absorption techniques in x-ray spectrometryabsorption techniques in x-ray spectrometry jun kawai in...

Absorption Techniques in X-ray Spectrometry

Jun Kawai

inEncyclopedia of Analytical Chemistry

R.A. Meyers (Ed.)pp. 13288–13315

John Wiley & Sons Ltd, Chichester, 2000

ABSORPTION TECHNIQUES IN X-RAY SPECTROMETRY 1

Absorption Techniques inX-ray Spectrometry

Jun KawaiKyoto University, Kyoto, Japan

1 Introduction 12 Acronyms and a Brief History 33 X-ray Absorption Near-edge Structure 5

3.1 Chemical Shift and Line Shape 53.2 Calculation Method for X-ray

Absorption Near-edge StructureSpectra 8

4 Theory of Extended X-ray AbsorptionFine Structure 84.1 Single Scattering Theory 84.2 Relation to Other Techniques

(X-ray Photoelectron Diffraction,Low-energy Electron Diffraction,X-ray Fluorescence Holography) 10

5 Data Analysis and Software Packagesfor X-ray Absorption Fine Structure 10

6 Instrumentation 136.1 Laboratory Extended X-ray Absorp-

tion Fine Structure 136.2 Synchrotron Radiation Extended

X-ray Absorption Fine Structure 136.3 Secondary Yield Techniques and

Applications 157 Sources and Databases 218 Alternative Methods 21

8.1 Electron Energy Loss Spectroscopy 218.2 Self-absorption 228.3 Extended X-ray Emission Fine

Structure 228.4 X-ray Raman Scattering 228.5 Diffraction Anomalous Fine

Structure 238.6 b-Environment Fine Structure 238.7 Inverse Photoemission Spectroscopy 23

9 Conclusion 23Abbreviations and Acronyms 23Related Articles 24References 24

X-ray absorbance depends on the wavelength of the X-rays,atomic number, chemical environment, and concentration

of analyte. X-ray absorption spectrometry is a techniquefor analyzing the chemical environment of an element inan unknown material. This method is closely related tophotoelectron spectroscopy, Auger electron spectroscopy,and X-ray fluorescence spectroscopy.

Chemical information in the chemical shift and lineshape of XANES (X-ray absorption near-edge structure)spectra is described. The history and theory of EXAFS(extended X-ray absorption fine structure) are discussedin relation to other experimental techniques. Data analysismethods, databases, software packages, instrumentation,and synchrotron radiation facilities for X-ray absorptionanalysis are overviewed. Alternative methods such aselectron energy loss spectroscopy (EELS), self-absorptioneffect, extended X-ray emission fine structure (EXEFS), X-ray Raman scattering, diffraction anomalous fine structure(DAFS),b-environment fine structure (BEFS), and inversephotoemission spectroscopy (IPES) are also described.

1 INTRODUCTION

X-rays are absorbed in matter and the energy ofthe X-rays is converted into the kinetic energy ofphotoelectrons, Auger electrons, secondary electrons,or fluorescent X-rays. The incident X-ray energy finallybecomes the thermal energy of the absorber.

The amount of energy absorbed by a matter is usuallyestimated by a transmission method, but can also beestimated by measuring these secondary phenomena,such as photoelectrons, Auger electrons, secondaryelectrons, fluorescent X-rays, thermal radiation, and drainelectric currents. The X-ray intensity of wavelength lbefore (I0) and after (I) the transmission of a thin film ofthickness d is expressed by I.l/ D I0.l/ exp[�µi.l/rid],where µi.l/ and ri are the mass absorption coefficientand mass density, respectively, of the ith element in thethin film and their dimensions are [cm2 g�1] and [g cm�3],respectively..1 – 7/ The mass absorption coefficient µ ofa specimen which contains n kinds of elements isexpressed by µ D µ1.l/W1 C µ2.l/W2 C Ð Ð Ð C µn.l/Wn,where W1,W2, . . . ,Wn are the weight fractions of element1, 2, . . . ,n in the specimen. The wavelength dependenceof the absorption coefficient µ.l/ is clarified when log µ.l/is plotted against log l as shown in Figure 1; µ.l/ valuesare taken from Henke et al..8/ in this plot. Henke et al..8/

tabulated µ.l/ from Z D 1 to 92 at energy from 50 eV to30 keV.

The plot of the mass absorption coefficients of matteragainst the incident X-ray energy or wavelength is calledan X-ray absorption spectrum (XAS), where we find somejumps at particular X-ray energy, corresponding to K, LI,LII, LIII, . . . electron shell binding energies as shown in

Encyclopedia of Analytical ChemistryR.A. Meyers (Ed.) Copyright John Wiley & Sons Ltd

2 X-RAY SPECTROMETRY

L edge

Zn Fe V

Zn Fe V

K edge

Wavelength (nm)

0.1100

101

102

103

104

105

106

1.0 10.0

µ (c

m2

g–1 )

Figure 1 Mass absorption coefficients of V, Fe, and Zn plottedagainst wavelength. Both axes are on a logarithmic scale.

Wavelength (Å)

L edge

K edge

0

Abs

orba

nce

1.4 1.2 1.0

L3

L2

L1

0.8 0.6 0.4 0.2

Figure 2 Platinum powder XAS. (Reproduced by permissionfrom Udagawa..9/)

Figure 2..9/ K, LI, LII, and LIII denote electron deficiencystates from 1s, 2s, 2p1/2, and 2p3/2 orbitals, respectively.Arabic numerals 1, 2, 3, . . . have more recently been usedin the subscript rather than Roman numerals I, II, III, . . .The electron configuration of one electron deficiencyfrom the 2p orbital is expressed as 1s22s22p53s23p6 for anAr atom. This state has two energy levels corresponding toj D 1

2 and 32 states, where j is the eigenvalue of vector sum

sC l, where s and l are called spin and angular momentumvectors, respectively, and the vector j is called the totalangular momentum. These two states are written as 2p�1

1/2

and 2p�13/2, or [2p1/2] and [2p3/2].

The relation between the electron deficient state andthe electron configuration is listed in Table 1. The s�1 hole

Table 1 Relation between the holestate and the electron configuration

Hole state Electron configuration

K [1s]L1 [2s]L2 [2p1/2]L3 [2p3/2]M1 [3s]M2 [3p1/2]M3 [3p3/2]M4 [3d3/2]M5 [3d5/2]

state has a total angular momentum 12 , and the subscript 1

2is usually omitted. The multiplicity of the state, which iscrudely proportional to the spectral intensity, is 2jC 1.

The jump is called the absorption edge, and thewavelength is highly correlated with the atomic numbersimilarly to Moseley’s law.1/ in X-ray emission spectra.Moseley’s law in emission spectra is expressed asEquation (1):

1plD K.Z� s/ .1/

where l is the X-ray wavelength, Z is the atomic number,and K and s are constants for a spectral series. Theabsorption coefficient is crudely proportional to Z4l3

except for the edge jumps. The energy at which the jump isobserved is called the threshold energy, but the definitionof the threshold is not exact, because it corresponds to thetransition from a core orbital to the lowest unoccupiedorbital. The ionization limit is a few or a few tens ofelectron-volts higher than the edge energy.

The mass per unit area is given by rd, where r isthe mass density. The linear absorption coefficient µl isdefined by µl D µr, and its dimension is [cm�1]. The X-ray attenuation length 1/µl is the length at which theX-ray intensity becomes 1/e after traveling in matter. Theattenuation length of Cu Ka1 X-rays (8047.8 eV) is 79 µmin aluminum, 4.2 µm in iron, 24 µm in copper, and 3.9 µm inlead. The attenuation length of Al Ka X-rays (1486.7 eV)is 9.2 µm in aluminum and 0.4 µm in iron. The intensity ofX-rays emitted by a copper target X-ray tube is, however,attenuated by only half after transmission through 2-mm-thick aluminum, but depends on the applied poweron the X-ray tube, because the X-rays emitted froman X-ray tube are not monochromatic. Thus it shouldbe noted that the X-ray shielding thickness for safetycannot be determined only from the monochromatic X-ray attenuation length.

The linear absorption coefficient can otherwise beexpressed as µ1 D 4pb/l, where l is the X-ray wavelengthand b the imaginary part of the complex refractive index


(n D 1� d� ib)..7/ The atomic form factor, f D f1 C if2,which is used in the analysis of X-ray diffraction, is relatedto the refractive index (Equations 2 and 3):

d D Nr0l2f1

2p.2/

b D Nr0l2f2

2p.3/

where N is the number of atoms in unit volume andr0 D e2/.mc2/ D 2.818ð 10�13 cm is the classical electronradius (e the electron charge, m the mass, and c the speedof light). The real part f1 is the Fourier transform ofthe electron density distribution in an atom. The relationbetween the absorption coefficient and atomic form factoris used in DAFS described below.

The mass absorption coefficient is the sum of twoeffects: photoelectric absorption and scattering of X-rays. The photoelectric absorption is the ionization of aninner-shell electron. Therefore, the absorption coefficientdue to the photoelectric part can be calculated by thephotoionization cross-section..8/ The scattering part isdue to the Rayleigh (coherent) and Compton (inelastic)scattering of X-rays, but X-ray absorption spectra areoften taken as if they represent only the photoelectricabsorption effect, although the experimental spectracontain both effects.

