07f. EXAFS_short

8/7/2019 07f. EXAFS_short

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EXAFS Extended X-Ray

Absorption Fine Structure

By: Ramon Adler

Lecturer: Prof. Jacob Zabicky


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What is X-Ray Absorption ?

When a beam of high energy photons penetrates

a solid, there are 3 major photon interactions:


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An atom absorbs an X-ray when the photon energy is sufficient to eject a

photoelectron, Below this threshold energy there is no absorption. Photons with

energies greater than the threshold energy to produce a photoelectron areabsorbed because the excess energy is conserved by transferring it to kinetic

energy of the photoelectron. However, the probability of the absorption

occurring decreases as the photon energy increases above the threshold

Ee = h - Eb

We saw this part of absorption in the primary ionization step of XPS and AES


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Compton Effect:

X-rays are scattered by the electrons of an

absorbing material, the problem is generallytreated as an elastic collision between a photonwith momentum p = h/ and a stationaryelectron with rest energy mc

2

. After scatteringat an angle , the photon wavelength is shiftedto larger values by an amount =(h/mc)(1-cos), where h/mc=0.0243 is known as theCompton wavelength of the electron.


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Pair Production:

If the photon energy is grater than

2mc2=1.02MeV, the photon can annihilate with

the creation of an electron-positron pair.

We saw this part of absorption when dealing with the PIPA method


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What is X-Ray Absorption Fine Structure ?

The x-ray absorption spectrum is typically considered in two parts. X-ray

Absorption Near Edge Structure and Extended X-ray Absorption Fine

Structure.

XANES contains information about the valence and density of states of theabsorber, as well as the chemical state of the sample.

EXAFS contains detailed information about the local atomic structure.


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In an EXAFS experiment, we measure an absorption spectrum, and use

the oscillations just above the edge.

These oscillations are understood theoretically to be a final-state

electron effect resulting from the interference between the

outgoing photoejected electron and that fraction of the

photoejected electron that is backscattered from the neighboring

atoms.


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X-RayPhoton

-Atom 1

-Atom 2

-Atom 3

-Atom 4



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The EXAFS phenomenon is due to :

a. The creation of a photoelectron by photon absorption

above the ionisation threshold.

b. Interference effects in the electronic wave function,

between the outgoing wave from the central atom and the

scattered waves from its neighbors.

Absorption edge energies are characteristic of the absorbing

element. XAFS allows you to tune into different types of atoms by

selecting the energy.


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The simplest XAFS experiments are done in transmission mode. Polychromatic X-

rays are produced by a synchrotron radiation source or by brems-strahlung

(rotating-anode X-ray tube) output, and a desired energy band of approximately 1eV bandwidth is then selected by diffraction from a silicon double crystal

monochromator. Only those X-ray photons that are of the correct wavelength to

satisfy the Bragg condition n= 2dsin at the selected angle will be reflected fromthe crystal; the others are absorbed. The parallel second crystal is used as a mirrorto restore the beam to its original direction. The monochromatic X-rays are then

allowed to pass through the sample, which should absorb approximately 50%-90%

of the incident X-rays. The incident and transmitted X-ray axes are monitored,

usually with gas ionization chambers

XAFS experiments


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XAFS is applicable to condensed matter (both crystalline and amorphous), and

molecular gases. No special conditions are required . Data analysis involves a

sequence of corrections to account for the background and factors of instrumental

and sample origin. Some of the problems that can be addressed are listed below.

Examples are shown in the following slides.

Data analysis

1. Bond lengths

2. Number of neighbors

3. Coordination

4. Partial pair distributions

5. Three-body effects

6. Structural evolution in amorphous to crystalline

transformation can be followed.

7. In situ studies under real-life conditions.


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Data analysis

The energy scale is converted

to k-scale using k=[0.263(E-E0)]

1/2

)(

)()( 0

kMS

kk

)(

)()( 0

kMS

kk

(k)=(k)

0(k)

SM(k)

E0 the energy threshold of the absorption edge

k

kk

k

)(kMS

0(k) post edge background

(k) Extrapolation of pre-edge

S step jump

M(k) McMaster correction


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Data analysis

)(

)()( 0

kMS

kk

)(

)()( 0

kMS

kk

Fourier Transform to r Space


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The EXAFS can be analyzed using curve fitting, yieldingquantitative information about the structural environment of

the absorber. In this approach, an unknown structure is

understood by comparison with a known standard.

This is the background-subtracted data in

photoelctron wavenumber.

This is the magnitude of thecomplex Fourier transform (FT)

with parts of the fit

Data analysis


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Time Resolution of Chemical Reaction

In this example tetrahedral CrVI is reduced to octahedral CrIII.

The large peak evolves into a much smaller one.


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STRUCTURE OF NANOCRYSTALLINE

ZIRCONIA AND YTTRIA

Structural analysis of nanosized powders as well as

compacted and sintered samples are essential to

understand property changes, e.g., diffusion, innanostructured materials compared to coarse grained

(~1m) materials. Structural defects inside the grains,

grain boundary structure in compacted samples and

degree of disorder of atoms in surfaces and interfaces

are of particular interest.


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STRUCTURE OF NANOCRYSTALLINE

ZIRCONIA AND YTTRIA

In a n-ZrO2 (about 80 nm) ceramic sample which was sintered 90 minutes at 1000 C invacuum the magnitude of the FT is nearly identical to coarse grained powder. Fitting the

Fourier filtered and back transformed oxygen shell in n-ZrO2 powder we can distinguishbetween the smaller distance (average value = 2.16 ) in the seven fold coordinatedmonoclinic part and the larger distance (2.35 ) of the eight fold coordinated tetragonalpart of the n-ZrO2 powder. The calculated ratio of polymorphs from diffraction results(tetragonal:monoclinic = 3:2) were used to calculate the coordination numbers. In n-ZrO

2the Zr-Zr-distance is significantly shifted. One reason for this shift is the larger Zr-Zr-distance in the tetragonal polymorph (3.62) and additionally it is assumed that this shiftis caused by a distortion of the monoclinic lattice. The coordination number of the firstoxygen shell decreased from 7.1 in coarse grained ZrO2 to 5.4 in n-ZrO2, with an increase

of the Debye-Waller-factor (thermal effects).


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In 1992 there was an effort to producenanoparticulate, metallic iron for use inbiosensor applications. The idea was tocover an iron core with an antioxidation

layer.

Characterizing Nanoparticles

Unfortunately the effort to datehas not been successful. TheXAS clearly shows that the ironportion of the sample (violet) is

well oxidized and similar to Fe3O4(red), but not to Fe metal (blue).

07f. EXAFS_short

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