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Joachim Stöhr Stanford Synchrotron Radiation Laboratory

X-Ray Absorption Spectroscopy. Joachim Stöhr Stanford Synchrotron Radiation Laboratory. http://www-ssrl.slac.stanford.edu/stohr J. Stöhr, NEXAFS SPECTROSCOPY, Springer Series in Surface Sciences 25, ( Springer, Heidelberg, 1992). J. Stöhr and H. C. Siegmann

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Joachim Stöhr Stanford Synchrotron Radiation Laboratory

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  1. X-Ray Absorption Spectroscopy Joachim Stöhr Stanford Synchrotron Radiation Laboratory • http://www-ssrl.slac.stanford.edu/stohr • J. Stöhr, • NEXAFS SPECTROSCOPY, • Springer Series in Surface Sciences 25, (Springer, Heidelberg, 1992). • J. Stöhr and H. C. Siegmann • MAGNETISM: FROM FUNDAMENTALS TO NANOSCALE DYNAMICS, • Springer Series in Solid State Sciences 152, (Springer, Heidelberg, 2006)

  2. Physical Processes

  3. Quantum Theoretical X-Ray Interactions with Matter: The Basic Processes Kramers- Heisenberg relation Fermi's Golden rule

  4. Tunable x- rays offer large interaction cross sections electrons optical light Photoemission neutrons

  5. X-Ray Absorption and Scattering Cross Sections per Atom Thomson Cross section Fe atom Experiment Fe metal Kortright and Kim, Phys. Rev. B 62, 12216 (2000) f1 and f2 tabulated Henke-Gullikson factors http://www-cxro.lbl.gov/index.php?content=/tools.html

  6. X-ray Absorption Spectra in a Nutshell tabulated Henke-Gullikson

  7. Absolute absorption coefficients from experimental spectra (from Henke-Gullikson compilation)

  8. Names: XAFS – NEXAFS –XANES - EXAFS or XANES Interference of outgoing photoelectron and scattered waves Nearest neighbor distances Number of neighbors

  9. Tunable x-rays offer elemental specificity

  10. Experimental Soft X-rayTechniques

  11. Experimental X-Ray Absorption Techniques

  12. X-Ray Absorption versus Photoemission

  13. Electron Yield Sampling Depth

  14. Surface sensitivity of total and Auger yield

  15. total yield

  16. Some Fundamental X-Ray Absorption Spectra -- soft x-rays --

  17. NEXAFS spectra of polymers: building block picture

  18. Chemical Sensitivity Core level shifts and Molecular orbital shifts

  19. NEXAFS of Transition Metals Dipole selection rule 2p 3d - strong 2p 4s - weak Ebert et. al. Phys. Rev. B 53, 16067 (1996). “white lines” Total intensity reflect number of empty holes

  20. Polarized X-Rays - Dichroism “dichroism” = pol. dep. absorption

  21. Polarization definitions (high energy physics) Historical note: different “handedness” definitions in optics (space) versus high energy physics (time)

  22. Polarized x-rays offer orientation sensitivity Antiferromagnetic order Orientational order Chirality Ferromagnetic order

  23. X-Ray Natural Linear Dichroism

  24. Linear Charge Dichroism in a d-electron system C. T. Chen et al. PRL 68, 2543 (1992)

  25. J. Stöhr et al., Science 292, 2299 (2001)

  26. X-Ray Magnetic Linear Dichroism

  27. Magnetic field splits p-orbitals sum

  28. XMLD – spectra below and above TN Lüning et al. Phys. Rev. B 67, 214433 (2003)

  29. XMLD spectra of two oxides

  30. XMLD effects especially strong in multiplet peaks (Ni2+,d8) (Ni1+,d9)

  31. X-ray Magnetic Circular Dichroism

  32. Magnetic Circular Dichroism

  33. Soft x-rays are best for magnetism

  34. XMCD spectra of the pure ferromagnetic metals

  35. The sum rules

  36. s X-ray Natural Circular Dichroism l k=k0 w/c Dipole Chirality =0 for x-rays

  37. Pasteur’s and Faraday’s experiments

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