Post on 22-Jun-2018
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