IFEFFIT

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IFEFFIT

• XANES Simulations e.g. FEFF8

Dr. Moniek Tromp 14 May 2008
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X-ray Absorption Spectroscopy
Pd K edge XAFS excitation 1s electron
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XANES oxidation state

• Initial state
• Increasing oxidation state gt orbitals contract gt
  lower energy initial state (XPS)
• Final state
• Mainly felt by metal
• Ucv increases with oxidation state, shift
  EvUcv may be larger than for Ec gt energy
  difference decreasing
• b. Localised on ligand gt does not experience
  full ox state change and remains unshifted

UCV core hole electron attraction EC core hole
energy
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Oxidation state Examples (1)
Cr and Cu Increasing edge energy with increasing
oxidation state
Chem. Phys. 2004, 300, 13 and JACS 1987, 109,
6433.
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Oxidation state Examples (2)
Vanadium Increasing edge energy with increasing
oxidation state
Phys. Rev. B 1984, 30, 5596
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Oxidation states Example (3)
Palladium Decreasing edge energy with increasing
oxidation state
JACS 2005, 127(2), 777
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XANES Selection rules

• Dipole selection rules ?l1 with orbitals
  s(l0), p(l1), d(l2), etc
• K edge s?p (e.g. Pd/Rh 1s?5p, Cu 1s?4p, Cr
  1s?4p)
• L edges p?d (e.g. Pt 2p3/2?5d5/2)
• Etc

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Octahedral
No mixing/hybridisation of p and d (and s)
orbitals only s?p transition visible
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Tetrahedral geometry
Hybridisation of d and p orbitals transition to
d orbitals becomes visible via a pre-edge
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Square planar D4h geometry
No mixing/hybridisation of p and d (and s)
orbitals only s?p transition visible
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Vanadium Structures
Octahedral
C3
C1
Cs
Lowering symmetry, increasing pre edge
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Cr complexes
1st feature 1s?3d Mainly in tetrahedral
complexes Cr(IV), Cr(V), Cr(VI) 2nd feature
1s?4s Absent in octahedral as Cr(III) Jahn-Teller
distortion in Cr(II) lowers coordination
symmetry and allows transition
Cubic
Square Planar
Tetrahedral
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Resume

• XANES direct probes the empty density of states
• Remember selection rules and thus allowed and
  forbidden transitions
• Lowering symmetry structure enables hybridisation
  of orbitals and thus (partly) allow originally
  dipole forbidden transitions

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DOS Organometallic Complexes
JACS 2005, 127(2), 777
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XANES (PP)Pd(XX)
JACS 2005, 127(2), 777
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DOS (PP)Pd(allyl)
Pd(II) complex 4d8
JACS 2005, 127(2), 777
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Transitions
Distorted square planar geometry
JACS 2005, 127(2), 777
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XANES features

• Pd2 Kr 4d8
• 1s ? 5p dipole allowed 2nd peak P p peak
• 1s? 4d dipole forbidden hybridisation with Pd d
  orbitals enable transition to occur 1st peak
  Pd d peak

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Simulations (FEFF8)
JACS 2005, 127(2), 777
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Transitions
Distorted square planar geometry
JACS 2005, 127(2), 777
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Oxidation state vs bite angle
JACS 2005, 127(2), 777
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5 pz
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Overview results
JACS 2005, 127(2), 777
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Understanding Materials

• Heterogeneous catalysts
• Metal nanoparticles on and within nanoporous
  silica

J Phys Chem B, 2001, 105, 5244
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DOS to probe particle size

• Probing DOS which reflexes metal particle sizes

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Rh clusters XANES
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Rh/Al2O3 XANES
EXAFS 2wt Rh/Al2O3 CN(Rh-Rh) 3.5-4 5wt
Rh/Al2O3 CN(Rh-Rh) 7-8
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5wtRh/Al2O3, NO/He, 300oC
1 Rh-NO 2.7 Rh-Rh
Rh-NO
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• Direct probing empty density of states
• Oxidation state
• Geometry
• Density of states (bonding information, if
  possible from different sites)
• Emission
• Warnings
• XANES very sensitive, good references required
• Average technique..