The mass absorption coefficients or physically equiva-lent parameters.8,9 – 13/ and the absorption edge energy orwavelength.14,15/ can be found in the literature. The valueof the absorption edge energy is close to the electron bind-ing energy, which is used in electron spectroscopy, ESCA(electron spectroscopy for chemical analysis) or XPS (X-ray photoelectron (photoemission) spectroscopy)..16,17/

The absorption edge jump is not exactly the same asthe electron binding energy, because the absorption edgeenergy corresponds to the excitation of core electronsinto the lowest unoccupied molecular orbital (LUMO)in the molecular orbital picture, or Rydberg state in theatomic orbital picture. The Rydberg state and contin-uum state threshold are clearly seen in rare gas X-rayabsorption spectra but are not clear for condensed mat-ter. The difference between the vacuum level and Fermienergy, which defines the highest energy of electrons ina conduction band, is called the work function, . Thisis another source of the difference between the elec-tron binding energy observed in XPS and the absorptionedge. The photoionization cross-sections.18,19/ and theelectron binding energies.20,21/ can be found in the liter-ature. The relation between the electron photoionizationcross-section (barns) and mass absorption coefficient issimple when the angular dependence is averaged..11/

Absorption techniques in X-ray spectrometry areused to measure the X-ray absorption spectra using

various methods described below, and to analyze theobtained spectral line shapes to obtain information on theelement, oxidation state, concentration, atomic distance,coordination number, surface geometry, and reaction onsolid surfaces, catalysts, or electrodes.

2 ACRONYMS AND A BRIEF HISTORY

The mass absorption coefficient plotted against the X-ray energy is called the XAS. The X-ray absorptionspectra of condensed matter near the threshold energyhave fine structures as shown in Figure 2. Fine struc-tures are sometimes observable at energies less thanthe threshold energy, and are called the pre-edge struc-ture (Figure 3)..22/ These fine structures are called theXANES, usually pronounced as ‘‘zaenz’’. The absorp-tion fine structure will extend up to 1000 eV above thethreshold energy, and thus it is called the EXAFS,.23 – 26/

pronounced ‘‘eksafs’’. XANES is restricted from thethreshold to ca. 50 eV above (this energy approximatelycorresponds to kR D 2p, where k is the ejected photo-electron momentum and R the nearest-neighbor atomic

0 10 20

KMnO4

MnO

Energy (eV)

X-r

ay a

bsor

ptio

n in

tens

ity

30 40 50

Figure 3 Mn K edge XANES spectra of MnO (octahedral, Oh,symmetry) and KMnO4 (tetrahedral, Td, symmetry). Pre-edgepeak is found in the KMnO4 spectrum. Chemical shift of theedge is found; the edge of KMnO4 is higher than that of MnO.(Reproduced by permission from Pandya et al..22/)


X-rayabsorbing

atom

Electronscattering

atom

R

Shape resonance

XA

NE

S

EX

AF

S

k > kcrt

k < kcrt

kcrt = 2π /R

Figure 4 Illustration of electron standing wave between theX-ray absorption atom and its neighboring atom. (Reproducedby permission from Udagawa.9/ and Ishii..26/)

distance as shown in Figure 4.9,26/). The momentum of thephotoelectron is k D [2m.E� E0/]1/2/h, where E is thephotoelectron kinetic energy, E0 the threshold energy, mthe electron mass, and h Planck’s constant. The photo-electron matter wave in a condensed system propagatesas a spherical wave and forms a standing wave as shownin Figure 5..9/ Recently, XANES has come to be callednear-edge X-ray absorption fine structure (NEXAFS),.27/

pronounced ‘‘neksafs’’. All the fine structures includingNEXAFS and EXAFS are grouped into the term X-rayabsorption fine structure (XAFS), pronounced ‘‘zafs’’.

The history of the development of the understand-ing and application of XAFS has an interesting fea-ture, as stated by Lytle et al.,.28/ Shiraiwa,.29/ Stern,.30/

and Lytle..31/ Barkla (after Stern.30/) or de Broglie(after Lytle.31/) firstly found the X-ray absorption edge.Although XANES was found for both solids and gases,EXAFS was found only for condensed matter such asmolecules, solids and liquids. EXAFS was first reportedby Fricke in 1920.32/ and was theoretically interpretedby Kossel..33/ He explained that the fine structure wasdue to the excitation of inner-shell electrons to anunoccupied level. This theory was valid for XANES,

Photoelectron wave Scattered electron wave

Figure 5 Schematic illustration of electron wave propagatingand scattering in a solid. (Reproduced by permission fromUdagawa..9/)

and thus XANES was called the Kossel structure. TheKossel theory was called short-range order (SRO) the-ory, because the electronic structures of unoccupied levelsare mostly determined by orbital hybridization betweenthe center atom and the nearest-neighbor atoms. Onthe other hand, Kronig.34/ explained that fine structurewas the result of the diffraction of photoelectrons as amatter wave when moving in a conduction band of asolid. The electron matter wave travels in a solid whenthe wavelength of an electron le does not satisfy theBragg condition, 2d sin q D nle. When the Bragg condi-tion is satisfied, then the electrons are scattered and leavethe solid. His theory explained EXAFS and thus EXAFSwas called the Kronig structure. His theory was calledthe long-range order (LRO) theory because the bandstructure is determined by the long-range periodic bound-ary conditions. Hayasi.35/ considered that the electronwaves that satisfied the Bragg condition form a standingwave in a solid, and thus the electron transition from aninner orbital to a standing wave state yields a maximumof X-ray absorption. Shiraiwa et al..36/ and Kozlenkov.37/

improved the SRO theory to explain the EXAFS, buttheir method needed to solve a Schrodinger equation toobtain the EXAFS. Sayers et al..38/ proposed a Fouriertransform method to obtain local structural informationon condensed systems. Owing to their Fourier analysis,we do not need to solve the Schrodinger equation directlyto obtain the local structure of matter. EXAFS had at thattime great potential to be developed as a powerful methodof analyzing the local structure of matter. The inelasticmean free path (IMFP) of a photoelectron is usually2 nm. When the photoelectron is scattered inelastically,the coherence is forgotten. The coherent length, i.e. thelength within which the electron matter waves emittedfrom a single source can interfere with each other, isan important length to apply in the EXAFS method toanalyze a condensed system. When the IMFP is includedin the LRO theory, it is equivalent to the SRO theory.


EXAFS XANES

Valenceband

1sUnoccupied pstate density

0Energy

εp

ε

Figure 6 Schematic illustration of X-ray absorption and pho-toelectron excitation from the 1s to the unoccupied p state.

The XAFS represents the unoccupied electron densityof states for atoms, molecules, solids, or liquids. One ofthe inner shell electrons, say a 1s electron, is excited into adiscrete or continuum unoccupied state by the incident X-ray photon. The transition probability from the 1s to theunoccupied state equals the X-ray absorption intensity(only the photoelectric part is considered here), andthus the plot of the intensity against the incident X-rayenergy is the XAS of a specimen. XANES is chiefly dueto the transition from the inner shell to the unoccupieddiscrete level (Figure 6), and EXAFS is to the unoccupiedcontinuum level.

3 X-RAY ABSORPTION NEAR-EDGESTRUCTURE

3.1 Chemical Shift and Line Shape

The XANES spectra show both the line shape modifi-cation and chemical shift.39/ of the absorption edge orpeak. Figure 7.40/ shows typical examples for the S Kedge for Na2SO4, Na2SO3, and Na2S2O3. The sharp andprominent absorption peak shown in Figure 7.40/ is calledthe ‘‘white line’’. This is because in the early days of X-rayexperiments a white line developed on the X-ray film wasobserved. The white line for insulators is usually sharperthan that for metals, because it corresponds to a 1s! pŁ

electron transition, where the asterisk denotes an unoccu-pied antibonding orbital. The pŁ state is usually a sharplylocalized state. The metal has a broad conduction band,and thus the absorption spectra show an edge jump butnot a white line.

The white line energy plotted against the oxidationnumber of sulfur is shown in Figure 8..41/ The source ofthe chemical shift is both the unoccupied level shift andcore level shift. The range of the unoccupied level shiftranges from the Fermi level (D0 eV) to the band gapenergy (Da few electron-volts). The core level shift is dueto the screening of core electrons by valence electrons;

Tota

l ele

ctro

n yi

eld

(arb

itrar

y un

its)

Energy (eV)2460 2470 2480 2490 2500 2510

S2–

S4+

S6+

S6+

S2O32–

2–

SO32–

2–

SO42–

2–

S S OO

O

S

O OO

O

S

O OO

Figure 7 Sulfur K edge absorption spectra of Na2SO4, Na2SO3,and Na2S2O3. (Reproduced by permission from Sekiyamaet al..40/)

if the atom is negatively charged then the core level isshifted to a shallower binding energy, and if an atom ispositively charged then it is shifted to a deeper energy.The source of the core level shift is the same as that of anESCA chemical shift.

In Figure 9.42/ is shown another example of a chemicalshift of the absorption edge for Al compounds: Al metal,AlN, and four- and six-fold coordinated oxides. TheAl�O distance of four-fold coordinated aluminum oxide(0.17 nm) is shorter than that of six-fold coordinatedoxide (0.19 nm), because the oxygen ions interfere witheach other and cannot be close to the Al atom for six-foldcoordinated oxide. Thus the orbital hybridization of four-fold coordinated oxide is stronger than that of six-foldcoordinated oxide, and consequently the six-fold form is


–22465

2470

2475

2480

2485

0 2

2–

Nominal oxidation number

X-r

ay a

bsor

ptio

npe

ak e

nerg

y (e

V)

4 6

2–

S S OO

O

S

O OO

2–S

O OO

Figure 8 Relation between X-ray absorption peak and nominalsulfur oxidation number. (Reproduced by permission fromKawai et al..41/)

0–10

Nor

mal

ized

abs

orba

nce

Energy – E0 (eV)

0

1

2

3

10

AI metal

20 30 40

AIN4 in AIN

AIO4 insodalite

AIO6 inkyanite

Figure 9 Al K edge XANES spectra of Al metal, AlN, sodalite,and kyanite. AlXn denotes the first shell coordination ofAl in each material. (Reproduced from J. Wong et al., ‘NewOpportunities in XAFS Investigation in the 1–2 keV Region’,Solid State Commun., 92, 559–562, 1994, with permissionfrom Elsevier Science.)

ionic and the four-fold form is covalent. The effectivepositive charge of six-fold coordinated Al3C is larger thanthat of four-fold coordinated oxide. The chemical shiftof six-fold coordinated oxide is larger than that of four-fold coordinated oxide. The shift is strongly correlatedto Pauling’s electronegativity.43/ of the neighboring atom,because the electronegativity determines the effectivecharge of the ion.

