1. XAFS: Study of the local structure around an X-ray absorbing atom (1) Principle of XAFS (2) Instrumentation (3) XAFS spectral analysis (4) XAFS applications (5) New directions of XAFS T.Ohta, Univ.Tokyo, Japan

2. (1) Principle of XAFS

3. X-ray Diffracted X-rays Photoelectrons Transmitted X-rays Diffracted X-rays Scattered X-rays Fluorescent X-rays Desorbed ions Phenomena caused by X-ray irradiation

4. L3 L2 L1 K X-ray absorption spectrum from Pt foil wavelength

5. XAFS X-ray Absorption Fine Structure : Local electronic and geometric structures around the x-ray absorbing atom

6. XAFS spectrum from an atom

7. XAFS spectrum from a diatomic molecule

8. X-ray absorption ： Fermi’s Golden Rule i: wave function of the initial state  1s f: wave function of the final state  superposition of the ejected wave and back-scattered waves EXAFS function Point atom, plane wave, and single scattering approximations

9. EXAFS oscillation Ri:bond distance Phase shift ii

10. Ｒ

11. Pb Sn Ge Si C Phase shift of the X-ray absorbing atom k/A -1

12. EXAFS amplitude Effective Coordi- nation number Back scattering amplitude Debye-Waller factor Electron mean free path

14. EXAFS amplitude Effective Coordi- nation number Back scattering amplitude Debye-Waller factor Electron mean free path High coordination number Low temperature High Z scattererShort distance

15. Au K-EXAFS of Au foil 278K 77K

16. EXAFS oscillation of Au K-edge 77K 278K

17. Photon Energy If the coordination number decreases,

18. Photon Energy If the bond distance increases,

19. (2) Instrumentation

20. monochromator IoI sample Data processing X-rays Ion chamber 《 transmission method 》 Experimental method of XAFS

21. X-ray escape depth>1000A Electron escape depth ＜ 50 A Fluorescent X-rays Secondary electron photoelectron Auger electron

22. sample X -ray Ionization chamber Filter and solar slit

23. Fluorescence Absorption Cu K-XAFS of CuSO 4 10mMol aq. solution

24. Transmission Fluorescence 0.5 mm film 6  m film

25. Partial electron yield  x-ray absorption of surface atoms

26. Total electron yield Photon energy Sample leak current Partial electron yield

27. (3) XAF S spectral analysis

28. Fourier transform Amplitude EXAFS function

29. Back Fourier Transformation k Amplitude Envelope Kind of scatterer Coordina- tion number Polarization Direction of Bond, Adsorp- Tion site Temperature Bond stiffness EXAFS function

30. Polarization Dependent EXAFS K-absorption(1s  p-like continuum)

31. Polarization Dependent EXAFS K-absorption(1s  p-like continuum)

32. Temperature dependence

33. c-As Determination of  2 (T)

34. What can we get from  2 (T) u0u0 u0u0 uiui RiRi R0R0 RiRi OEinstein model Einstein frequency

35. c-As As-As: 216 cm -1 a-As As-As: 234 cm -1 c-As 2 S 3 As-S: 332 cm -1 g-As 2 S 3 As-S: 330 cm -1 c-As 4 S 4 As-As: 222 cm-1 As-S : 342 cm-1

36. Data acquisitionDetermine E 0, Convert from eV to kBackground subtraction and normalization Multiply k n Fourier transformation Fourier filteringBack Fourier transformation Curve fitting in real spaceModel structure

37. X-ray energy(eV) EXAFS  (k) Atomic distance(Å) EXAFS  (k)

38. EXAFS analysis-1 Phase shift Atomic distance amplitude 1st nearest 2nd nearest 3rd nearest Fourier transformation

39. EXAFS analysis-2 Effective Coordination Number Debye- Waller factor Back scattering amplitude Direction of chemical bond Pair Potential Function Kind of scattering atom

40. Limitation and Improvement of XAFS theory Multiple scattering effect which is enhanced at XANES region and also at longer distance above 3 A.  FEFF program developed by J.Rehr can be used for spectral simulation. Shadowing effect negligible Non-negligible

41. Limitation and Improvement of XAFS theory Vibrational anharmonicity The formula assumes a Gussian distribution.  Cumulant expansion method has been developed to take into the anharmonicity, which gives the information of real bond distance, thermal expansion coefficients, radial distribution curve. where

42. (4) XAFS applications Catalysis Amorphous systems Material physics(High Tc, CMR,….) Magnetic materials  XMCD Thin films and Surface science Environmental science Biological materials

43. (5) Challenge of XAFS Time-resolved XAFS spectroscopy Micro XAFS or Nano XAFS

44. Summary--Features of XAFS Applicable to any phase (amorphous, liquid, gas), surface/interface and biomaterials Measurable under various conditions  under high pressure, gaseous atmosphere, for real catalysis Polarization dependence  direction of the bond Temperature dependence  strength of any specific bond Combined with microbeam  local structure of a local area Pump-probe experiment  dynamics of local structure