XANES measurements and Interpretation 2003

EXAFS Data Collection and Analysis Course, NSLS, July 14-17, 2003

XANES Measurements andInterpretation

Simon R. Bare

UOP LLC, Des Plaines, IL

[email protected]


Acronyms

XANES

• X-ray Absorption Near Edge Structure

NEXAFS

• Near-Edge X-ray Absorption Fine Structure

The two acronyms should be interchangeable but overthe years NEXAFS has become terminology for “low Z”elements - C, N, O...


What Is XANES?

AB

SO

RP

TIO

N

214002120021000208002060020400202002000019800

PHOTON ENERGY (eV)

XANESEXAFS

X-ray absorptionspectrum ofmolybdenum metalfrom 19.8 to 21.5 keV

• XANES is region of x-ray absorption spectrum within ~50eV of the absorption edge.

• Suggested that division is that at which wavelength of excited electron is equal todistance between absorbing atom and its nearest neighbor. (λ (Å) ≈ 12/[e(eV)] ½.


What Is XANES?

-20 0 20 40 60 80 100 120Photon Energy (E-E0)

0

1

Nor

mal

ized

Abs

orpt

ion

(X-ray absorption spectrum of Ti K-edge of Ba2TiO4)

XANES= Pre-edge + Edge + XANES


Terminology

• Traditionally, set energy scale and calibrate energy by takingmaximum of first derivative of edge as “E0”.

• Then any peaks below this energy are “pre-edge” peaks.

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

Abs

orpt

ion

2.8502.8452.8402.8352.8302.8252.820

Photon Energy, keV

0.3

0.2

0.1

0.0

-0.1E0

Cl K-edge XANESof Cl(aq)

First derivative


“Pre-edge” Peaks2.0

1.5

1.0

0.5

0.0

Abs

orpt

ion

2.8502.8452.8402.8352.8302.8252.820

Photon Energy, keV

K2PtCl6

E0

Pre-edge peak Cl K-edge XANESof K2PtCl6

• True energy zero is the Fermi energy: there can be no transitions below this energy.

• If “edge” energy correctly defined as EF there are, by definition, no “pre-edge”features!

• Often difficult to distinguish between bound (localized) and unbound (delocalized)states.


Ba2TiO4

K2TiSi3O9

Why Are We Interested In XANES?Local Coordination Environment

Both Ti4+

Ba2TiO4

K2TiSi3O9

• Ti K-edge XANES shows dramatic dependence onthe local coordination chemistry.


Why Are We Interested In XANES?Oxidation State

• Many edges of many elements show significant edgeshifts (binding energy shifts) with oxidation state.

4.0

3.5

3.0

2.5

2.0M

n va

lenc

e6552655065486546

Mn K-edge Energy, eV

MnO Mn2O3 MnO2


What Is XANES and Why Are We Interested?XANES is strongly sensitive to the chemistry (formaloxidation state and geometry) of the absorbing atom.

Region Transitions Information Content

Pre-edge Features caused byelectronic transitions toempty bound states.Transition probabilitycontrolled by dipolarselection rules.

Local geometry around absorbing atom.Dependence on oxidation state and bondingcharacteristics (chemical shift).

Edge Defines ionizationthreshold to continuumstates.

Dependence on oxidation state (chemicalshift), main edge shifts to higher energy withincreased oxidation state. (As much as 5 eVper one unit change).

XANES Features dominated bymultiple-scatteringresonances of thephotoelectrons ejected atlow kinetic energy. Largescattering cross section.

Atomic position of neighbors: interatomicdistances and bond angles. Multiplescattering dominates but ab initiocalculations providing accessible insight (e.g.FEFF8).


XANES Transitions• XANES directly probes the angular momentum of the

unoccupied electronic states: these may be bound or unbound,discrete or broad, atomic or molecular.

• Dipole selection rules apply*: ∆l = ±1, ∆j = ±1, ∆s = 0.

• Primary transition will be:

• s → p for K (1s core electron) and L1 (2s core electroninitial state) edges

• p → d for L2 (2p½) and L3 (2p3/2) edges

• But…..final state usually not atomic-like and may havemixing (hybridization) with other orbitals. This is often theinteresting part of the XANES!

* Some transitions are true quadrupolar transitions. These are usually very weak.


XANES Interpretation• The EXAFS equation breaks down at low-k, which complicates

XANES interpretation.

• We do not have a simple equation for XANES.

XANES can be described qualitatively (and nearly quantitatively) in terms of:

coordination chemistry regular, distorted octahedral, tetrahedral…

molecular orbitals p-d hybridization, crystal field theory

band structure the density of available occupied electronic states

multiple scattering multiple bounces of the photoelectron

• These chemical and physical interpretations are all related:

What electronic states can the photoelectron fill?