The unoccupied discrete level is composed of Rydbergstates in the atomic picture, pŁ and sŁ orbitals inthe molecular-orbital picture (the asterisk denotes anantibonding molecular orbital), or conduction bands incrystals. The sŁ transition, which is formed in a potential

well of neighboring atomic potentials, is called the shaperesonance (Figure 4).

While the 1s! pŁ transition is a sharp white line, the1s! sŁ transition usually results in a broad and weakhump at higher energy,.44/ which is called the shaperesonance. The term shape resonance is used in the fieldof atomic spectra. The excited state or ionized state isbound in a potential wall, because of the centrifugalforce potential of a high angular momentum orbital suchas an f orbital, or surrounding potential such as F inSF6. However, as shown schematically in Figure 4, such asurrounding potential does not have sufficient height toenclose the electron, but a weak resonance is observable.This is the origin of the term shape resonance.

The pre-edge structure shown in Figure 3 above isobserved for the K edge of transition metal compoundswhose local symmetry around the X-ray absorbing atomis Td (tetrahedral). On the other hand, it is not observablefor locally Oh (octahedral) symmetry solids. This pre-edgeis sometimes said to be an electric quadrupole transitionfrom 1s to 3d, whereas ordinary optical absorption is theelectric dipole transition (1s! 2p or 2p! 3s, 3d). Thequadrupole transition probability is, however, very weak,as shown in Table 2, where the probability is calculated bythe Dirac–Fock method..45/ The origin of such a strongabsorption as shown in Figure 3 is due to the electricdipole transition. The unoccupied p orbitals stronglyhybridize with the d band for tetrahedral symmetrycompounds based on the group theory as shown inTable 3,.46/ where both p and d orbitals belong to thet2 orbital. Thus the electric dipole transition is stronglyobserved at the energy of an empty d band. On the other

Table 2 Calculated transitionprobability for Cu

K–L1 0.00000038K–L2 0.19K–L3 0.37K–M1 0.000000072K–M2 0.022K–M3 0.043K–M4 0.000025K–M5 0.000036

Table 3 Part of the character table of Td

Td p d

a1 x2 C y2 C z2

a2

e (2z2 � x2 � y2, x2 � y2)t1t2 (x, y, z) (xy, xz, yz)


Table 4 Part of the character table of Oh

Oh p d

a1g x2 C y2 C z2

a2g

eg (2z2 � x2 � y2, x2 � y2)t1gt2g (xy, xz, yz)a1ua2ueut1ut2u (x, y, z)

Energy (eV)7700 7750

DC

E

BA

Abs

orpt

ion

(arb

itrar

y un

its)

[Co(NH3)6]l3

[Co(NH3)6]Br3

[Co(NH3)6]Cl3

Figure 10 Co K edge spectra of [Co(NH3)6]X3 (X D Cl,Br, and I). The vertical bars show calculated spectra for[Co(NH3)6]3C. Peak A is due to the electric quadrupoletransition but still mixed with the p orbital owing to theskewing of the molecular structure from exact octahedralsymmetry. (Reproduced by permission from Sano..47/ 1988The American Chemical Society.)

hand, p and d orbitals never mix with each other for Oh

symmetry, as shown in Table 4,.46/ where the p orbitalbelongs to t2u and d belongs to eg and t2g. Therefore, thep transition emerges at a different energy from the emptyd band for octahedral symmetry compounds. The trueelectric quadrupole transition is very weak, as shown inFigure 10..47/ However, even in this case, the observablestrength is due to the hybridization of the p characterinto the empty d band. The intensity and energy shift ofthe pre-edge peak are good indices for fingerprinting thechemical environment in the compounds, especially forbiological samples..48/

The chemical shifts of reference samples are measuredand plotted against the electronegativity, and then theneighboring atom type is estimated for an unknownmaterial from the chemical shift of the absorption edge.

After the discovery of high-temperature superconduc-tors,.49/ the understanding of the electron correlationeffect of transition metal compounds.50/ and rare earthcompounds has been greatly improved by the study ofXPS. Consequently, the understanding of the correla-tion effect, i.e. how the hole left in the final state ofphotoionization interacts with d holes in transition metalcompounds, has developed substantially. Many reportshave been published concerning the electron correla-tion effect on the XANES line shape of complicatedmaterials..51/

Mixed-valence rare earth compounds are clearlyobserved by the measurement of XANES, as shown inFigure 11,.52/ but the intensity ratio sometimes does notdirectly represent the mixed-valence components becauseof a dynamic electron transfer, i.e. correlation effect, dueto the core hole screening..53/ The peak decompositionof XANES spectra into Eu2C and Eu3C, as shown inFigure 11, yields a rough estimate of the mixed-valencestate. However, the core hole created by the X-rayabsorption rearranges the valence electrons and thusthe peak intensity does not always represent the exact

273 K

19 K

7.006.986.966.94

0.0

0.5

1.0

1.5

0.0

0.5

1.0

1.5

Energy (keV)

Abs

orpt

ion

(arb

itrar

y un

its)

Figure 11 XANES spectra of EuNi2Si0.5Ge1.5 at 19 and 273 K.The dashed-dotted and the dashed spectra indicate the Eu2C

and Eu3C final state components, respectively. (Reproducedby permission from Wortmann et al..52/ 1991 The AmericanPhysical Society.)


portion of the Eu2C and Eu3C states before the X-ray isabsorbed.

3.2 Calculation Method for X-ray AbsorptionNear-edge Structure Spectra

The electronic states of photoelectrons whose kineticenergy is from a few electron-volts to a few tens ofelectron-volts are treated as conduction electrons in aconduction band. Thus a multiple scattering (MS) methodor Green’s function method, which has been used tocalculate the electronic structure of conduction electronsnear the Fermi energy in metals, is applicable to calculatethe XANES of materials. The line shape of a XANESspectrum represents the partial and local electron densityof states of the X-ray absorbing atom..54/ Hence anykinds of electronic structure calculations other than theMS theory, such as the LCAO-MO (molecular orbitalderived from a linear combination of atomic orbitals)method or the APW (augmented plane wave) method, arealso applicable to interpret the near-edge fine structure.One of the most popular methods for calculating XANESspectra is the MS theory.

In the MS theory, a sphere of radius ri centered at the ithatom is considered, and the solid is divided by spheres. Aspherically symmetric atomic potential V.r/ is put insideeach sphere and the potential equals zero or constantoutside the spheres. This is called the muffin-tin (MT)potential. The wave function in the solid is expressed asthe overlap of spherical Bessel functions (radial part ofthe wave function) multiplied by the spherical harmonicfunctions (angular part of the wave function).

The wave function y.r/ of a photoionized electron isscattered by an atomic potential V.r/ near the ionizedatom, and finally it becomes itself after being scatteredmany times (Equation 4)

y.r/ D � 14p

∫exp.ikjr � r0j/jr� r0j V.r0/y.r0/ dr0 .4/

where k2 D e is the kinetic energy of a photoelec-tron and k is real for e > 0 (photoionized electron),exp.ikjr � r0j//.jr � r0j/ represents a spherically expand-ing wave, and V.r/ is the MT potential. This methodis called the MS method, Green’s function method, orKorringa–Kohn–Rostker (KKR) method..55/ The KKRmethod is only exact for solids that have translationalsymmetry, or periodic boundary conditions. Small clus-ters, molecules, amorphous or surface adsorbates have alower symmetry, and it is difficult to apply directly theKKR method. Thus the cluster calculation method wasproposed by Johnson.56/ and was called the multiple scat-tering Xa (MS-Xa) method, because Slater’s Xa exchangepotential.57/ is used in place of the Hartree–Fock (HF)exchange integral. The Xa method is also called the

Energy (eV)650

E

T

640

Inte

nsity

Figure 12 Mn L2,3 XAS (E) of MnF2 compared with atomic3d5 multiplet calculation including the crystal field splitting (T).(Reproduced by permission from de Groot et al..60/ 1990 TheAmerican Physical Society.)

Hartree–Fock–Slater (HFS) method, and recently it hasbeen developed as a local density approximation (LDA)theory. The calculation method for XANES spectra isa modified MS-Xa method..58/ In another way, the MSmethod is the expansion of the wave function of positiveenergy by an infinite sum of the spherically outgoing andincoming scattering waves. The electrons excited into thecontinuum level have a wave function of a standing waveformed by the infinite number of incoming and outgo-ing spherically traveling waves. That is to say, a wavewhose intensity is V.r0/y.r0/ coming from every point r0

in space is synthesized and forms a wave y.r/ at pointr. This method produces a wave function similar to theAPW method, which is an appropriate method to cal-culate a metallic band structure, but APW requires agreater number of basis functions than the MS method.The wave function at point r is the sum of all the scatteredwaves multiplied by the phase factor. The LCAO-MOmethod is another choice for calculating the electronicstructures of solid or molecules, and is thus applicable tothe calculation of XANES spectra..59/

An atomic calculation yields a satisfactory agreementbetween experiment and theory, as shown in Figure 12,.60/

after the inclusion of the perturbation of crystal fieldsplittings. Bragg reflection of electrons in a crystalreproduces a rough XANES spectrum..61/

4 THEORY OF EXTENDED X-RAYABSORPTION FINE STRUCTURE

4.1 Single Scattering Theory

Whereas MS of photoelectrons in a solid is a goodapproximation to treat XANES, because the electronkinetic energy of the EXAFS region is very high, singlescattering is a good approximation to EXAFS except for


special cases. The wave function fk.r/ of a photoelectronscattered by a single atom is asymptotically expressed byEquation (5):

fk.r/ ��! exp.ikz/C f .#/r

exp.ikr/ .5/

where f .#/ is the scattering amplitude and # is thescattering angle (# D 0° for forward scattering and# D 180° for backscattering). The scattering amplitudeof an electron of velocity v scattered by an atom of atomicnumber Z is expressed by the first Born approximation(Equation 6):

f .#/ D e2

2mv2[Z �A.#/]

