XAFS: Study of the local structure around an Xray absorbing atom

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XAFSStudy of the local structure around an
X-ray absorbing atom
T.Ohta, Univ.Tokyo, Japan

• (1) Principle of XAFS
• (2) Instrumentation
• (3) XAFS spectral analysis
• (4) XAFS applications
• (5) New directions of XAFS

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(1) Principle of XAFS
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Phenomena caused by X-ray irradiation
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X-ray absorption spectrum from Pt foil

L1
L2
L3
K
wavelength
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XAFS
X-ray Absorption Fine Structure Local
electronic and geometric structures around the
x-ray absorbing atom
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XAFS spectrum from an atom
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XAFS spectrum from a diatomic molecule
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X-ray absorption Fermis 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

Point atom, plane wave, and single scattering
approximations
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EXAFS oscillation
Ribond distance
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Phase shift
R
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Phase shift of the X-ray absorbing atom
Pb
Sn
Ge
Si
C
k/A-1
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EXAFS amplitude
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EXAFS amplitude
High coordination number
Low temperature
High Z scatterer
Short distance
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Au K-EXAFS of Au foil
278K
77K
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EXAFS oscillation of Au K-edge
77K
278K
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If the coordination number decreases,
Photon Energy
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If the bond distance increases,
Photon Energy
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(2) Instrumentation
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Experimental method of XAFS
transmission method
monochromator
Io
I
X-rays
Ion chamber
sample
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Auger electron
photoelectron
Fluorescent X-rays
Secondary electron
Electron escape depth lt50 A
X-ray escape depthgt1000A
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Filter and solar slit
X -ray
sample
Ionization chamber
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Fluorescence
Absorption
Cu K-XAFS of CuSO4 10mMol aq. solution
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0.5 mm film
6mm film
Fluorescence
Transmission
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Partial electron yield ? x-ray absorption of
surface atoms
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Partial electron yield
Total electron yield
Sample leak current
Photon energy
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(3) XAF S spectral analysis
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EXAFS function
Fourier transform
Amplitude
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Back Fourier Transformation
EXAFS function
k
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Polarization Dependent EXAFS K-absorption(1s
? p-like continuum)
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Polarization Dependent EXAFS K-absorption(1s
? p-like continuum)
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Temperature dependence

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Determination of s2(T)
c-As
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What can we get from s2(T)
u0
ui
u0
Ri
R0
Ri
O
Einstein model
Einstein frequency
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c-As2S3 As-S 332 cm-1 g-As2S3 As-S 330
cm-1
c-As4S4 As-As 222 cm-1 As-S 342 cm-1
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X-ray energy(eV)
EXAFS c(k)
Atomic distance(Å)
EXAFS c(k)
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EXAFS analysis-1
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EXAFS analysis-2
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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.

Non-negligible
negligible
Shadowing effect
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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
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(4) XAFS applications

• Catalysis
• Amorphous systems
• Material physics(High Tc, CMR,.)
• Magnetic materials ? XMCD
• Thin films and Surface science
• Environmental science
• Biological materials

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(5) Challenge of XAFS

• Time-resolved XAFS spectroscopy
• Micro XAFS or Nano XAFS

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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