Advantages of XANES vs. EXAFS• Spectra simpler to measure than EXAFS: features intense, concentrated

in small energy region.

• Weak temperature dependence (Debye-Waller), so spectra can berecorded at reaction temperature (in situ).

• Faster to measure than full spectrum: <msec demonstrated.

• Sensitive to chemical information: valence, charge transfer.

• Probes unoccupied electronic states: important in chemistry.

• Often used as simple “fingerprint” to identify presence of a particularchemical species.

• Beamlines with micro-probe capabilities can also scan energy and obtainXANES spectra with elemental distribution.


XANES Analysis: Oxidation StateSulfur

• Sulfur K-edge XANES used toidentify and quantify the form ofsulfur in heavy petroleum, coals, soilsetc.

• 11 eV edge shift from S2- to S6+.

• Spectra of S in similar environmentssimilar: thiophene, benzothiophene.

• Can be used as fingerprint.

Reference: George and Gorbaty, J. Am. Chem. Soc. 101 (1979) 3182


XANES Analysis: Oxidation StateSulfur

12

10

8

6

4

2

0

Nor

mal

ized

Abs

orpt

ion

2.4902.4852.4802.4752.4702.465

Photon Energy, keV

Sulfide Thiophene Sulfoxide Sulfone Sulfate

2,479.0

2,476.4

2,473.0

2,471.2

2,470.6

Dodecyl sulfide, thiophene, tetramethylene sulfoxide, tetramethylene sulfone, sulfate(aq)


XANES Analysis: Oxidation State

Many, many examples in the literature…...

Mo K-edgeV K-edge

Ref: Cramer et al., JACS, 98 (1976) 1287

Ref: Wong et al., Phys Rev. B 30 (1984) 5596

Re L3-edge

10534

10535

10536

10537

10538

10539

10540

10541

0 2 4 6 8

Re Oxidation State

Re

L3-

edg

e E

ner

gy,

eV


-20 -10 0 10 20 30 40 50

Photon Energy (E-E0) eV

0.0

0.5

1.0

1.5N

orm

aliz

ed A

bsor

ptio

n

Ti_refs1.axg

Ti K-edge XANES: Reference Compounds

E0 = 4966.0 eV

Anatase - 6 coordinate

Fresnoite -5 coordinate

Bariumorthotitanate -4 coordinate

1s → 3d

• Symmetry around absorbing atom strongly affects pre-edge transition:ability to differentiate 4, 5, 6-fold coordination.

Local Site Symmetry in Ti-containing Compounds


4969 4970 4971 4972

Absolute Position (eV)

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

Hei

ght

[4]

[5]

[6]

Ba2TiO4

This data

Farges dataJDF-L1

This dataFarges data

Anatase

Ti umbite

• Correlation between absolute position and peak height of pre-edgepeak: all 4-fold, 5-fold and 6-fold coordinated Ti compounds fall intoseparate domains.

• Ability to distinguish Ti coordination from pre-edge peak information.

Local Site Symmetry in Ti-containing Compounds

Reference: Farges et al., Geochim.Cosmochim. 60 (1996) 3023


XANES of 3d Transition Metals:Coordination

• For Td symmetry 1s to 3d pre-edge peak sharp and intensefrom Ti→Mn, decreases Fe →Cu, absent for Zn.

• Decrease in intensity due toprogressive filling of the 3dband.

• Oh symmetry shows only a smallpre-edge peak throughout series.

Td Oh

Ref: Lytle et al. Phys. Rev. B 37 (1988) 1550.


“White line” Intensity of Group VIII Metals• Peak historically called a “white line” as when it was detected by x-

ray film it showed up as a white line due to the strong absorption.

Reference: G. Meitzner et al., J. Phys. Chem. 96 (1992) 4960

• Transition from 2p3/2 to 5d states.

• Absence of peak for Au: 5d states almost completely occupied.

• For others Pt<Ir<Os<Re, corresponding to increase in number ofunoccupied 5d states on the atoms.

L3 edge XANESfor 5d metals


Quantification of “White Line”• Fit to combination of Lorentzian and

arctangent functions.

• Determine: area, fwhm, position.

Ref: Lytle et al., Proc. 9th Int. Congr. Catal, Vol 5 (1988) 54

• Linear correlation betweenwhite line area and number of5d-holes for Au-Re


“White Line” Intensity: OxidesRe L3-edge - Transition from 2p3/2 to 5d states.

Re metal (Re0) - 5d5

ReO2 (Re4+) - 5d1

NH4ReO4 (Re7+) - 5d0

• Intensity of Re L3 white line probes Re LDOS


“White Lines”: large change in absorption coefficient

• Step height linear with concentration from 100-10,000 ppm S - butrelative white line intensity constant only for <2000 ppm.

• Important if using a “reference” spectrum for fitting or fingerprinting.