1

sin2.#/2/.6/

where e and m are the charge and the mass of an electron,respectively, and A.#/ is the atomic structure factor forX-rays, given by Equation (7):

A.#/ D 4p∫ 1

0

sin krkr

r.r/r2 dr .7/

where k D .4pmv/h/ sin.#/2/ is the change in electronmomentum before and after the scattering and r.r/ is thecharge distribution in an atom. The forward scatteringamplitude crudely depends on the atomic number in away such that (Equation 8)

f .0°/ D 13

∫ 10

4pr.r/r4 dr D 13

Zhr2i .8/

in atomic units, because (Equation 9).62/

Z D∫ 1

04pr2r.r/ dr .9/

where h i denotes an average. The calculated scatteringamplitude is shown in Figure 13..63/

The EXAFS is expressed by Equation (10):.64/

c.k/ D �∑

j

Nj

kR2j

jfj.k,p/j exp.�2s2j k2/ sin[2kRj C fj.k/]

.10/where k D p2m.hn� E0//h is the photoelectron wavevector, Nj is the number of nearest neighbors, jf .k,p/jis the backscattering amplitude, and Rj is the distancefrom the center atom. The exponential term containsthe Debye–Waller-like vibrational effect and dumping.The dumping due to the finite coherent length ofthe photoelectron, exp[�2Rj/l.k/], is multiplied fora more exact expression. The Debye–Waller factorcontains both effects of thermal vibration and geometricrandomness. The oscillating part of the EXAFS equation,� sin 2kR/.kR/2, if plotted as a function of kR, is theEXAFS oscillation.

0°0

1.0

Forward Back45° 90° 135° 180°

50 eV50 eV

100 100

505505

13201320

140

140 285

285

200

200

Scattering angle (θNi)

f(θ

Ni)

(arb

itrar

y un

its)

Figure 13 Calculated plane-wave scattering factor amplitudejf j of nickel as a function of both the scattering angle qNiand the photoelectron kinetic energy. (Reproduced fromM. Sagurton et al., ‘Derivation of Surface Structures fromFourier Transforms of Photoelectron Diffraction Data’, Phys.Rev. B, 30, 7332–7335, 1984, with permission from ElsevierScience.)

The EXAFS oscillation amplitude is larger when theatomic number of neighboring elements is higher. Forexample, the Si K edge EXAFS oscillation amplitudeof Si is stronger than that of SiO2, because the atomicnumber of Si is higher than that of O. The white lineof SiO2 is sharper and stronger than that of Si. Hencethe EXAFS oscillation and the white line intensity donot directly indicate the concentration of the atom in theanalyte. However, the edge jump is a good measure ofconcentration, and the measurement of edge jump coulddetermine the concentration without a working curve, asshown in Table 5..65/

The effect of thermal vibration on the line shapeof X-ray absorption spectra is shown schematically inFigure 14..44/ This is the line in the XANES region.Similarily, the EXAFS oscillation becomes unclear owingto thermal vibration. As the atomic number becomes

Table 5 Results of copper–zincsolution to test the trace elementanalysis

Zn (µg mg�1 Edge jumpCu solution)

0.092 0.00590.049 0.00350.020 0.00210.000 0.0010

Reproduced by permission fromNomura..65/ 1992 The AmericanChemical Society.


Inte

nsity

(ar

bitr

ary

units

)

Asymmetric lineshape

Photon energy (arbitrary units)

Figure 14 Asymmetric line shape of X-ray absorption spectracaused by the vibration of a diatomic molecule. (Reproduced bypermission from D.A. Outka, J. Stohr, ‘Curve Fitting Analysisof Near-edge Core Excitation Spectra of Free, Adsorbed andPolymeric Molecules’, J. Chem. Phys., 88, 3539–3554 (1988). 1998 American Institute of Physics.)

higher, the core hole lifetime becomes shorter. Ashorter lifetime of the inner shell level indicates thatthe energy of the inner shell becomes vague becauseof Heisenberg’s uncertainty principle. Consequently, theEXAFS oscillation of the K spectrum for higher atomicnumber elements is not clear compared with that of theL edge spectrum of the same element.

The IMFP of photoelectrons is a function of kineticenergy for a particular material, as shown in Figure 15..66/

It is 1–2 nm for the usual EXAFS experiments. Hence thephotoelectron is only coherent within a few nanometers,

1

0

2

3

4

5

10 100

Electron energy (eV)

Mea

n fr

ee p

ath

(nm

)

1000 10 0001

Figure 15 Calculated IMFP of photoelectrons in gold. (Repro-duced by permission from Kuzmin..66/)

and it probes the local structure within the IMFP.Thus 1–2 nm regularity, usually up to the next-nearestneighbors, in the structure is sufficient for the EXAFSoscillation to emerge.

4.2 Relation to Other Techniques (X-ray PhotoelectronDiffraction, Low-energy Electron Diffraction, X-rayFluorescence Holography)

X-ray photoelectron diffraction (XPD).62/ is used to studythe local structure of surfaces. The photoelectron intensityas a function of detected polar and azimuthal angles ismeasured in this technique. The photoelectron intensityis anisotropic in its detection angle. This effect is due tophotoelectron diffraction, but roughly speaking it is dueto the photoelectron’s forward scattering by the nearest-neighbor atoms around the photoelectron-emitting atom.XPD uses forward scattering of photoelectrons; EXAFSuses backscattering of photoelectrons.

Recently XPD has been treated as photoelectronholography (PEH)..67/ The intensity distribution of thephotoelectrons of a single crystal is measured and Fouriertransformed, and then a local atomic structure of thesingle crystal near the photoelectron-emitting atom can beconstructed. The phase shift of electron waves scatteredby a neighboring atom makes the analysis of the Fouriertransform for PEH complicated. The angular distributionof the X-ray fluorescence intensity is measured and theFourier transform of the angular distribution reconstructsthe crystal image. This is called X-ray fluorescenceholography (XFH)..67/ XFH is free from the phase shiftproblem, but this method is bulk sensitive, whereas XPDand PEH are surface sensitive.

Low-energy electron diffraction (LEED).68/ is a surfacecrystallography experimental method where electronsof a few hundred electron-volts impinge on a singlecrystal and a diffracted electron pattern is observed.The penetration depth of these energy electrons is afew nanometers, hence this method is surface sensitive.Electron diffraction requires a periodic structure of atleast 10 nm on the surface, hence the LEED methodcannot probe the structure clusters of a few nanometerson a surface.

5 DATA ANALYSIS AND SOFTWAREPACKAGES FOR X-RAY ABSORPTIONFINE STRUCTURE

The measured (Figure 16a) X-ray absorption spectralenergy is converted into photoelectron momentum. Thesmooth background µ0.E/, which means an isolated atomabsorption coefficient, is subtracted from the observed


FYTEY

Abs

orpt

ion

(arb

itrar

y un

its)

Photon energy (eV)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1800

(a)

1900 2000 2100 2200 2300 2400 2500

FYTEY

(b) Wavenumber, k (Å–1)

–2.54 6 8 10 12

0.0

2.5

k2χ(

k)

FYTEY

(c)

0 1 2 3

Distance, R (Å)

4 5 60.0

0.5

1.0

1.5

2.0

F(R

)

2.5

3.0

3.5

Figure 16 (a) Si K edge EXAFS spectra of an Si(001) wafermeasured using the X-ray fluorescence yield (XFY) methodand total electron yield (TEY) method. (b) Oscillation functionof (a). (c) Fourier transforms of (b). (Reproduced by permissionfrom Y. Kitajima, ‘Fluorescence Yield X-ray Absorption FineStructure Measurements in the Soft X-ray Region’, Rev. Sci.Instrum., 66, 1413–1415 (1995). 1995 American Institute ofPhysics.)

value µ.E/ and normalized according to Equation (11):

c.k/ D µ.E/� µ0.E/µ0.E/

.11/

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(a)

600 700 800

Photon energy (eV)

900 1000

0.0

1.0

2.0

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c. E

XA

FS

inte

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3.0

4.0 E C

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FS

inte

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2.0

E

R2

CuO2.0 A

Figure 17 Schematic illustration of diatomic molecule (fulland open circles) adsorbed on a four-fold coordinated siteof metal substrate (hatched circles). When the incident X-raybeam electric vector is parallel to the diatomic molecular axis,the absorption spectrum of the molecule is obtained (a strongwhite line due to an insulating compound is observed). Whenthe incident X-ray beam electric vector is perpendicular tothe molecular axis, information on the metallic bond betweenthe adsorbed atom and the substrate metal is obtained.(Reproduced by permission from J. Stohr, ‘Geometry and BondLengths of Chemisorbed Atoms and Molecules: NEXAFS andSEXAFS’, Z. Phys. B: Condens. Matter, 61, 439–445 (1985). 1985 Springer-Verlag.)

This is the EXAFS oscillation. If we want to enlarge theoscillation for a larger k region, we sometimes plot k3c.k/or k2c.k/ in place of c.k/ as shown in Figure 16(b)..69/


Then this is Fourier transformed as shown in Figure 16(c),and a radial distribution function is obtained. Toobtain the coordination geometry, measurement of thepolarization-dependent EXAFS oscillation is important,as shown in Figure 17(a) and (b)..70/ An example of S Kedge spectra of CS2 on a Cu(111) surface is shown inFigure 18..71/

A spline function or higher order polynomial deter-mines the smooth background. Theoretically, the smoothbackground has a shape of tan�1 q at the threshold,because the discrete absorption line shape is a Lorentzianfunction and its sum in a Rydberg series becomes tan�1 qas shown in Figure 18.

