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Tartu. March 2004 G. Öhrwall, University of Uppsala, Sweden X-ray photoemission studies of free molecular clusters using synchrotron radiation.

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Presentation on theme: "Tartu. March 2004 G. Öhrwall, University of Uppsala, Sweden X-ray photoemission studies of free molecular clusters using synchrotron radiation."— Presentation transcript:

1 Tartu. March 2004 G. Öhrwall, University of Uppsala, Sweden X-ray photoemission studies of free molecular clusters using synchrotron radiation

2 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?

3 Tartu. March 2004 Cluster production ions, e - SR Skimmer Turbo- pump P≈10 -3 mbar P≈1-10 bar Nozzle d≈100 mm = (T, D, p, k)

4 Tartu. March ≈2000 XPS of Ar clusters 2p 3/2 XPS PRL 74, 3017 (1995), JCP 104, 1846 (1996)

5 Tartu. March 2004 CO 2 cluster XPS Same shift for C 1s and O1s van der Waals bonded - mainly final state relaxation Core hole screening

6 Tartu. March 2004 C 2 H 5 OH cluster XPS

7 Tartu. March 2004 Core level shifts for ethanol clusters Chemical shifts  E(O 1s)=1.3 eV  E(C 1s, intermediate)=0.95 eV  E(C 1s, methyl)=0.9 eV

8 Tartu. March 2004 Core level shifts for ethanol clusters Chemical shifts  E(O 1s)=1.3 eV  E(C 1s, intermediate)=0.95 eV  E(C 1s, methyl)=0.9 eV Weak initial state effects - hydrogen bonding

9 Tartu. March 2004 Core level shifts for ethanol clusters Chemical shifts  E(O 1s)=1.3 eV  E(C 1s, intermediate)=0.95 eV  E(C 1s, methyl)=0.9 eV Weak initial state effects - hydrogen bonding Chemical shift predominantly relaxation effect

10 Tartu. March 2004 Core level shifts for ethanol clusters Chemical shifts  E(O 1s)=1.3 eV  E(C 1s, intermediate)=0.95 eV  E(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

11 Tartu. March 2004 CH 3 OH cluster XPS Size dependent vertical shift Difference C1s-O1s similar to ethanol: eV

12 Tartu. March 2004 Molecule Cluster Locailzed or delocalized final states?

13 Tartu. March 2004  E=2 2 X=4X +1 core ionized state Molecule  E=X Cluster +2 valence ionized state  E Auger =4X-X=3X XPS and Auger shifts

14 Tartu. March 2004 Ar cluster Auger Intensity (arb. Units) Kinetic Energy (eV) Ar LMM ≈200 h =310 eV Cluster spectrum (surface and bulk) modelled as shifted and broadened version of atomic Auger spectrum.  E=3X works for surface and bulk!

15 Tartu. March KE (eV) Molecule theory Ice theory Ice exp Cluster exp Molecule exp ?  E≈3eV  E≈8eV H 2 O Auger Liegner and Chen JCP 88, 2618 (1988) Localized picture insufficient. Misinterpreted solid AES?

16 Tartu. March 2004 Ultra fast dissociation in resonant Auger decay SR |i> (ground state) |i> (intermediate state) |f> (final state) Dissociation can occur on same time scale as core hole life time - few fs for k-shell in second row elements. Ultra fast dissociation gives rise to features constant in kinetic energy

17 Tartu. March 2004 Ultra fast dissociation in CH 3 Br clusters 4a 1 resonance known to give rise to ultra fast dissociation (Nenner & al., J. Electron Spectrosc. Relat. Phenom. 52, 623 (1990)) Br 3d 5/2 -> 4a 1

18 Tartu. March 2004 CH 3 Br cluster RAS UFD features as intense in molecules and clusters - not surface effect!

19 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”

20 Tartu. March 2004 Acknowledgements Maxim Tchaplyguine MAX-lab Joachim Schulz Olle Björneholm Uppsala University Marcus Lundwall Andreas Lindblad Torbjörn Rander Svante Svensson

21 Tartu. March 2004 Acknowledgements Dept. of Physics, Uppsala Olle BjörneholmMarcus Lundwall Svante SvenssonAndreas Lindblad Raimund FeifelTorbjörn Rander MAX-lab, Lund Maxim TchaplyguineAndreas Lindgren Stacey Sorensen Financial Support KAW, SSF

22 Tartu. March 2004 Cluster beam size Atomic Ar 3p-lines Cluster Ar 3p-lines Scienta SES-200 detector image Slit 25 mm Magn.=5x Atomic Ar width ≈5 mm Cluster Ar width >1 mm 5 cm from nozzle Kinetic Energy Pos.

23 Tartu. March 2004 CO 2 cluster valence PES Shifts depend weakly on electronic state Vertical shifts similar to core level shifts (screened 1-hole states)

24 Tartu. March 2004 H2O cluster valence PES h =60 eV Cluster+Mol. Mol. A-state ((3a 1 ) -1 ) more affected by cluster formation than X or B. X A B

25 Tartu. March 2004 Clustering from a binary gas mixture Pure expansion: = (T, D, p, k) Mixed A B expansion: = (T, D, p, k A, k B, r A/B ) Present experiment: T, D, p fixed k A, k B, r A/B varied

26 Tartu. March 2004 Valence PES (UPS) Core-level PES (XPS) NEXAFS PE(PI) n CO TOF-MS Homogenous mixing Radial segregation Radial layering Non-mixing

27 Tartu. March 2004 Relative binding energy (eV) Ar 2p 3/2 Kr 3d 5/2 0.0 Surface: more Ar, less Kr Bulk: less Ar, more Kr Ar/Kr clusters from 1.8% Kr in Ar 50 eV E k

28 Tartu. March 2004 Structure of Ar/Kr mixed clusters Ar/Kr radial gradient Surface: more Ar, less Kr Bulk: less Ar, more Kr

29 Tartu. March 2004 Ar/Xe clusters 50 eV E k Xe 4d 5/2 Ar 2p 3/2 100% 3.2% 2.1% 2.7% 5.3% 0%

30 Tartu. March 2004 O 2 cluster XPS Exchange splitting same in molecule and cluster

31 Tartu. March 2004 O 2 cluster NEXAFS Valence orbitals less affected by cluster formation than Rydberg states. Recorded RAS on top of  *

32 Tartu. March 2004 O 2 cluster RAS Cluster spectrum contains features consistent with ultra fast dissociation


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