TUS (JP)

Solid state, gas phase3 – 150 μm10-50 MeVCLIO (F)Solid state4 – 280 μm17-38 MeVFELBE (D)Mol. spectroscopy3 – 250 μm15-50 MeVFELIX (NL)

Mol. & cluster Spec., surface science

5 – 300 μm20-50 MeVn.c.FHI-FEL

There are FELs for all possible wavelengths!

X-rays

0.1 nm10000 eV

Microwaves

10 mm10-4 eV

Most recent family members: X-ray FELs

FLASH, DESY (Hamburg, Germany), SC Linac, 6.5-45 nm

XFEL, 0.1–6 nm, first light planned for 2014

LCLS, Stanford (USA), SC Linac, 0.15–1.5 nm (0.8-8 keV)working since April 2009

The Laser

resonator

gain medium

Energy source

Light Amplification by Stimulated Emission of Radiation

Processes in a light field (2-level system)

stimulatedabsorption

stim.emission

spontan.emission

Rates for induced processes depend on the spectral energy density ρ(ν)

Amplification

Free Electron Laser (FEL)

Medium and energy source:High energy (relativistic) electron beam

Interaction of the light field with the medium ?Amplification mechanism

Motion of electrons in a periodic magnetic field

N S N S N S N S NS

N S N S N S N S N SB

laboratorysystem

internalsystem

Oscillation frequency

cf Uλ

=

Hertz Oscillator radiating with f

λU

Relativistic electron beams

cve ≈ 1. radiation characteristics

at rest relativistic

( )2,1 cv ze−≈ϑΔ

Relativistic electron beams

2. Lorentz contraction cve ≈

γλ

=λ′ UU

( )220 ,1

1

cv

kin

zecmE

−==γ

λU

MeV5.020 ≈cm 40 MeV beam γ ≈ 80

Relativistic electrons in the undulator I

γλ

=λ′ UU

λγ

≈λ

λ+

−=λ

211

1,

,

obs

cv

cv

obsze

ze

Oscillation (=radiation) frequency γ⋅

λ=c

f U

Corresponding wavelength (internal, moving system)

Wavelength in the laboratory system is detected with a strong Doppler shift

2221

γλ

=γγ

λ=λ UU

Relativistic electrons in the undulator II

But: electrons do not move straight along the z-axis in the undulator

Wiggling motion reduces speed in forward (z) direction Effective γ is reduced

2221

z

U

zz

U

γλ

=γγ

λ=λ

20cmEkin

z =γ<γ

Reduction depends on the deflection and thereby the magnetic field strength B )1( 2K

z+

γ=γ ( )B∝K

( )22 1

2K

z

U +γλ

=λSpontaneously emitted radiation from electrons in a periodic magnetic field

Setting the wavelength

( )22 1

2K

z

U +γλ

=λ

FHIFEL:Ekin= 50 MeVmid-IR Undulator: λU=0.04 mWavelength for K=1 ?

K: typically 0.5…3

Setting the wavelength

( )22 1

2K

z

U +γλ

=λ K: typically 0.5…3

( )( )

eV) (0.3 µm4

11MeV5.0MeV502

m04.0 22

≈

+⋅

≈λ

Wavelength and small signal gain

FHIFEL:Ekin= 50 MeVmid-IR Undulator, λU=0.04 mWavelength for K=1 ?

Setting the wavelength

( )22 1

2K

z

U +γλ

=λ

BESSY II: Ekin=1.7 GeVUndulator UE56

K: typically 0.5…3

( )( )

eV) (250 nm5

11MeV5.0GeV7.12

m056.0 22

≈

+⋅

≈λ

Ways for tuning the wavelength

( )22 1

2K

z

U +γλ

=λ magnet field BBeam energy Ekin

Adjusting theundulator gap d

d

Changing accelerator parameters and electron optics

Interaction of the electrons with the light field

Ponderomotive force acts on the electrons depending on relative phase Acceleration or slowing down

Initial situation: incoherent spontaneously emitted radiation captured in the resonator

Interaction of the electrons with the light field

Microbunching of the electron packets through momentum transfer between light field and electrons

Result: electrons move in phaseRadiations from single bunches can interfere constructively

Oszillator FEL

Stored light field increases with timeNew electron bunches traverse the undulatorMore efficient bunching enhances energy transfer into the light field

exponential rise until saturation

X-rays can not be stored…

“Self amplified stimulated emission”(SASE)<

No effective mirror materials

Amplification in a single pass along the undulator

FLASH, DESY (Hamburg, Germany)

A train of short light pulses

Electron beam determines temporal characteristics of emitted light:Short pulses of a few fs – ps length