An incident X-ray beam forms a standing wave ina large-sized single crystal. In this case, additional finestructure, the 1985-eV structure in Figure 19,.72/ forexample, depending on the incident angle of X-rays,is observable. The standing wave profile is sensitive tothe location of impurity atoms in a crystal, i.e. which sitein the lattice. The use of standing waves is one of the

92 K 30 L

92 K 0.02 L

CS2/Cu(111)π∗

σ∗Exp.Calc.

2460 2470 2480 2490 2500 2510

Photon energy (eV)

Flu

ores

cenc

e yi

eld

Tota

l ele

ctro

n yi

eld θ = 55°

θ = 15°

θ = 55°

θ = 90°

Figure 18 Experimentally obtained S K-edge XANES spectra(dots) of a CS2 multilayer (30 L) at an X-ray incident angleof 55°, and submonolayer (0.02 L) at 15°, 55°, and 90°.(Reproduced from S. Yagi et al., ‘Structural and ElectronicProperties of Molecularly Adsorbed CS2 on Cu(III) Studied byX-ray Absorption and Photoelectron Spectroscopies’, Surf. Sci.,311, 172–180, 1994, with permission from Elsevier Science.)

Tota

l ele

ctro

n yi

eld

(arb

itrar

y un

its)

1840 1860 1880 1900 1920 1940 1960 1980 2000

Photon energy (eV)

Figure 19 TEY spectrum of partially oxidized Si(111) wafer.Additional structure at 1985 eV is due to the incident X-rayBragg diffraction (standing wave). (Reproduced from T. Ohtaet al., ‘A Possible Use of the Soft X-ray Standing Wave Methodfor Surface and Interface Structure Analysis’, Nucl. Instrum.Methods Phys. Res. A, 246, 760–762, 1986, with permissionfrom Elsevier Science.)

surface analysis methods, but sometimes interferes withobtaining c.k/, as shown in Figure 19.

Another effect interfering with the observation of c.k/is the multiple ionization effect. The effect of an additionalone or two electrons ionized from outer shell(s) is notnegligibly small..73/ The double ionization probability issometimes more than 30% of single K shell ionization.This is a source of error in EXAFS analysis.

EXAFS Fourier analysis is sometimes not easy whenadditional peaks such as multiple ionization, standingwave structure, and impurity peaks originating from theanalyte, X-ray source, or X-ray optics emerge.

Data analysis methods have been developed andseveral standard computer programs are now available.

Table 6 XAFS analysis computer programs

ATOMS FUSEAUTOBK G4XANESAUTOFIT GNXASBAN LASECDXAS MacXAFSCERIUS2 MURATAEDA REDUCEEX.TR.As REXEXAFIT REX2EXAFS and FITEX SEDEMEXAFS (for Mac) TT-MULTIPLETSEXAFSPAK UWXAFSEXBACK WinXASEXBROOK XAFSEXCALIB XAIDEXCURVE98 XANADUFEFF XDAPFEFFIT XFIT


FEFF is the most popular program that is used by EXAFSusers. The EXAFS analysis program is not difficult tocode, and a laboratory that studies EXAFS may haveits own program, but not always published. Some of theprograms listed in Table 6 can be down-loaded from Websites.

6 INSTRUMENTATION

6.1 Laboratory Extended X-ray Absorption FineStructure

To measure the X-ray absorption spectra, a strong X-raysource of continuous energy is required, such as whiteradiation from an X-ray tube or synchrotron radiation(SR). A metallic wheel is rotated in vacuum and ahigh electric potential is applied between the wheeland a filament. Thermal electrons are emitted from thefilament and bombard the wheel target. Water flowsinside the wheel to cool it against heating by the electronbombardment. To eliminate the heating, the wheel isrotated. Therefore, this type of rotating anode X-ray tubeproduces one order of magnitude stronger X-rays than theordinary sealed X-ray tubes. The electron deceleration atthe metal target converts the electron kinetic energy intoX-ray energy. The X-rays thus produced are continuumX-rays in addition to characteristic X-rays and themaximum energy is the acceleration electric potentialapplied. The X-rays from the X-ray tube are thenmonochromated by a crystal monochromator using theBragg diffraction condition. Then the monochromaticX-rays are incident on the specimen as shown inFigure 20..74/ Sometimes, to compensate for the weaksource intensity, a position-sensitive proportional counteris used as shown in Figure 21..75/ However, the very simpleexperimental set-up shown in Figure 22.76/ is sometimesused.

X-ray sourceBent crystal

FluorescentX-ray detector

Rowland circle

Sample

I0

I

Figure 20 Example of EXAFS spectrometer using a rotatinganode X-ray tube. (Reproduced by permission from Rigaku..74/)

Sample

Monochromator

X-ray source

Pos

ition

-sen

sitiv

e de

tect

or

Figure 21 Example of EXAFS spectrometer using a posi-tion-sensitive proportional counter. (Reproduced by permissionfrom Maeda et al..75/)

Sample

Ionization chamberMono-chromator

X-ray tube

I

I0

Figure 22 Laboratory EXAFS using a q–2q goniometer.(Reproduced by permission from K. Sakurai, ‘High-intensityX-ray Line Focal Spot for Laboratory Extended X-ray Absorp-tion Fine-structure Experiments’, Rev. Sci. Instrum., 64, 267–268(1993). 1993 American Institute of Physics.)

6.2 Synchrotron Radiation Extended X-ray AbsorptionFine Structure

Recently, SR has frequently been used as an X-ray source.Synchrotron is the name of an electron (or positron)accelerator made of an ultrahigh vacuum (UHV) ringand electrons rotating inside the ring. High voltage isapplied to the rotating electrons using variable-frequencyradio waves and the frequency is synchronized with theelectron rotation during the acceleration of the electronsin the ring. When the electron speed reaches close to thespeed of light, a very sharp X-ray beam is emitted in atangential direction because of the relativistic effect. ThisX-ray beam is called the SR. Usually SR from a storagering is used. A ring in which electrons (or positrons) arerotated at a certain constant speed is called the storagering. Usually electrons accelerated to a sufficient speed bya synchrotron or a linear accelerator are injected into thestorage ring, and the tangential radiation when a magnetbends the electron beam is used as an X-ray source..77/ TheSR thus produced from a bending magnet is continuousover a wide range (more than 1000 eV) of X-ray energyand a few orders of magnitude stronger than the rotating


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7-element Si(Li)detector

Sagittally bentsecond crystal

Verticalfocusing mirror

Directlywater-cooledfirst crystal

2.5-GeV storage ring

27-pole wiggler

e+

Sample

Slits

i0monitor

αω

φ

Figure 23 Schematic view of X-ray wiggler beam line. (Reproduced by permission from Oyanagi et al..78/)

anode X-ray tube. The smaller emittance indicates theelectron orbit stored in a storage ring being sharpened,but practically it indicates a smaller X-ray beam sizeand higher photon density. The emittance is measured inmeter Ð radians (m Ð rad). The emittance is a measure ofbeam quality. The smaller the emittance, the more thebeam becomes parallel. The SR is monochromated by asingle crystal.

Continuous X-rays from a bending magnet are mostconvenient for X-ray absorption spectroscopic experi-ments. The rotating axis of monochromator crystals isparallel to the electric vector of the SR X-rays to makegood use of the X-rays. To obtain stronger X-rays, SRfrom an undulator or wiggler beam line is used. Theundulator and wiggler are insertion devices in the storagering, and made of many strong permanent magnets. Theelectron beam in the storage ring is undulating whenit goes through an undulator, and a coherent quasi-monochromatic X-ray beam is produced. It consists ofmany harmonics and each harmonic has a narrow (say100 eV) bandwidth. By changing the magnet gap width,the peak energy of the X-rays from the undulator iscontrolled; the wider the gap, the lower is the energy.Thus, to scan the energy over 1000 eV continuously usingan undulator, both the undulator gap and monochroma-tor crystal rotation should be controlled simultaneously.This is a difficult task but now routinely done in someSR undulator beam lines. An undulator can produce afew orders of magnitude stronger X-rays than a bendingmagnet beam line. If the X-rays produced by undulat-ing by magnets are not coherently interfered, such aninsertion device is called a wiggler. X-rays from a wigglerare not strong compared with those from an undulatorbut are continuous in energy, and thus much easier to

use in X-ray absorption experiments than those from anundulator. An example of the experimental set-up of awiggler is shown in Figure 23..78/

SR facilities are classified into first-, second-, and third-generation sources. The first-generation synchrotronswere particle accelerators, and spectroscopists parasiti-cally used the SR. Such SR was unstable. The second-generation synchrotrons use a storage ring to obtain SRbut the emittance is still not small enough, i.e. 10�7 mrad.The emittance of the third-generation synchrotrons haveemittance as small as 10�9 m Ð rad. Such a small-emittancestorage ring requires a large-radius electron orbit becausea large number of magnets are required to keep the elec-tron beam cross-section small. The fourth generation ofsynchrotron has not yet been constructed and not defined,but may be a free electron laser facility using an electronaccelerator. The third-generation SR facilities are tabu-lated in Table 7, and the second-generation SR facilitiescan be reached via the links of the Web pages listed.

The first crystal in the monochromator used in an SRbeam line experiences a high heat load, and the latticeconstant is slightly different from the second crystal.The crystal optics should be cooled by flowing water.To adjust the difference in the lattice constant betweenthe first and the second crystals, usually one of the twocrystals is independently finely moved by a piezoelectric

Table 7 Third-generation SR facilities and Websites

ALS http://www-als.lbl.gov/als/APS http://epics.aps.anl.gov/welcome.htmlESRF http://www.esrf.fr/SPring-8 http://www.spring8.or.jp/


mechanism. The adjustment of the two crystals shouldalways be monitored for the measurement of EXAFS forthe 1000-eV range. The monitoring of the adjustment isdone by measuring the incident X-ray intensity, which ismaximized at every energy point scanned.

