Xray photoemission studies of free molecular clusters using synchrotron radiation

1
X-ray photoemission studies of free molecular
clusters using synchrotron radiation
2
Why clusters?
Bridge between the isolated atom and the infinite
solid Size-dependent physical and chemical
properties Microscopic origin of macroscopic
properties Applications?
3
Cluster production
Skimmer
P1-10 bar
ions, e-
P10-3 mbar
SR
Nozzle d100 mm
ltNgt ltNgt(T, D, p, k)
Turbo- pump
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XPS of Ar clusters
ltNgt2000
2p3/2 XPS

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PRL 74, 3017 (1995), JCP 104, 1846 (1996)
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CO2 cluster XPS
Same shift for C 1s and O1s
van der Waals bonded - mainly final state
relaxation Core hole screening
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C2H5OH cluster XPS
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Core level shifts forethanol clusters
Chemical shifts DE(O 1s)1.3 eV DE(C 1s,
intermediate)0.95 eV DE(C 1s, methyl)0.9 eV
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Core level shifts forethanol clusters
Chemical shifts DE(O 1s)1.3 eV DE(C 1s,
intermediate)0.95 eV DE(C 1s, methyl)0.9
eV Weak initial state effects - hydrogen bonding
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Core level shifts forethanol clusters
Chemical shifts DE(O 1s)1.3 eV DE(C 1s,
intermediate)0.95 eV DE(C 1s, methyl)0.9
eV Weak initial state effects - hydrogen
bonding Chemical shift predominantly relaxation
effect
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Core level shifts forethanol clusters
Chemical shifts DE(O 1s)1.3 eV DE(C 1s,
intermediate)0.95 eV DE(C 1s, methyl)0.9
eV Weak initial state effects - hydrogen
bonding Chemical shift predominantly relaxation
effect Different screening implies different
coordination for O and C atoms - depends on
geometry
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CH3OH cluster XPS
Size dependent vertical shift
Difference C1s-O1s similar to ethanol 0.3-0.4 eV
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Locailzed or delocalized final states?
Cluster
Molecule








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XPS and Auger shifts
Molecule
Cluster
DEX
1 core ionized state
DEAuger4X-X3X
DE22X4X
2 valence ionized state
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Ar cluster Auger
Ar LMM ltNgt200 hn310 eV
Cluster spectrum (surface and bulk) modelled as
shifted and broadened version of atomic Auger
spectrum. DE3X works for surface and bulk!
Intensity (arb. Units)
208
210
212
214
202
204
206
200
Kinetic Energy (eV)
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H2O Auger
Molecule theory
Localized picture insufficient. Misinterpreted s
olid AES?
Liegner and Chen JCP 88, 2618 (1988)
Ice theory
Ice exp
?
DE3eV
Cluster exp
DE8eV
Molecule exp
470
480
490
500
510
KE (eV)
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Ultra fast dissociation inresonant Auger decay
igt (intermediate state)
Dissociation can occur on same time scale as
core hole life time - few fs for k-shell in
second row elements.
SR
fgt (final state)
Ultra fast dissociation gives rise to
features constant in kinetic energy
igt (ground state)
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Ultra fast dissociation in CH3Br clusters
Br 3d5/2 -gt 4a1
4a1 resonance known to give rise to ultra
fast dissociation (Nenner al., J.
Electron Spectrosc. Relat. Phenom. 52, 623 (1990))
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CH3Br cluster RAS
UFD features as intense in molecules and clusters
- not surface effect!
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Summary
Third generation synchrotron radiation offers new
possibilities to study free clusters Core level
PES on clusters gives information on local
surrounding - surface/bulk, geometry Localization
/delocalization of two-hole final states in
AES Possible to observe femtosecond nuclear
dynamics in core excited state in solid
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Acknowledgements
Maxim Tchaplyguine MAX-lab Joachim Schulz Olle
Björneholm Uppsala University Marcus
Lundwall Andreas Lindblad Torbjörn Rander Svante
Svensson
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Acknowledgements
Dept. of Physics, Uppsala Olle Björneholm Marcus
Lundwall Svante Svensson Andreas
Lindblad Raimund Feifel Torbjörn
Rander MAX-lab, Lund Maxim Tchaplyguine Andreas
Lindgren Stacey Sorensen Financial Support KAW,
SSF
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Cluster beam size
Scienta SES-200 detector image
Pos.
Slit 25 mm
Magn.5x
Atomic Ar width 5 mm
Cluster Ar width gt1 mm 5 cm from nozzle
Kinetic Energy
Atomic Ar 3p-lines
Cluster Ar 3p-lines
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CO2 cluster valence PES
Shifts depend weakly on electronic state
Vertical shifts similar to core level
shifts (screened 1-hole states)
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H2O cluster valence PES
hn60 eV
X
A-state ((3a1)-1) more affected by cluster
formation than X or B.
ClusterMol.
B
A
Mol.
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Clustering from a binary gas mixture
Pure expansion ltNgt ltNgt(T, D, p, k)
Mixed A B expansion ltNgt ltNgt(T, D, p, kA,
kB, rA/B)

• Present experiment
• T, D, p fixed
• kA, kB, rA/B varied

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Valence PES (UPS) Core-level PES
(XPS) NEXAFS PE(PI)nCO TOF-MS
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Ar/Kr clusters from 1.8 Kr in Ar XPS _at_ 50 eV Ek
Bulk less Ar, more Kr
Surface more Ar, less Kr
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Structure of Ar/Kr mixed clusters
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Ar/Xe clusters XPS _at_ 50 eV Ek
Xe 4d5/2
0
2.1
2.7
3.2
5.3
100
Ar 2p3/2
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O2 cluster XPS
Exchange splitting same in molecule and cluster
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O2 cluster NEXAFS
Valence orbitals less affected by cluster
formation than Rydberg states.
Recorded RAS on top of s
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O2 cluster RAS
Cluster spectrum contains features consistent
with ultra fast dissociation