X-Ray Absorption Spectroscopy

EXAFS

Extended X-ray Absoprtion Fine Structure

XANES

X-ray Absorption Near-Edge Structure

• Potential of XAFS

• Synchroton radiation

• Photon absorption

• Scattering of the photoelectron

• XANES and EXAFS regions

• Measurement setups

• Basics of photon absorption

• The EXAFS formula

• Examples

Content

Uniqueness of XAFS

• Atom specific

• Highly sensitive to short and medium range order

- Coordination number (EXAFS)

- Bond distance (EXAFS)

- Type of neighboring atoms (EXAFS)

- Coordination geometry (XANES, EXAFS)

• Applicable to all states of matter

- Solids: crystalline and amorphous

- Liquids

- Molecular gases

- Bulk, dilute, surface and buried interfaces

X-ray Absorption Spectroscopy

• broad spectral distribution

- tunability: IR (0.1 mm) to hard x-rays (0.01 nm)

• high intensity

- > 106 times those of x-ray tubes

• high natural collimation

- high spatial resolution probe

• plane polarization

- εεεε-vector in electron orbital plane

• sharply pulsed time structure

- pulse length ~ 0.3 ns

- pulse interval ~ 800 ns

• properties can be calculated

Properties of synchrotron radiation

Laser driven fast and ultraLaser driven fast and ultra--fast fast xx--ray stations around the worldray stations around the world

High Harmonics Generation

X-ray lasers

Incoherent x-ray sources

Larmor Radiation

Insertion devices

• wiggler

• undulator

FEL: free electron laser

Brightness (intensity) of

different X-ray sources

Brightness

undulator

wiggler

wiggler

undulator

focussing

magnets

bending

magnet

injectionmagnet

electrons

wiggler

beam

undulator beam

acceleration

section

3rd. Generation Synchrotron

Principle of a linear accelerator

Electron Injector Main accelerator Electronsource distribution

Charged particles are accelerated by a

sinusoidal RF-field : U = Uo sin(ϖϖϖϖHF t)

≈≈≈≈e- source RFgenerator

Extract ≥≥≥≥ 1010 e- withinvery small phase volume• Solid angle• small E spread

Pre-accelerate withoutloss of electronsMatch machine param.

Provide final energyto the electrons

Tschentscher, DESY

Linac vs. Storage ringX-ray generation

Storage ring sources

Electron beam parameters are

determined by the storage ring. Equilibration is reached within few

turns.

- Energy spread ∆∆∆∆Ee/Ee ~ 10-3

- Horizontal emittance εεεεn ~ 10-4 m rad

+ Vertical emittance εεεεn < 10-6 m rad

- Pulse duration ∆∆∆∆t ~ 30-100 ps

+ Repetition rate

+ Multiple use

+ Beam stability

Linac sources

Electron beam parameters are determined

by the source and their preservation during

acceleration.

+ Energy spread ∆Ee/Ee ~ 10-4

+ Horizontal emittance εn ~ 10-6 m rad

+ Vertical emittance εn ~ 10-6 m rad

+ Pulse duration ∆t ~ 100-200 fs

- Repetition rate

- Multiple use

? Beam stability

Energy Recovery Linac sources

Tschentscher, DESY

Bending Magnet

Storage Ring

Synchrotron

Kyungmin, Chung

SASE FEL

Undulatore-

Synchrotron radiation

low emittance electron beam

relativistic electron energy

periodic acceleration of electron in magnetic field of an undulator

collimated radiation

tunable by electron energy & magn. field

The Undulator

From: Tschentscher, DESY

15 m

An Undulator at the HASYLAB/Hamburg

From: Tschentscher, DESY

The 50 GeV Linac at SLAC

Tschentscher, DESY

Michael Wulff, European Synchrotron Radiation Facility, Grenoble, Cedex 38043, France

The ESRF in Grenoble, France

DESY in Hamburg, Germany

DORIS at DESY in Hamburg, Germany

Relaxation of the excited atom

X-ray fluorescence Auger transition

M. Newville

The X-ray absorption coefficient µµµµ

Z = atomic number

A = atomic mass

E = X-ray energy

ρ = density

µµµµ has sharp absorption edges = characteristic core-level energies of the atom

M. Newville

Elements with K- or L-edge between 3 and 35 keV

T. Ressler

The X-ray absorption by a free atom

X-ray absorption: an available

state for the emitted electron

is needed

No available state:

no absorption

X-ray energy large enogh

to promote a core-level

electron to continuum:

sharp increase in absorption

Isolated atom: µµµµ(E) has a sharp step at the core-level binding energy, issmooth as function of energy above the edge

M. Newville

What is the Origin of EXAFS ?What is the Origin of EXAFS ?

X-ray photons

XX--ray Absorption ray Absorption with Neighboring atomswith Neighboring atoms ((PhotoPhoto--Electron Scattering)Electron Scattering)

7

Kyungmin, Chung

The X-ray absorption with photo-electron scattering

Neighbouring atom: photo-electron can scatter and return back to the absorbing atom

Photo-electron scattered

back will interfere

with itself

Amplitude of back-scattered photo-electron at the absorbing atom will oscillate with energy

causing oscillations in µ(E). Oscillations are an interference effect due to the presence of neighbours

M. Newville

Interference between the outgoing wave and the wavescattered at the neighboured atom

Model: 1. Photo-electron leaves the absorbing atom: 2. It is scattered at the neigbhour atom;

3. It returns to the absorbing atom