6.3 Secondary Yield Techniques and Applications

The intensity of the monochromated X-rays is moni-tored by an ionization chamber. X-rays pass throughthe ionization chamber (I0) and are then incident on aspecimen, and the transmitted X-ray intensity is mea-sured by another ionization chamber. The XAS isthe plot of � log.I/I0/ against the X-ray energy. Theabsorption spectra are usually measured by this trans-mission method as shown in Figure 24(a)..79/ When theX-rays are absorbed strongly by the sample, the sec-ondary responses such as photoelectron intensity, sampleelectric current intensity, secondary electron intensity(electrons whose kinetic energy < 50 eV is mostly thesecondary electrons), Auger electron intensity, and X-rayfluorescence intensity become strong. Therefore, equiv-alent spectra to the absorption spectra are measurableby these secondary phenomena such as photoelectronintensity, secondary electron intensity, Auger electronintensity (Figure 24b), sample drain current (Figure 24cand d), X-ray fluorescence intensity (Figure 24e), ionintensity due to photostimulated desorption, and othersecondary techniques.

The photoelectron intensity for a single crystal hasan anisotropy with respect to the observed directionbecause of the photoelectron diffraction. The angularaverage of the photoelectron intensity measured asthe change in incident X-ray energy is the XAS. Thismethod is called the photoelectron yield method. IfAuger electrons are detected, this is the Auger electronyield method. The intensity of Auger electrons froma single crystal also has an angular anisotropy. Thedetection angle is therefore important for interpretingthe observed data for these electron yield methods.Many kinds of electrons are detected, such as secondaryelectrons, core photoelectrons, valence photoelectrons,and Auger electrons, as shown in Figure 25(a–c)..80/

The different kinetic energy electrons correspond to thedifferent probing depths. Thus the electron yield spectraare a mixture of various depth spectra as well as a mixtureof various processes of electron production. The relationbetween the X-ray absorption process and the electronemission process is neither direct nor clear.

The electric drain current is of the order of 10�9 Awhen using a bending magnet SR beam line. The electricdrain current is a measure of X-ray absorption, and thismethod is called the TEY method. This is because thedrain current represents the sum of all the electrons

Sample

X-ray(a)

I0 I

X-ray(b)

e

Electrondetector

Grid

X-ray(c)

e

A

X-ray

(d)

A

X-ray

X-raydetector

FluorescentX-ray

(e)

Figure 24 Various methods of measuring X-ray absorptionspectra. (a) Transmission method; (b) partial electron yieldmethod; (c, d) TEY method; and (e) XFY method. (Repro-duced from J. Kawai et al., ‘Depth Selective X-ray AbsorptionFine Structure Spectroscopy’, Spectrochim. Acta Part B, 49,739–743, 1994, with permission from Elsevier Science.)

emitted from the sample. The electrons produced in asolid are scattered in the solid as shown in Figure 26,.81/

and only electrons produced near the surface contribute tothe electric drain current. When we detect ions desorbedfrom the surface due to X-ray absorption, the ion intensityrepresents the surface top layer.

These methods of detecting electrons (or electriccurrent) or ions are surface sensitive and therefore arecalled surface extended X-ray absorption fine structure(SEXAFS). If the sample current is measured in anair or helium atmosphere, then the ejected electrons areconverted into O2� or other ions and a positive or negativecurrent is observable depending on the sample electricpotential with respect to the ground. This method is calledthe conversion electron yield (CEY) method.

The XFY method is not surface sensitive, becausethe fluorescent X-rays originate from as deep a locationas the X-ray attenuation length. However, if XFY iscombined with the grazing-incidence X-ray technique(Figure 27),.82/ where the total reflection X-ray techniqueis combined with the TEY and CEY methods, it becomesmore surface sensitive than normal-incidence TEY or


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(b)

(a)

Yield window settings

SecondaryElastic AugerPartial Auger

Total

Binding energy (eV) Kinetic energy (eV)

Photoemission

Auger

AugerA

B

VB

A B VB

EF EV

φ

O

EPES EA

h 1ν

h 2ν

h 3ν

Figure 25 Schematic photoelectron spectra for (a) the excitation energy below the excitation threshold of core level A, (b) justabove the absorption threshold but below the photoionization threshold, and (c) far above the threshold. The contributions to thephotoelectron intensity from different level photoelectrons and Auger electrons are indicated. At the bottom the window settingsfor various electron yield X-ray absorption techniques are shown. (Reproduced by permission from Stohr et al..80/ 1984 TheAmerican Physical Society.)

CEY. The X-rays are totally reflected on the surfaceand the evanescent X-ray wave penetrates only a fewnanometers from the surface. X-ray reflectivity is also ameasure of XAFS.

The grazing exit angle method is also possible, as shownin Figure 28,.83/ because of the reciprocal theorem ofoptical beams. The XFY method has atomic selectivitybecause an X-ray detector usually has energy resolution,hence the signal-to-noise ratio is better than in theTEY method, and minor elements adsorbed on thesurface can be detected by the XFY method. X-ray absorption spectra of very dilute analytes can be

detected using the XFY method; the detection limit is<1012 atoms cm�2.

The X-rays emitted from a deep location in a specimensuffer from the self-absorption effect and the spectralshape is different from that of an absorption-freespectrum..84/ The XFY method combined with the grazingincidence method does not suffer heavily from the self-absorption effect compared with the normal-incidencemethod.

Some materials emit luminescence in the visiblewavelength range when irradiated with X-rays. Thisoptical luminescence signal intensity corresponds to the


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hν hν

ElectronescapedepthL~50 Å

VacuumAdsorbate

atomsA

Substrateatoms

B

e−

e−

Photonmean freepath1/µ>1000 Å

~

Figure 26 Photoabsorption and electron production in a solidconsisting of substrate atoms B and an adsorbed layer A. Onlyelectrons originating within a depth L from the surface willcontribute to the TEY signal. (Reproduced from J. Stohr et al.,‘Surface Crystallography by Means of Electron and Ion YieldSEXAFS’, Surf. Sci., 117, 503–524, 1982, with permissionfrom Elsevier Science.)

amount of X-rays absorbed by the specimen. Variousprocesses of optical luminescence de-excitation are shownin Figure 29..85/ Optical luminescence is usually strongfor rare earth compounds, but some crystals whichhave defects in their crystal structure emit strongerluminescence, although a perfect single crystal of thesame material does not emit optical luminescence.

The photoacoustic (PA) effect produces sound onirradiating a sample surface by a chopped photon beam.This effect was discovered by Alexander Graham Bell.

Sample

Proportionalcounter

ETEY

A

Eθ

hν

Figure 28 Schematic experimental set-up of grazing exit angleexperiment for XFY and TEY. (Reproduced by permissionfrom Kitajima..83/)

Heat is produced while the sample is irradiated by anoptical beam and it diffuses during the beam chopping.The chopping frequency usually ranges from a few toa few hundred hertz. The sound wave is detected by amicrophone or a piezoelectric device. When the choppingfrequency is low, the heat diffuses into deeper location inthe sample compared with when it is high. Thus theprobing depth is variable by changing the choppingfrequency. All the incident photon energy is finallyconverted into thermal energy through nonradiativetransition processes in the solid. The PA process in thevisible wavelength range is used for the very sensitiveabsorption spectrometry of thick bulk samples, which arenot transparent to an optical beam. This method has beenapplied to the measurement of the X-ray absorption,.86/

where the incident X-ray beam should be chopped toproduce acoustic waves in the sample. This method is athermal yield method.

Biasvoltage

Ionizationchamber

Ionizationchamber

Currentamp.

Currentamp.

Currentamp.

V/F conv.

V/F conv.

ImageintensifierBe

windowBe

window

Sample chamber

Cu meshSlit

Sample

Beamstop

SR

XYslit

Si(111) mono.

Figure 27 Schematic experimental set-up for TEY and CEY at grazing angles. (Reproduced by permission from Zheng andGohshi..82/)


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Figure 29 Schematic diagram of X-ray absorption and opticalluminescence processes. Three different excitation processes,from the 1s orbital to a continuum state (absorption coeffi-cient µ1) and to a bound state (µ2), and from the 2s orbitalto a continuum state (µ3), give rise to a single luminescencewith luminescence yields h1, h2, and h3, respectively. X-ray flu-orescence, KLL Auger, electron multiscattering, nonradiativedecay, and radiative decay processes are schematically shown.(Reproduced by permission from Emura et al..85/ 1993 TheAmerican Physical Society.)

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l

hν

Figure 30 Schematic illustration of a liquid cell. (Reproducedby permission from T.K. Sham, R.A. Holroyd, ‘Photoconduc-tivity Measurements of X-ray Absorption of Liquids: Fe K-edgeSpectrum of Ferrocene in 2,2,4-Trimethylpentane’, J. Chem.Phys., 80, 1026–1029 (1983). 1983 American Institute ofPhysics.)

Liquid samples are inserted into a cell shown inFigure 30,.87/ and an electrode is used to measure

the photoconductive spectra; electrical conductivity isinduced by the incident X-rays. Spectra thus measuredare sometimes the inverse of the transmission spectra andsometimes similar to the transmission spectra, dependingon the concentration..88/ Electrode surface reactionprocesses can be measured using X-ray absorption incombination with the total reflection X-ray technique.

X-ray submicrometer beams are now available in majorSR facilities, and using these beam lines micro- ornanobeam techniques are now applicable, as shown inFigure 31,.89/ where the XFY is measured by a solid-statedetector (SSD). The energy resolution of an SSD is of theorder of 100 eV, while the energy shift of an absorptionedge is a few electron-volts. If the incident X-ray energy isbetween the edge energies of two chemical states (say FeOand Fe2O3), then only one kind of chemical state (FeO)can emit the X-ray fluorescence. Using this technique,chemical state mapping is possible.

Using a bent crystal monochromator as shown inFigure 32,.90/ multiple-energy X-rays can be focusedon a sample and the transmitted X-rays are detectedby a position-sensitive detector, which is made of aphotodiode array (PDA). Using such a kind of energy-dispersive optics, one spectrum can be measured withina few milliseconds..91/ This method is called quick X-rayabsorption fine structure (Q-XAFS).

