X-Ray Spectroscopy at LCLS

Transcription

1 LCLS proposal preparation workshop for experiments at XPP, June 21, 2008, SLAC, Menlo Park, CA ħω ħω e - X-Ray Spectroscopy at LCLS Uwe Bergmann SSRL Stanford Linear Accelerator Center bergmann@slac.stanford.edu

2 Outline why x-ray spectroscopy x-ray techniques concepts of instrumentation conclusions

3 Probing Valence Electrons the hydrogen bond is directional probing of valence electrons local structure of water configurations molecular orbitals of the water molecule occupied unoccupied 1b 2 3a 1 1b 1 4a 1 2b 2

4 XANES Probing Unoccupied Molecular Orbitals 0 Mn 4p 0-1 Mn 3d -2

5 EXAFS - Probing the Local Structure Fourier Transform Magnitude EXAFS x k k (Å -1 ) 1 Fe-O at 2.00 Å 1 Fe-O at 2.50 Å Distance (Å) EXAFS x k k (Å -1 ) 1 Fe-O at 2.5 Å 1 Fe-Fe at 2.5 Å Å EXAFS x k k (Å -1 ) 1 Fe-O at 2.00 Å 5 Fe-O at 2.00 Å EXAFS x k k (Å -1 ) σ 2 = Å 2 σ 2 = Å 2 σ 2 = Å Å

6 XFEL Spectroscopy Experiments multi-photon excitation ultra-fast pump probe biological pump probe (μs ms)

7 Multi-Photon Effects (Numbers) unfocused XFEL pulse (~100 fs): photons / (100 * 100 μm 2 ) ~ 1-10 photons / Å 2 x-ray attenuation lengths μm => photons / Å 3 interaction sphere (< 3 Å radius) < 100 Å 3 ~ photon / interaction sphere interaction time scale (core-hole life time) ~ 1-10 fs < photons / (interaction sphere * core-hole life time) multi-photon effects not an issue for unfocused XFEL pulse gain from focusing ~ multi-photon effects best observed with soft x-rays - probably possible with hard x-rays

8 Realities of SASE XFEL shot-to-shot noise of intensity ~ 10 % energy jitter ~ 30% of band width up to 100% for monochromatic beam pulse width ΔE/E ~ 0.1 % (with electron beam chirp ~ 1%) XANES very difficult, scanning required EXAFS might be impossible alternative techniques that do not require scanning

9 Photon-in Photon-out out X-ray X Spectroscopy monochromatic analyzer (Rowland geometry) monochromator detector sample X-ray beam dispersive analyzer (von Hamos geometry) monochromator PSD sample X-ray beam

10 XES Probing Occupied Molecular Orbitals chemical sensitivity of K fluorescence level diagram valence levels Kβ 2,5, Kβ 3p 2p 1s Kβ 1,3, Kβ Kα 1.2 Intensity [a.u.] 0 Mn(II)O Mn(IV)O 2 KMn(VII)O 4 Kα 2 Kβ' Kβ'' Bergmann et al, Chem. Phys. Lett. 302, 1999 Kα 1 Kβ 1,3 Kβ 2,5 Relative Energy [ev] 35 Relative Integrated Intensity Normalized Intensity [a.u.] K 2 MnF 6 Mn salen Nitrido KMnO 4 F O N Energy Shift [ev] Mn-O Distance [A] o

11 XES of Mn Halide Models Kβ'' region Mn 2 X 2 (X = Cl, F) Intensity [a.u.] F O N Cl (3s) Energy [ev] N X N N N N O X Mn Mn O N N N Mn 3 X [X = Cl, F] N N N Mn O X O Mn O N Mn O N N N

12 Oxygenic Photosynthesis photosynthesis: - only fundamental source of food on earth - has created our atmosphere and ozon layer - has created fossil energy sources (crude oil, coal, gas) - shows alternative ways to obtain energy in the future Bavaria Buche', ~ year old beech, Altmühltal, Germany, leave area ~ 8500 m 2

13 Oxygenic Photosynthesis Where do plants split water? Mn 4 O x Ca cluster

14 Kok Cycle of Water Splitting B. Kok et al. Photochem. and Photobiology 11, 457 (1970) Oxygen Evolving Complex ћω e - ћω O 2 S 0 30 μs S 1 70 μs e ms ћω S 4 e - S μs ћω e - S 2 Transition times are from Haumann et al. Science 310, 1019 (2005)

15 X-ray Raman Process (Non Resonant) ΔE = E 0 -E e - quasi elastic peak graphite sample photon θ photon Intensity [log scale] Compton peak Raman scattering E E 0 ΔE scattering probability (dipole limit): w cos 2 θ sin 2 (θ/2) (angular dependence, horizontal scattering) w Z -4 (element dependence) w E 3 (energy dependence at constant q) E 0 [ev] w ΔE -1 (energy transfer dependence)

16 Isotope Effect on the Structure of Water Bergmann et al, PRB 76, , 2007

17 Polycyclic Aromatic Hydrocarbons (PAHs( PAHs) N N H Intensity [arbitrary units] N S N Energy [ev] Energy [ev] Bergmann, U; Mullins, OC; in Asphaltene, Heavy Oils and Petroleomics, Springer, New York, 2006

18 XRS Based EXAFS Bergmann et al, J. Chem. Phys. 127, (2007)

19 EXAFS Based O-O O O RDF

20 Schematic Setup of Dispersive Optics PSD analyzer top view sample x-ray pulse laser pulse analyzer x-ray and laser pulse vertical cut

21 Wavelength Dispersive Analyzer von Hamos geometry Hayashi et al, J. Electron Spectr. Rel. Phenom. 136, 191 (2004)

22 New Spectrometer at SSRL

23 Conclusion photon-in photon-out spectroscopy very powerful tool for XPP experiment at XFEL - wavelength dispersive optics: required for single shot studies crucial practical advantages for pump-probe studies optimized spectrometer detector setup right detector is most important problem to solve! - XES: easiest setup (no mono needed) -spin state, chemical state, neighbor distance and type - XRS: low Z systems, non dipole DOS, EXAFS possible - R-XES: isolate LUMO resonances L-edge/M-edge like information with hard x-rays

24 Collaborators Kelly Gaffney (group) Pieter Glatzel Anders Nilsson (group) Lars Pettersson (group) Philippe Wernet Andrea di Cicco (group) Vittal Yachandra (group) Oliver Mullins Steve Cramer Stanford ESRF, Grenoble Stanford/Stockholm Stockholm BESSY, Berlin Camerino Berkeley Schlumberger-Doll LBNL/Davis Beamline Staff SSRL 10-2, 6-2, NSLS X-25, APS 18ID

25 Multi-Crystal Spectrometer DuMond, Rev. Mod. Phys., 5, 1, 1933

26 THANK YOU