X-ray photoemission studies of
free molecular clusters using
synchrotron radiation
G. Öhrwall, University of Uppsala, Sweden
Tartu. March 2004
Why clusters?
Bridge between the isolated atom and the infinite
solid
Size-dependent physical and chemical properties
Microscopic origin of macroscopic properties
Applications?
Tartu. March 2004
Cluster production
Skimmer
P≈1-10 bar
ions, e-
P≈10-3 mbar
SR
Nozzle
d≈100 mm
Turbopump
Tartu. March 2004
<N> = <N>(T, D, p, k)
<N>≈2000
XPS of Ar clusters
2p3/2 XPS
+
-+
-+
-+
+
+
++-
+
+
PRL 74, 3017 (1995), JCP 104, 1846 (1996)
Tartu. March 2004
+
CO2 cluster XPS
Same shift for
C 1s and O1s
van der Waals
bonded - mainly
final state relaxation
Core hole screening
Tartu. March 2004
C2H5OH cluster XPS
Tartu. March 2004
Core level shifts for
ethanol clusters
Chemical shifts
DE(O 1s)=1.3 eV
DE(C 1s, intermediate)=0.95 eV
DE(C 1s, methyl)=0.9 eV
Tartu. March 2004
Core level shifts for
ethanol 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
Tartu. March 2004
Core level shifts for
ethanol 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
Tartu. March 2004
Core level shifts for
ethanol 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
Tartu. March 2004
CH3OH cluster XPS
Size dependent
vertical shift
Difference
C1s-O1s similar
to ethanol:
0.3-0.4 eV
Tartu. March 2004
Locailzed or delocalized final states?
Molecule
Cluster
+
+
++
++
+
+
Tartu. March 2004
XPS and Auger shifts
Molecule
Cluster
DE=X
+1 core ionized state
DEAuger=4X-X=3X
DE=22X=4X
+2 valence ionized state
Tartu. March 2004
Ar cluster Auger
Intensity (arb. Units)
Ar LMM
<N>≈200
hn=310 eV
Cluster spectrum
(surface and bulk)
modelled as shifted
and broadened
version of atomic
Auger spectrum.
DE=3X works for
surface and bulk!
200
202
204 206 208 210
Kinetic Energy (eV)
Tartu. March 2004
212
214
Liegner and Chen
JCP 88, 2618 (1988)
H2O Auger
Molecule theory
Ice theory
Ice exp
Localized
picture
insufficient.
?
DE≈3eV
Cluster exp
Molecule exp
DE≈8eV
470
480
Tartu. March 2004
490
500
KE (eV)
510
Misinterpreted
solid AES?
Ultra fast dissociation in
resonant Auger decay
SR
|i> (intermediate state) Dissociation can occur on
same time scale as core
hole life time - few fs
for k-shell in second
row elements.
|f> (final state)
|i> (ground state)
Tartu. March 2004
Ultra fast dissociation
gives rise to features
constant in kinetic energy
Ultra fast dissociation in CH3Br
clusters
Br 3d5/2
-> 4a1
4a1 resonance
known to give rise
to ultra fast
dissociation
(Nenner & al., J. Electron
Spectrosc. Relat.
Phenom. 52, 623 (1990))
Tartu. March 2004
CH3Br cluster RAS
UFD features
as intense in
molecules and
clusters - not
surface effect!
Tartu. March 2004
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”
Tartu. March 2004
Acknowledgements
Maxim Tchaplyguine MAX-lab
Joachim Schulz
Olle Björneholm Uppsala University
Marcus Lundwall
Andreas Lindblad
Torbjörn Rander
Svante Svensson
Tartu. March 2004
Acknowledgements
Dept. of Physics, Uppsala
Olle Björneholm
Marcus Lundwall
Svante Svensson
Andreas Lindblad
Raimund Feifel
Torbjörn Rander
MAX-lab, Lund
Maxim Tchaplyguine
Stacey Sorensen
Financial Support
KAW, SSF
Tartu. March 2004
Andreas Lindgren
Cluster beam size
Scienta SES-200 detector image
Slit 25 mm
Magn.=5x
Pos.
Atomic Ar
width ≈5 mm
Cluster Ar
width >1 mm
5 cm from nozzle
Atomic Ar 3p-lines
Tartu. March 2004
Cluster Ar 3p-lines
Kinetic
Energy
CO2 cluster valence PES
Shifts depend
weakly on
electronic state
Vertical shifts
similar to core
level shifts
(screened 1-hole
states)
Tartu. March 2004
H2O cluster valence PES
hn=60 eV
X
A-state ((3a1)-1)
more affected by
cluster formation
than X or B.
Cluster+Mol.
B
A
Mol.
20
18
16
14
Binding Energy (eV)
Tartu. March 2004
12
10
Clustering from a binary gas mixture
Pure expansion:
<N> = <N>(T, D, p, k)
Mixed A B expansion:
<”N”>= <”N”>(T, D, p, kA, kB, rA/B)
Present experiment:
• T, D, p fixed
• kA, kB, rA/B varied
Tartu. March 2004
Valence PES (UPS)
Core-level PES (XPS)
NEXAFS
PE(PI)nCO TOF-MS
Homogenous
mixing
Radial
segregation
Tartu. March 2004
Radial
layering
Non-mixing
Ar/Kr clusters from 1.8% Kr in Ar XPS @ 50 eV Ek
+ +
- + - + -+
+ +
+
Ar 2p3/2
Bulk: less Ar, more Kr
Surface: more Ar, less Kr
Kr 3d5/2
0.0
-1.0
Relative binding energy (eV)
Tartu. March 2004
Structure of Ar/Kr mixed clusters
Bulk: less Ar, more Kr
Surface: more Ar, less Kr
Ar/Kr radial gradient
Tartu. March 2004
Ar/Xe clusters XPS @ 50 eV Ek
Xe 4d5/2
0%
2.1%
2.7%
3.2%
5.3%
100%
Ar 2p3/2
Tartu. March 2004
O2 cluster XPS
Exchange splitting
same in molecule
and cluster
Tartu. March 2004
O2 cluster NEXAFS
Valence orbitals
less affected by
cluster formation
than Rydberg
states.
Recorded RAS
on top of s*
Tartu. March 2004
O2 cluster RAS
Cluster spectrum
contains features
consistent with
ultra fast dissociation
Tartu. March 2004