Circularly polarized X-rays can be produced bylinearly polarized X-rays transmitted through a diamondsingle crystal at a special angle depending on itswavelength, which is called the phase retarder, as shown inFigure 33..92/ The vertical and horizontal components, theratio of which is called helicity, of the X-ray electric vectorcan be controlled by the small rotation and tilting of thediamond crystal. Absorption spectra of a magnetic sampleare measurable. This is X-ray magnetic circular dichroism(XMCD) X-ray absorption. The magnetic S and N polesapplying the magnetic field to the sample are inverted

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Figure 31 Schematic illustration of an X-ray fluorescencemicroprobe at the Photon Factory. (Reproduced from Nakaiet al.,.89/ courtesy of Marcel Dekker, Inc.)


Sample

SR

Monochromator

Position-sensitivedetector

Figure 32 Energy-dispersive system at a synchrotron facility.(Reproduced by permission from Derbyshire et al..90/)

or the circularly rotating direction of the X-ray electricfield is inverted and the difference in the absorbanceis measured. The former is usually used to measurethe dichroism. Magnetic thin multilayers have recentlybecome important for information mass storage devices,and these materials are characterized with microbeamXMCD X-rays. The difference in the absorption coeffi-cients for the left and right circularly polarized X-raysis illustrated schematically in Figure 34..93/ The details ofMCD are described in several books..94 – 97/

High-resolution X-ray fluorescence spectra of transitionmetal compounds show multiplet splitting due to theexchange interaction between the unoccupied 3d leveland the core X-ray hole. Thus the XFY absorptionspectrum of each multiplet line provides spin-selective

Energy

Fermi energy

Magnetic field

L3

L2

Figure 34 One-electron model used to explain the X-rayabsorption dichroism process and intensities. The d band is splitinto spin-up and spin-down bands. The absorption of circularlypolarized X-ray photons by the spin–orbit split 2p shell createsa spin-polarized core hole. (Reproduced by permission fromDuda et al..93/ 1994 The American Physical Society.)

absorption spectra, as shown in Figure 35..98/ This methodcan measure spin-selective X-ray absorption spectrawithout applying a magnetic field to the sample. Thismethod is useful for characterizing mixed-valence proteincompounds.

The phase transition due to the temperature change isobservable, as shown in Figure 36..99/ The phase transitionis a small change in bond distance and bond angle,and consequently the electronic structure of the samplechanges. Thus both XANES and EXAFS region spectrachange their line shapes.

The surface of water, where a liquid monolayer ispresent, could be analyzed by grazing incidence X-rayreflection XAFS..100/ When the monolayer absorbs metal

Si(111) double-crystalmonochromator

X-ray fromundulator

Diamondphase retarder

Ionizationchamber (I0) Ionization

chamber (I )

Electromagnet

To current amplifierTo current amplifier

Sample

From piezo driver

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Figure 33 Experimental set-up for XMCD measurements with the helicity-modulation technique. (Reproduced by permissionfrom Suzuki et al..92/)


Photoionizationof 1s electron byX-ray absorption

X-ray fluorescence

3d

3p

3d

3p

1s

Kβ1,3 X-rays

Incident X-rays

1s

7P

Figure 35 Schematic illustration of absorption and emission(Kb1,3 lines) of X-rays due to 1s down-electron photoionizationof Mn2C ion; 1s up-electron photoionization produces the Kb0

X-ray emission spectral line. (Reproduced by permission fromGrush et al..98/)

530 535

Nor

mal

ized

inte

nsity

Photon energy (eV)

RT

120°C

σ*

π* d

Figure 36 O 1s absorption spectra of VO2 measured at roomtemperature and at 120 °C. (Reproduced by permission fromAbbate et al..99/ 1991 The American Physical Society.)

ions from the water solution, the concentration of themetal ion on the surface is slightly higher than that in thewater. The coordination structure around the metal ionis analyzed by the EXAFS method.

When a powder is measured by the XFY and TEYmethods on a substrate, then the depth-selective chemicalstate analysis of a fine particle can be performed. Fly ashis a powder of micrometer-sized particles, which are asource of acid rain when they are dispersed in the air. Theparticles are put on an aluminum foil and irradiatedby monochromated X-rays, and XANES spectra aremeasured by TEY and XFY methods..101/ The TEYmethod is sensitive to the surface chemical state of thepowder particle, and XFY is sensitive to deep parts of theparticle (micrometers). The chemical shift of the white

line in the absorption spectra is a measure of the oxidationstate of an element in the particle.

The surface catalyst process could be elucidated by theanalysis of EXAFS spectra. The incident X-ray polariza-tion dependence is an important parameter for the deter-mination of the geometry of a reactant and the surface..102/

X-ray detectors in the XFY method have beendeveloped for X-ray absorption experiments, such asa 19-element Ge detector array; a schematic illustrationof a seven-element detector is shown in Figure 23; a 100-element detector array can be used for more efficientdetection of X-rays.

Transition metals are usually a target of XAFS analysis,the energy range of which is from 5 to 20 keV. Thebeamline for X-rays of this energy range uses Be andpolymer films as windows to separate the vacuum systemfrom the atmosphere. Both lower and higher energyXAFS experiments require different techniques.

Soft X-ray XAFS experiments, ranging from 0.1 to5 keV, require UHV techniques. This is because anywindows between the SR storage ring and the sampleheavily absorb X-rays, hence a windowless beamline isrequired. Consequently, the sample chamber is madeof UHV components and must be baked out up to200 °C. The contaminants in X-ray windows and otherX-ray optics are carbon and oxygen, which are in thesoft X-ray region (250–600 eV). Hence a windowlesssystem is appropriate for the analysis of these elements.The shortcoming of the soft X-ray region experimentis the UHV system, because samples requiring X-rayabsorption analysis could not always be inserted intothe UHV system because they would emit gas into theclean system. A vacuum of 10�2 Pa is sufficient to avoidthe absorption of X-rays in the X-ray path. The XAFSspectra of this soft X-ray region are surface sensitive andthe profile changes of the spectra due to chemical stateare very large. The spectra measured can be used for thesame purpose (chemical state analysis) as XPS or ESCA.The detection limit of XAFS is usually lower than that ofESCA, hence XAFS has an advantage over ESCA if SRis available.

XAFS experiments higher than 20 keV require differ-ent experimental techniques. The number of applicationexamples is not large, mainly because high-energy X-raysources are limited without using third-generation SRfacilities. In place of measuring this energy range K-edgeXAFS, the L-edge XAFS, the energy of which usuallyfalls in the range 5–20 keV, is measured. The L XAFSspectra are composed of L1, L2 and L3 edge jumps, whichinterfere with each other. To avoid this, the K edge isuseful for the analysis. The lifetime of the K hole stateof high-energy K-edge XAFS of higher atomic numberelements is, however, shorter than the long-wavelengthregion. Because of the Heisenberg uncertainty principle,


the line widths of the absorption spectral componentsare as large as 100 eV. Both the EXAFS oscillation andXANES lose fine structure owing to this lifetime broad-ening. High-energy XAFS has recently been measuredwith the development of third-generation SR facilities,because the numerical analysis overcomes the short life-time effect.

7 SOURCES AND DATABASES

Academic societies and E-mail lists discuss the standard-ization of X-ray absorption spectrometry. As XAFS isused in many areas of research, standardization has beenrequired. To achieve standardization, the InternationalXAFS Society (IXS) was established in the 1990s. Thepurpose of the IXS is stated as follows:

The International XAFS Society represents all thoseworking on the fine structure associated with inner-shellexcitation (near-edge and extended) by various probes(e.g. X-rays and electrons), and related techniques forwhich the data are interpreted on the same physical basis.The purpose of the IXS is to oversee activities whichbenefit the community as a whole, to establish operationalcommittees, to provide for education in the field, todisseminate relevant information, to work with otherrelated regional, national and professional organizationsin promoting and developing XAS and related disciplines,and to act as representative for the community to otherprofessional organizations.

This society has a close relation to the International Unionof Crystallography (IUCr). The WWW home page of theIXS is at Illinois Institute of Technology, http://ixs.iit.edu/,where a large number of XAFS databases are presented.This WWW page links to other related WWW homepages. National society and working groups are activein many countries. All the information concerning theseactivities is obtainable at the international conferences onXAFS. The first XAFS international conference was heldat Daresbury, UK, in 1981, and subsequent conferencesare listed in Table 8.

Many kinds of activity reports published by SR facilitiesare useful sources of experimental methods and standardspectra. Journal sources are listed in Table 9. Fundamen-tal reviews in Analytical Chemistry published in evenyears relate to X-ray absorption spectrometry..113 – 120/

In a book by Meisel et al.,.121/ references are classifiedby atomic number and spectral series. Recently severalbooks which treat newer X-ray techniques.122 – 126/ andconcerning X-ray absorption.127,128/ have been published.The Materials Research Society has held symposia onapplications of SR in materials science..129 – 131/

The Denver X-ray analysis conference and interna-tional conferences on electron spectroscopy, on X-ray

Table 8 List of international XAFS conferences andproceedings

Conf. no. Date Location Ref.

1st March 1981 Daresbury 1032nd September 1982 Frascati 1043rd July 1984 Stanford 1054th July 1986 Fontevraud 1065th August 1988 Seattle 1076th August 1990 York 1087th August 1992 Kobe 1098th August 1994 Berlin 1109th August 1996 Grenoble 111

10th August 1998 Chicago 11211th August 2000 Ako –

Table 9 Source of information (journals)

Advances in X-ray AnalysisAnalytical Chemistry, Fundamental Review, even years.113 – 120/

Journal of X-ray Science and TechnologyJournal of Electron Spectroscopy and Related PhenomenaJournal of Synchrotron RadiationPhysical Review BSurface ScienceSynchrotron Radiation NewsX-ray Spectrometry

and inner-shell processes and on vacuum ultravioletphysics are sources of X-ray absorption spectroscopyand spectrometry.

