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Applications of X-Ray Absorption

Spectroscopy in Physics and

Material Science.

Joaquin Garcia Ruiz

Instituto de Ciencia de Materiales de

Aragón, CSIC-Univ. de Zaragoza, Spain

IUCr 2011 XAFS Tutorial, Madrid

Introduction.

Experimental aspects.

Regions of the XAS spectrum. (EXAFS,XANES)

Multiple scattering description

Extended X-ray Absorption Fine Structure (EXAFS) spectra -EXAFS formula, Fourier transform, determination of the structural parameters Applications: Materials Science (high Tc, CMR,..), Magnetic materials, Amorphous and liquid systems, Thin films and Surface Science. Some examples

XANES spectra (Soft and Hard X-rays)

-Information contained in XANES -Theoretical approaches, Multiple scattering and Multiplets - Some applications.

Related techniques

Time-resolved XAFS

Differencial EXAFS

Micro- or Nano-XAFS

X-ray magnetic circular dichroism (XMCD)

Anomalous scattering

DAFS (Diffraction anomalous Fine Structure)

Resonant X-ray scattering (RXS)

IUCr 2011 XAFS Tutorial, Madrid

X-ray absorption process

X-rays are absorbed by all matter through the photoelectric-effect: The excited core-hole will relax back to the ground state of the atom by two main mechanisms: X-ray Fluorescence, for hard x-ray regime (> 2 keV) and Auger Effect for lower energy x-ray absorption.

When the incident x-ray has an energy

equal or greater than the binding energy of a core-level electron (K=1s; LI=2s; LII,

LIII=2p, etc.), the x-ray photon is absorbed

and the core electron is ejected from the atom.

The atom is left in an excited state with an

empty electronic level (core hole) and any excess energy is given to the ejected photo-electron.

IUCr 2011 XAFS Tutorial, Madrid

The probability that an x-ray beam passing through a material of thickness t will be absorbed is given by the linear absorption coefficient,P:

I = I0 e-t

t I0 I (E) Z4 / E3 + 5 10 15 20 25
30
35
40

102030405060708090

E (keV)

Z K LIII

E ~ Z2

10-1 100
101
102
103
104

110100

(cm 2/gr)

E (keV)

Ag Fe O

X-ray absorption

IUCr 2011 XAFS Tutorial, Madrid

X-ray absorption measurements

XAFS measures the energy dependence of (E) near and above the absorption edge of a selected element. Transmission: The intensity transmitted through the sample is measured (direct method) (E)t = ln(I0/I) Fluorescence / Total-electron yield: The fluorescence x-ray or the electron emitted is measured (indirect methods) (E) ~ If / I0 0 0.5 1 1.5 2

71007200730074007500

(E)

E (eV)

X-ray absorption fine structure ?

IUCr 2011 XAFS Tutorial, Madrid

Multiple scattering description.

Absorption coefficient can be factorized in an atomic part and a term which represents the scattering of the photoelectrons by the sourronding atoms = 0(E) (1 + n(E) )

EXAFS XANES

n= 2 n> 2 (E)-(E) 00

0IUCr 2011 XAFS Tutorial, Madrid

X-ray Absorption Fine Structure (XAFS -ray absorption coefficient at energies near and above an x-ray absorption edge due to the chemical and physical state of the atom. XAFS is also referred to as X-ray Absorption Spectroscopy (XAS) and is divided into two regions:

XANES X-ray Absorption Near Edge Structure

EXAFS Extended X-ray Absorption Fine Structure

environment and electronic state.

XAFS Capabilities :

local atomic probe (short-range order) composition/coordination chemical/oxidation state applies to any element/any phase (gas, liquid, crystalline, amorphous ...) minimal sample requirements (low concentrations, wide range of sample environments 0 0.5 1 1.5 2

70007200740076007800

(E)

E (eV)

Fe K-edge XAFS of Fe3O4

XANES (region within ~ 50 eV of the absorption edge)

EXAFS (region well above the absorption edge,

up to 1 keV)

IUCr 2011 XAFS Tutorial, Madrid

R1 R2

Single and multiple scattering paths

))(2(),()(2 2)(22 0 22
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