How to analyse or simulate ??
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Simulation Methods

• K edge XAFS spectra above the pre-edge
• Well-reproduced using FEFF codes
• Ab initio real-space multiple scattering of
  electronic structure and XAS
• Includes many-body effects e.g. self energy and a
  screened core hole, often good approx. to
  anti-bonding resonances, spin-orbit coupling
• NO non-spherical potentials, inter-atomic
  electron-electron interactions that lead to
  multiplet structure, charge transfer
• Multiplets important i.e. in case where core
  hole other than 1s is present in initial or final
  state
• Atomis-multiplet/ligand field approach
• Spherical symmetry of the atomic-wavefunctions is
  reduced by point group symmetrry of metal in
  compounds
• Group theoretical considerations allow modelling
  of orbital splitting, wavefunction mixing,
  electron transition rules, charge transfer
• NO complete description chemical bonding,
  pi-bonding and back-bonding effects
• Density Functional calculations
• Reproduce bonding orbitals of a system well
• NO anti-bonding and continuum orbitals 10-50 eV
  above the Fermi level, multiplet structure
• Approach desired which combines full potentials
  and many-body effects with multiplet theory
• Problem for most theoreticians How to deal with
  core hole(s).

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XANES FEFF8
License

• Muffin Tin potentials
• Full multiple scattering
• Self consistent field
• EXAFS
• XANES
• Angle, polarisation dependent
• Density of States
• X-ray Natural and Magnetic Circular Dichroism
• Spin Polarized X-ray Absorption Spectra
• Non-Resonant X-ray Emission Spectra (XES)
• X-ray scattering amplitude (Thomson and anomalous
  parts)
• Electron Energy Loss Spectroscopy (EELS)

http//leonardo.phys.washington.edu/feff/wiki/inde
x.php?titleMain_Page
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FEFF8 Overview

• Calculate scattering potentials using atomic
  overlap or self-consistently using an automated
  SCF look
• Scattering phase shifts, dipole matrix elements,
  x-ray cross section and angular momentum
  projected density of states calculated
• Full multiple scattering XANES calculations done
  for specified cluster
• Leading MS paths enumerated
• Effective scatterings amplitude and other XAFS
  parameters calculated for each scattering path
• XAFS parameters from one or more paths are
  combined to calculate a total XAFS or XANES
  spectrum

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Different Cards EXCHANGE

• EXCHANGE ixc Vr Vi
• ixc type of potential
• Vr E0 correction
• Vi Experimental resolution corrections

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Different cards

• XANES xkmax xkstep estep
• Calculation near edge structure including atomic
  background and absolute energies
• Levels beyond Fermi level, i.e. empty density of
  states
• Accuracy in absolute energies varies from few eV
  at low Z to few hundred eV for high Z
• FMS calculations not valid beyond k6
• LDOS emin emax eimag
• Angular momentum projected density of states
  (standard on a grid of 84 points)
• Eimag imaginary part of potential, i.e.
  Lorentzian broadening

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Different cards

• SCF rfms1 lfms1 nscmt ca nmix
• Rfms1 the radius of the cluster for FMS.
• Typically 30 atoms within the sphere. Usually
  this value is smaller than the value rfms used in
  FMS card, but should be larger than the radius of
  the second coordination shell.
• Lfms1 1 for solids, 0 fore molecular
  calculations
• FMS rfms1 lfms2
• Rfms Cluster radius used in all modules but
  POT.
• The FMS module sums all MS paths within the
  specified cluster. Typically, a converged XANES
  calculation requires about 50-150 atoms in a
  cluster, but sometimes more are needed.
• Lfms1 1 for solids, 0 fore molecular calculations

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Some more useful cards

• NOHOLE
• Roughly estimates the effect of complete
  core-hole screening
• Useful to test final state for calculated XAS
• Often better agreement for dDOS and L2 and L3
  (whitelines)
• Poor for K edges, especially in insulator
  materials
• NLEG nleg
• Limits the number of scattering paths to nleg
• Nleg2 only single scattering paths
• Default nleg8

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Some more useful cards Specific Calcu.