8 ALTERNATIVE METHODS

8.1 Electron Energy Loss Spectroscopy

An electron beam, with an energy from a few hun-dred electron-volts to a few hundred kiloelectron-volts,impinges on a sample and loses its kinetic energy. Whenthe sample is a bulk material, reflected electron energy ismeasured. Usually the loss of transmitted electron energyis measured for thin-film samples less than a few microm-eters or a few tens of nanometers thick. This is calledEELS..132/ EELS is usually combined with transmissionelectron microscopy (TEM). The electron energy lossstructure is similar to the XAFS. The EXAFS region inEELS is called the extended electron energy loss finestructure (EXELFS). Forward-scattered (0°) electronenergy loss spectra, formed when electrons are trans-mitted in a thin film, are approximately equivalent to theoptical spectra; the selection rule is the electric dipole.Energy loss spectra of electrons scattered at a large angleare not treated by the electric dipole transition, andsometimes include optically forbidden transitions. Thetransmission method used in TEM has a very high spatial


resolution, hence chemical state imaging by the chemi-cal shift of the absorption edge is possible..133,134/ Highenergy resolution and high spatial resolution are notalways achieved by a single instrument. The EELS spec-tra are sensitive for low atomic number elements such asboron and carbon. It is not easy to measure the XAFSspectra of these long wavelengths using an SR facility.

8.2 Self-absorption

Although the characteristic X-ray wavelength of anelement is usually separated from the absorption edgewavelength of the same element for hard X-rays, theyare very close to each other for the soft X-ray region.These close lines are, for example, transition metal La,bX-ray emission lines and L2,3 absorption edges. The Laand Lb X-ray emission lines emitted in a deep locationin a solid are absorbed during the travel in the solid.Hence the X-ray emission spectra have dips due to theX-ray absorption spectra. The profiles of the La,b X-rayemission lines of transition metals excited by differentelectron energies (3 and 16 keV) change because of theself-absorption effect..135/ If the electron energy is high(16 keV), then the penetration depth of the electronis deeper. Hence the X-ray emission spectrum suffersheavily from the self-absorption effect. In contrast, if theelectron energy is low (3 keV), then the penetration depthis shallow, and the X-ray emission spectrum is free fromthe self-absorption effect. The comparison of these twospectra yields an X-ray absorption. Similarly, one set ofX-ray emission spectra is obtainable by tilting the sampleto the X-ray detector or to the incident electron beam,when the electron energy is fixed.

8.3 Extended X-ray Emission Fine Structure

The radiative Auger effect (RAE) is always associatedwith the X-ray characteristic lines and this effect is anenergy loss structure in characteristic X-ray emission, asshown in Figure 37..136/ The second electron shaken upinto an unoccupied orbital has similar information to theXAFS. This is called EXEFS..137/ This method is used tomeasure low atomic number elements such as Na, Mg, Al,and Si, because the RAE satellite intensity is strong forthese elements. If wavelength-dispersive electron probemicroanalysis (EPMA) is available, XAFS spectra of 1 µmdiameter area are measurable using this method.

8.4 X-ray Raman Scattering

X-ray Raman scattering is the effect of energy losson X-ray scattering. Raman scattering is a similarphysical process to Compton scattering. The differenceis that Raman scattering involves scattering by coreelectrons whereas Compton scattering involves scattering

0

500

1000

1500

2000

2500

1200 1180

B

A

1160

Energy (eV)

Inte

nsity

(co

unts

per

4s)

1140 1120 1100

Figure 37 (A) Low-energy satellites (the radiative Augersatellites) of the Ka X-ray fluorescence spectrum of MgOcompared with (B) XANES measured at an SR facility.(Reproduced by permission from Kawai and Takahashi..136/)

8265

Energy (eV)

Inte

nsity

Rayleigh

Raman

Compton

×10

0

7765

500

θ = 60°

Figure 38 Rayleigh, Compton, and Raman scattering spectraof 8265-eV incident X-rays by graphite observed at q D 60°.(Reproduced by permission from Tohji and Udagawa..138/ 1987 The American Physical Society.)

by conduction band electrons. X-ray Raman scattering isa method for measuring soft X-ray absorption spectra(say of carbon) with a hard X-ray spectrometer (a fewkiloelectron-volts). Hard X-rays can be measured inair; soft X-ray absorption spectroscopy, which usuallyrequires UHV, is possible in air by this method. Hard X-rays (8265 eV) impinge on a carbon-containing sample,and the X-rays lose energy by the carbon K edge due to theRaman scattering (ca. 300 eV), as shown in Figure 38..138/


The X-ray that should be detected is at ca. 8 keV, whichstill falls in the hard X-ray region. The XAFS study ofcatalysts during reaction with gases is possible using theRaman effect.

8.5 Diffraction Anomalous Fine Structure

DAFS was proposed by Stragier et al..139/ XAFS usuallymeasures the wavelength dependence of f2, the imaginarypart of the atomic structure factor; DAFS measures f1.The wavelength dependence of f1 and f2 has a closerelation through the Kramers–Kronig transformation.In the DAFS experiment, the intensity of a diffractionpeak of a specimen is measured by the change in theincident X-ray energy. The sample and detector angles(q� 2q) are measured by the change in the incidentX-ray energy, or powder X-ray diffraction patterns aremeasured by the change in incident X-ray energy. Thismethod can measure site-selective XAFS of the sameelement, because the diffraction peak corresponds to adifferent site in the crystal. If the diffraction peaks whichoriginate from the surface and bulk phases are separated,space-selective EXAFS-like spectra are obtainable bythis method.

8.6 b-Environment Fine Structure

The b-electron emission process in a nuclear conversionprocess suffers interference by the crystal structure forthe same reason as EXAFS. This method is calledBEFS..140/

8.7 Inverse Photoemission Spectroscopy

IPES is an alternative method to measure the unoc-cupied electronic structure by irradiating electrons anddetecting photons..141/ This method is otherwise calledbremsstrahlung isochromat spectroscopy (BIS). Theextended structure like EXAFS is also observable in BISand this is called the extended X-ray bremsstrahlungisochromat fine structure (EXBIFS)..142/ The BIS isusually combined with an ESCA instrument, and thusoccupied and unoccupied electronic structures (similar toXANES) are measurable..143/

9 CONCLUSION

X-ray absorption spectroscopy is chiefly used in the areaof electronic structure study and structural analysis forthe study of new materials, surfaces, and catalysts. Thespectra measured are surface sensitive or bulk sensitivedepending on the detection method. Chemical shift andprofile changes are observable. Thus the spectral analysisis useful for materials characterization. This method is

also powerful for analyzing mixed chemical states inindustrial, environmental, and biological analytes. Thedevelopment of SR facilities will make it possible tomeasure nanometer-sized samples in less than a fewmilliseconds.

ABBREVIATIONS AND ACRONYMS

APW Augmented Plane WaveBEFS b-Environment Fine StructureBIS Bremsstrahlung Isochromat

SpectroscopyCEY Conversion Electron YieldDAFS Diffraction Anomalous Fine StructureEELS Electron Energy Loss SpectroscopyEPMA Electron Probe MicroanalysisESCA Electron Spectroscopy for

Chemical AnalysisEXAFS Extended X-ray Absorption Fine

StructureEXBIFS Extended X-ray Bremsstrahlung

Isochromat Fine StructureEXEFS Extended X-ray Emission Fine

StructureEXELFS Extended Electron Energy Loss

Fine StructureHF Hartree–FockHFS Hartree–Fock–SlaterIMFP Inelastic Mean Free PathIPES Inverse Photoemission SpectroscopyIUCr International Union of

CrystallographyIXS International XAFS SocietyKKR Korringa–Kohn–RostkerLCAO-MO Linear Combination of Atomic

Orbitals-Molecular OrbitalLDA Local Density ApproximationLEED Low-energy Electron DiffractionLRO Long-range OrderLUMO Lowest Unoccupied Molecular OrbitalMS Multiple ScatteringMS-Xa Multiple Scattering XaMT Muffin-tinNEXAFS Near-edge X-ray Absorption Fine

StructurePA PhotoacousticPDA Photodiode ArrayPEH Photoelectron HolographyQ-XAFS Quick X-ray Absorption Fine StructureRAE Radiative Auger EffectSEXAFS Surface Extended X-ray Absorption

Fine StructureSR Synchrotron Radiation


SRO Short-range OrderSSD Solid-state DetectorTEM Transmission Electron MicroscopyTEY Total Electron YieldUHV Ultrahigh VacuumXAFS X-ray Absorption Fine StructureXANES X-ray Absorption Near-edge StructureXAS X-ray Absorption SpectrumXFH X-ray Fluorescence HolographyXFY X-ray Fluorescence YieldXMCD X-ray Magnetic Circular DichroismXPD X-ray Photoelectron DiffractionXPS X-ray Photoelectron Spectroscopy

RELATED ARTICLES

Environment: Water and Waste (Volume 4)X-ray Fluorescence Spectroscopic Analysis of LiquidEnvironmental Samples

Surfaces (Volume 10)Auger Electron Spectroscopy in Analysis of Surfaces žSoft X-ray Photoelectron Spectroscopy in Analysis ofSurfaces ž X-ray Photoelectron Spectroscopy in Analysisof Surfaces

Electroanalytical Methods (Volume 11)X-ray Methods for the Study of Electrode Interaction

X-ray Photoelectron Spectroscopy and Auger ElectronSpectroscopy (Volume 15)X-ray Photoelectron and Auger Electron Spectroscopy žX-ray Photoelectron Spectroscopy and Auger ElectronSpectroscopy: Introduction

X-ray Spectrometry (Volume 15)Energy Dispersive, X-ray Fluorescence Analysis ž Struc-ture Determination, X-ray Diffraction for ž TotalReflection X-ray Fluorescence ž Wavelength-dispersiveX-ray Fluorescence Analysis ž X-ray Techniques:Overview

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