Transform limited bandwidth (FELIX: 0.4-7 %)

Repetition within a macropulse (also pulsed cw possible)

Pulse scheme of FELIX

Applications of Free Electron Lasers

high intensity

Weak absorbers(low density and/orlow cross sections)

Multiple photon processes

(ultra-) short pulses

Time resolution

Relaxation processes

To take snapshots

The Free Electron Laser for Infrared eXperiments (FELIX)FOM Institute for Plasma Physics “Rijnhuizen”, Nieuwegein, The Netherlands

tunable between 40-2400 cm-1

(up to ~3700 cm-1 on 3rd harmonic)

up to 100 mJ per macropulse(1010 W/cm2 in a micropulse)

bandwidth typically 0.5-2 %of the central wavelength

cluster source

IR Multiple Photon Dissociation (IR-MPD) Spectroscopy of neutral and charged clusters

reactive gas

gas pulse

UV laser(F2, 7.9 eV)

metal rod

Nd:YAGlaser

Reflectron time-of-flightmass spectrometer

Ion Detector

FELIX beam

FELIX4.92 μm

500 550 600 650 700m / z (amu)

Rh6CO+Rh5CO+

Probing the surface chemistry of the ligand

character of CO bindingdissociative molecular

Surf. Sci. 603 (2009) 1427.

CO at Rhn+: Size dependence of the binding site

• Observation of CO bound in 3-fold face capping (µ3), 2-fold bridging (µ2), and linear (µ1) geometries

• CO binding depends on cluster size

• mainly µ1 ligands

• Isomers, e.g. for n = 7

JACS 125 (2003) 15716.J. Phys. Chem. B 108 (2004) 14591.

Comparison of neutral, cationic and anionic RhnCO+/-

cations anionsneutrals

Effect of charge: example of Rh8CO+/0/-

C OCO

p

s p

ssσ(3σ)

pσ(4σ)

pπ(1π)

sσ∗(5σ)

pπ∗(2π)

pσ∗(6σ)

M(σ) CO(5σ)σ donation

M(δ) CO(2π)π back donation

CM O

CM O

Saturated Rhodium Cluster Carbonyl Cations

6*9 + 16*2 -1 = 85Rh6(CO)16+

5*9 + 14*2 -1 = 72Rh5(CO)14+

4*9 + 12*2 -1 = 59Rh4(CO)12+

3*9 + 9*2 -1 = 44Rh3(CO)9+

2*9 + 8*2 -1 = 33Rh2(CO)8+

9 + 5*2 -1 = 18Rh(CO)5+

CVEsSaturation composition

(18-3)*4 = 60

(18-3)*2+(18-2)*2 = 62

Inferring structures from counting electrons

18 electron rule, Rh: 4d85s1

“The Bonding Capabilities of Transition Metal Clusters”J. W. Lauher, J. Am. Chem. Soc. 100 (1978) 5305

Saturated Rhodium Cluster Carbonyl Cations

J. Am. Chem. Soc. 130 (2008) 2126

Rhodium cluster carbonyls: Cations vs. Neutrals

neutralcation

SAME

DIFFERENT

DIFFERENT

No bridging CO in the cations of Rh2(CO)8 and Rh4(CO)12

Neutrals in Xe, Allian et al., Vib. Spec. 41 (2006) 101

Rh

C

O

-10 -8 -6 -4 -2 0 2 4 6 8 10

x 10 -3

Destabilization of µ2-CO upon ionization comes from removal of electron density out of orbitals with Rh-µ2-CO binding character

Rhodium cluster carbonyls: Cations vs. Neutrals

Far-IR multiple photon dissociation spectroscopy of metal cluster rare-gas complexes

resonant absorption Fragmentation of the complexdepletion spectrum

wavelength (µm)

inte

nsity

(%)

frequency (cm-1)

IR absorption spectrum

cros

sse

ctio

n

Obtaining the structural information

Density Functional TheoryPredict stable isomers (may include a global optimization)Compute infrared spectrumEffects of rare gas atoms?Accounting for anharmonicity, fluctional behavior via MD simulations

exp. spectrum calculated IR spectra for different structures

structure assigment

frequency (cm-1)

rel.

cros

sse

ctio

n

Au7

Analyzing the structure vs. understanding the IR spectraExperiment: Au7Kr at ~100 K