10

8

6

4

2

0

Nor

mal

ized

Abs

orpt

ion

2.4902.4852.4802.4752.470

Photon Energy, keV

96 ppm 368 ppm 942 ppm 1840 ppm 3740 ppm 9000 ppm

0.06

0.05

0.04

0.03

0.02

0.01

0.00

Abs

orpt

ion

Ste

p H

eigh

t

1000080006000400020000

Wt ppm Sulfur as Sulfate

11

10

9

8

7

White line intensity


L3

L2

• Significant difference in L3 and L2 edge XANES: 2p to 5d transitions.

•Pt 5d3/2 filled, so no whiteline.L2 shifted to align with L3 edge.

EF5d5/2

5d3/2

L3 L2

2p1/22p3/2

•Same l=2 final density of states but because of selection rule, ∆j = ±1,different total quantum number probed.

• j=3/2 probed by L2-edge, j=5/2 probed by L3-edge.

Pt L3 and L2 Edge XANES


• Transition is 2p to 5d: Pt d-band full, so “no” intensity at edge.• PtGe intermetallics: charge transfer from d-band of Pt to Ge, resulting in

significant intensity at edge.• Use as signature of Pt-Ge intermetallic formation.

-30 -20 -10 0 10 20 30 40

Photon Energy (E-E0) eV

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Nor

mal

ized

Abs

orpt

ion

Platinum L2-edge XANES: PtxGey Intermetallics

Pt

Pt3Ge2

Pt3Ge2

Pt3Ge

XANES to Probe Charge Transfer in Alloys

Increasing %Gecontent PtGe2


Effect of Adsorbed Hydrogen on Pt L3 XANES

• White-line intensity decreasesand spectra broaden to higherenergies as H is added.

• Difference signal typically leadsto broad structure ~8 eV aboveabsorption edge.

• Several different interpretationsin the literature.

0.05

0.00

Abs

orpt

ion

40200-20

Photon Energy (E-E0), eV

Diff

eren

ce

40200-20

Photon Energy, E-E0, eV

Increasing H coverage10-15Å Pt clusterssupported on Al2O3


Which Edge to Choose: Energy ResolutionMo K-edge XANES of Na2MoO4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

Abs

orpt

ion

20.1520.1020.0520.0019.95

Photon Energy, keV

Td symmetrys-dtransition

• Mo K-edge at 20.00 keV, effective resolution of 10 eVdominated by core-hole lifetime.

s-ptransition


Which Edge to Choose: Energy Resolution

8

6

4

2

0

Nor

mal

ized

Abs

orpt

ion

0.300.250.200.150.100.050.00-0.05

Photon Energy, (E-E0) keV

• Comparison of normalized Mo L3-edge (2.5 keV) XANESof Na2MoO4 with that of Mo K-edge (20.0 keV).

L3

K

• Mo L3-edge at 2.5 keV, 0.5 eV spectral resolution!


Which Edge to Choose: Energy ResolutionIn

tens

ity (

arb.

uni

ts)

403020100-10-20

Photon Energy (eV)

Inte

nsity

(ar

b. u

nits

)

403020100-10-20

Photon Energy (eV)

Sodium Molybdate, NaMoO4

Mo atom tetrahedral coordinationCobalt Molybdate, CoMoO6

Mo atom octahedral coordination

2p 3/2 2p 3/2

e

t 2

eg

t 2g

Mo L3 -edge XANES Mo L3 -edge XANES

• ∆E tetrahedral = 2.2-2.5 eV; ∆E octahedral = 3.25-4.2 eV


0.0

1.0

2.0

3.0

4.0

0.0

10.54

10.56

10.58

20

40

60

80

Y

X

Z

Photon Energy, keV

Nor

mal

ized

Abs

orpt

ion

RT

500ºC

Time Evolution of XANES: Kinetics

Reductionin H2

Reduced Re

Oxidized Re

500

100

80

60

40

20

Per

cent

Re

2O7

rem

aini

ng

500400300200100

Temperature, ºC

Re

Al2O3

Al2O3

Re-O = 1.72Å

Re-Al = 3.05Å

Re7+In situ temperature programmedreduction of Re2O7/Al2O3


0.5

1.0

1.5

2.0

2.5

3.0

3.5

5.46

5.47

5.48

5.49

5.5

20

40

60

80

Y

X

Z

Time Evolution of XANES: KineticsTPR-XANES showingreduction of vanadium ascatalyst is heated in H2 to 500ºC

Heat inH2 flow

Vanadium K-edgeXANES

RT

500 º C

Recorded at X19A, 80 sec/scan


Q-XANES & D-XANESQuick XANES

• Slew monochromator continuously to obtain a XANESspectrum in few seconds.

• All modes of detection.

Dispersive XANES

• Polychromatic beam dispersed onto linear detector.

