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X-ray photoemission studies of free molecular clusters using synchrotron radiation

X-ray photoemission studies of free molecular clusters using synchrotron radiation. G. Öhrwall, University of Uppsala, Sweden. Why clusters?. Bridge between the isolated atom and the infinite solid Size-dependent physical and chemical properties Microscopic origin of macroscopic properties

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X-ray photoemission studies of free molecular clusters using synchrotron radiation

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

  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 P≈1-10 bar ions, e- P≈10-3 mbar SR Nozzle d≈100 mm <N> = <N>(T, D, p, k) Turbo- pump

  4. + XPS of Ar clusters <N>≈2000 2p3/2 XPS + - + - + - + + + - + + - - + + PRL 74, 3017 (1995), JCP 104, 1846 (1996)

  5. CO2 cluster XPS Same shift for C 1s and O1s van der Waals bonded - mainly final state relaxation Core hole screening

  6. C2H5OH cluster XPS

  7. 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

  8. 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

  9. 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

  10. 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

  11. CH3OH cluster XPS Size dependent vertical shift Difference C1s-O1s similar to ethanol: 0.3-0.4 eV

  12. Locailzed or delocalized final states? Cluster Molecule + + + + + + + +

  13. XPS and Auger shifts Molecule Cluster DE=X +1 core ionized state DEAuger=4X-X=3X DE=22X=4X +2 valence ionized state

  14. Ar cluster Auger 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! Intensity (arb. Units) 208 210 212 214 204 206 202 200 Kinetic Energy (eV)

  15. H2O Auger Molecule theory Localized picture insufficient. Misinterpreted solid AES? Liegner and Chen JCP 88, 2618 (1988) Ice theory Ice exp ? DE≈3eV Cluster exp DE≈8eV Molecule exp 470 480 490 500 510 KE (eV)

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

  17. 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))

  18. CH3Br cluster RAS UFD features as intense in molecules and clusters - not surface effect!

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

  21. 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

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

  23. CO2 cluster valence PES Shifts depend weakly on electronic state Vertical shifts similar to core level shifts (screened 1-hole states)

  24. 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.

  25. Clustering from a binary gas mixture Pure expansion: <N> = <N>(T, D, p, k) Mixed AB expansion: <”N”>= <”N”>(T, D, p, kA, kB, rA/B) • Present experiment: • T, D, p fixed • kA, kB, rA/B varied

  26. Valence PES (UPS) Core-level PES (XPS) NEXAFS PE(PI)nCO TOF-MS Radial segregation Homogenous mixing Radial layering Non-mixing

  27. + + - - + - - + + + - - - + + + + - + - + Ar 2p3/2 Kr 3d5/2 0.0 -1.0 Relative binding energy (eV) Ar/Kr clusters from 1.8% Kr in Ar XPS @ 50 eV Ek Bulk: less Ar, more Kr Surface: more Ar, less Kr

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

  29. Ar/Xe clusters XPS @ 50 eV Ek Xe 4d5/2 0% 2.1% 2.7% 3.2% 5.3% 100% Ar 2p3/2

  30. O2 cluster XPS Exchange splitting same in molecule and cluster

  31. O2 cluster NEXAFS Valence orbitals less affected by cluster formation than Rydberg states. Recorded RAS on top of s*

  32. O2 cluster RAS Cluster spectrum contains features consistent with ultra fast dissociation

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