PBE+D

i) Structural isomers ii) Effect of Kr atoms iii) Dynamics at 80 K

L. Ghiringhelli, M. SchefflerFHI Theory Dep. (DFT+D)

IR Spectrum and Structure of Neutral Nb8

1.4294.4302.9a10.0276.5284.4a14.1270.0277.8b10.0249.5256.7b11.3242.7249.7a10.4236.5243.3b20.3233.8240.5b20.5228.2234.8a11E-5187.8193.3b21.8185.3190.6a10184.1189.4a20.7183.4188.7b10181.7186.9a20.2144.8148.9a10.3140.5144.6b20.3134.7138.6b10.0117.1120.5a10111.5114.7a2

IntScaledcm-1

Calccm-1

Sym.

neutral: ν=180 cm-1

anion: ν=165 cm-1

Calc. spectrum (DFT)

T. P. Marcy and D. G. Leopold, Int. J. Mass Spectr. 196, 653 (2000).

Magnetism in small rhodium clusters

Y.-C. Bae et al. Phys. Rev. B 72 (2005) 125427.

108 9

► Cubic growth can explain magnetic properties

► Eight-center bonding through d orbitals

1312

A.J. Cox et al. Phys. Rev. B 49 (1994) 12295.

The (predicted) cubic structures of rhodium clusters

Rh8 cube, Oh symmetry1 IR active mode (t1u)

108 9 1312

0

0.5

1.0re

l. en

ergy

(eV

)

cube

anticube

bicap-ohdiamond-p

bicap-tp

Predicted isomers of Rh8+

(basin hopping DFT)

PBE PBE-hybrid

Dan Harding, Tiff Walsh, U Warwick, UKStuart Mackenzie, U Oxford, UK

cube

anticube

bicap-oh

bicap-tp

J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77, (1996) 3865 J.P. Perdew, M. Ernzerhof, K. Burke, J. Chem. Phys. 105, (1996) 9982

e

b2

bicapped octahedralstructure as identified also for other transition metals

0

+0.18 eV

+0.56 eV

+0.92 eV

Assignment of the structure of Rh8+

J. Chem. Phys. 132 (2010) 011101.

Structures of small transition metal clusters

Nbneutral

cation

Rh cation

n = 5 6 7 8 9

Polytetrahedral packings

Cooling processes following IR multiple photon excitation

IR multiple photon dissociation(IR-MPD)

IR resonance enhanced multiplephoton ionization (IR-MPD)

For most systems: BDE < IP

Exceptions: refractory materialsmetals (W, Mo, Nb…)

oxides (MgO, Al2O3, TiO2)carbides (TiC, NbC)fullerenes C60

IR Resonance Enhanced Multiple Photon Ionization(IR-REMPI)

Ionization of neutral NbCclusters with IR light of ~500 cm-1 (0.06 eV)NbC: cubic bulk structure(NaCl) geometric shell closings at

Nb14C13 (3x3x3 atoms)Nb18C18 (3x3x4 atoms)Nb24C24 (3x4x4 atoms)Nb32C32 (4x4x4 atoms)

Bulk niobium carbide: metallic, interstitiell C in octahedral holes of Nb fcc lattice

ionization energies ≈ 4 eV (67 photons@500 cm-1)binding energy per atom: > 6 eV

IR-REMPI spectra of niobium carbide clusters

Bands are observed close to the surface phonon modes:

S4 “Lucas“ mode at 640 cm-1

C atoms move within the surface plane

S2 “Wallis“ mode at 490 cm-1

C atoms move perpendicular to the surface plane

Small clusters (upto Nb9C9) show only bands around ~670 cm-1

C. Oshima et al. Phys. Rev. Lett. 56 (1986) 240

From FELIX to FELICE

100 mJ(1 %)

FELIX: Intensity still insufficient for many multiple photon excitation experiments

FELICE:Free Electron Laser for IntraCavity Experiments

10 J

FELICE: Free Electron Laser for IntraCavity Experiments

wavelength range 3-100 µmbeam energy 20-60 MeVmicropulse rep. rate 1 GHzmicropulse energy ≤1 mJmacropulse energy some 10J

Electron detachment and ionization by IR multiple photon excitation

Nb13 Nb13+

Nb13Ar Nb13+Ar

IR-REMPI of neutral niobium clusters

Ionization (IP=5.5 eV)

Dissociation (BDE<0.1 eV)

Vibrationally mediated electron detachment from anionic clusters

Nb13

Ta4C- Ta4C (ca. 1-2 eV)

(with V. Lapoutre, J. Bakker, FOM Rijnhuizen)

Techniques: Raman (matrix)Anion photoelectron spectroscopyPhotodissociation, IR-MPDIR-REMPI

Vibrational spectroscopy is a suitable tool for obtaining structural information for metal (compound) clusters, especially with complementary theoretical studies (DFT).

Surface chemistry and binding geometries of adsorbed molecules can be identified.

Summary