• XANES spectrum in msec.

• Transmission only.

• Need extremely uniform samples.


Micro-XANES• Use special optics to focus x-ray beam to 10µm diameter (X26A) or <1µm

diameter at APS.

• Combined with x-ray microprobe: elemental composition maps andoxidation state/local coordination.

• Applications:– Speciation of metals in soils, sediments and organisms

– Grazing incidence studies of cations and anions on surfaces

– Time-resolved studies of reactions on surfaces and interfaces

– High temperature studies (trace elements in melts)

– Oxidation states of planetary material

– High pressure phases (diamond anvil cell)

• See http://www.bnl.gov/x26a/ for information.


Analysis of Mixtures

• XANES useful technique to quantitatively determinecomposition of a mixture of species.

• Useful for following time evolution of species duringa chemical reaction.

• Two most common methods:

– Least squares linear combination fitting

– Principal component analysis


Least Squares Linear CombinationFitting

• Use a linear combination of spectra of various referencesamples.

• Allows quantification of species in multiple-componentmixture from their fingerprint in the XANES region.

• Use a least-squares algorithm to refine the sum of a givennumber of reference spectra to an experimental spectrum.

• Simple method, easy to implement.

• Must have good quality spectra of the referencecompounds recorded under similar conditions.


Linear Combination Fitting

100

80

60

40

20

0

% o

f Cer

ium

6004002000

Temperature, ºC

Ce(IV) Ce(III)

Nor

mal

ized

Abs

orpt

ion

5.825.805.785.765.745.725.705.68

Photon Energy, keV

Fit experimental data tolinear combination ofknown referencecompounds

LC-XANES fitto determineamount ofCe(III) andCe(IV) presentas function oftemperature

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Nor

mal

ized

Abs

orpt

ion

5.785.765.745.725.70Photon Energy, keV

Ce(IV) oxide Ce(III) sulfate

TPR-XANES of insitu reduction ofCe0.7Zr0.3 oxide

H2


Principal Component Analysis

• Used since 1970’s in other chemical spectroscopy†. Firstpublished reference in XANES 1992*.

• Traditional approach: choose pure model standard, fitedges to these standards, but…

• How many standards are needed?

• How do we know models are reasonable?

• If you have wrong group of standards…there is no wayto get the correct answer...

†Factor Analysis in Chemistry, 2nd Ed. John Wiley & Sons, NY, 1991

*Determination of molybdenum surface environment of molybdenum/titania catalysts by EXAFS,XANES and PCA. Mikrochimica Acta 109 (1992) 281.


Principal Component Analysis• PCA estimates number of distinct species in a series of spectra.

• Used as a first stage of analysis.

• Based on linear algebra - each spectrum represented as a vector.

• Goal is to find number of components that can reproduce the experimentalspectra to within experimental (statistical) error.

• No a priori assumptions on number/type of components.

• Growing popularity in XANES spectroscopy*.*”Principal component analysis approach for modeling sulfur K-XANES spectra in humic acids”, S. Beauchemin etal., Soil. Sci. Soc.Am.J., 66 (2002) 83.

“Quantitative speciation of Mn-bearing particulates emitted from autos burning mcp-Mn gasolines using XANESspectroscopy”, T. Ressler et al., Environ. Sci. Technol., 34 (2000) 950.

“EXAFS and principal component analysis: a new shell game”, S. Wasserman et al., J. Synch. Rad., 6 (1999) 284.

“The kinetic significance of V5+ in n-butane oxidation catalyzed by vanadium phosphates”, G.W. Coulston et al.,Science, 275 (1997) 191

“XANES-TPR study of Cu-Pd bimetallic catalysts: application of factor analysis”, M.Fernandez-Garcia et al., J.Phys. Chem 99 (1995) 12565.


Feff8.1 and XANES

• Dramatic progress in past decade both in theory and abinitio calculations.

• Significant progress has been made in understandingXANES.

• Quantitative theory central to quantitative interpretationof XAFS spectra in terms of local geometric andelectronic structure.

• Will hear more about Feff tomorrow!


Feff8.1 and XANES: Pt L-edgesFeff reproduces differences in

white line at Pt L3,2-edges Pt L3-edge XANES as functionof Pt cluster size


SummaryXANES is a much larger signal than EXAFS

XANES can be done at lower concentrations, and less-than-perfectsample conditions.

XANES is easier to crudely interpret than EXAFS

For many systems, the XANES analysis based on linear combinations ofknown spectra from “model compounds” is sufficient.

More sophisticated linear-algebra techniques, such as principalcomponent analysis can be applied to XANES spectra.

XANES is harder to fully interpret than EXAFS

The exact physical and chemical interpretation of all spectral features isstill difficult to do accurately, precisely, and reliably.

This situation is improving…..

XANES measurements and Interpretation 2003

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