• POLARIZATION x y z
• Specifies direction of incident beam or main axis
  of ellips
• Card omitted spherically averaged XAFS
  calculated
• ELLIPTICITY ellipticity x y z
• Use with POLARIZATION card
• ellipticity ratio of amplitudes of electric
  field in the two orthogonal directions of
  elliptically polarized light
• No distinction can be made between left and right
  circular polarized light
• Zero value corresponds to linear polarization, 1
  to cicular
• X,y,z coordinated of any nonzero vector in the
  direction of the incident beam ( normal to the
  polarization vector)

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• Upper limit of XANES calculation. This
  must be
• uncommented to make Feff calculate full
  multiple
• scattering rather than a path expansion
• kmax delta_k delta_e
• XANES 6.0 0
• Radius of cluster for Full Multiple
  Scattering calculation
• l_fms 0 for a solid, 1 for a molecule
• r_fms l_fms
• FMS 6.88929 0
• Energy grid over which to calculate DOS
  functions
• emin emax eimag
• LDOS -20 30 0.1
• for EXAFS RMAX 6.0 and uncomment the
  EXAFS card
• RPATH 0.1
• EXAFS 20

• TITLE name MnO
• Mn K edge energy 6539.0 eV
• EDGE K
• S02 1.0
• pot xsph fms paths genfmt ff2chi
• CONTROL 1 1 1 1 1 1
• PRINT 1 0 0 0 0 0
• ixc0 means to use Hedin-Lundqvist
• ixc Vr Vi
• EXCHANGE 0
• Radius of small cluster for self-consistency
  calculation
• A sphere including 2 shells is a good choice
• l_scf 0 for a solid, 1 for a molecule
• r_scf l_scf n_scf ca
• SCF 4.5 0

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Other programs e.g. Charge Transfer Multiplet
program
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X-ray Absorption Spectroscopy
Excitations of core electrons to empty
states The XAS spectrum is given by the Fermi
Golden Rule
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X-ray Absorption Spectroscopy

• Element specific DOS
• L specific DOS
• Dipole selection rule (?L 1)

oxide
1s
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XAS core hole effect
TiSi2

• XAS probes empty DOS
• Core Hole pulls down DOS
• Final State Rule Spectral shape of XAS looks
  like final state DOS
• Initial State Rule Intensity of XAS is given by
  the initial state

Phys. Rev. B. 41, 11899 (1991)

• Dipole selection rule (?L 1)
• Element specific DOS
• L specific DOS

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XAS multiplets and charge transfer
Multiplet effect Strong overlap of 2p-core and
3d-valence wave functions Single Particle model
breaks down Necessary to use atomic-like
configurations. Charge Transfer Core hole
potential causes reordering of configurations
3d
ltpd1/rpdgt 10 eV
2p3/2 2p1/2
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X-ray Absorption Spectroscopy

• Single Electron Excitation
• K edges
• (WIEN, FEFF, .)
• Many Body Excitation
• Other edges
• (CTM)

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X-ray Absorption Spectroscopy
No Unified Interpretation!

• Single Electron Excitation
• K edges
• (WIEN, FEFF, .)
• Many Body Excitation
• Other edges
• (CTM)

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Multiplet-Charge Transfer Theory

• To account for the ligand environment of the
  transition metal, the spherical symmetry of the
  atomic-wave functions is reduced by the point
  group symmetry of the metal ion in the compound.
• Group theoretical considerations then allow one
  to model the orbital splitting, wave function
  mixing, and electron transition selection rules
• Determine symmetry and term symbols initial and
  final state(s)
• Introduce crystal field and charge transfer
  effects

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XANES

• Not a unified and accepted theory available yet
  which includes all effects observed in XANES!
• Approximations only!
• Strengths and limitations of different
  theoretical approaches for different edges and
  materials