Anatomy of an XAFS Measurement

Anatomy of an XAFS Measurement

(Parte 1 de 5)

Anatomy of an XAFS Measurement

Matt Newville Consortium for Advanced Radiation Sources University of Chicago

Experiment Design

Transmission v. Fluorescence modes

X-ray detectors Data Collection Strategies

Sample Preparation Problems to Avoid

(version 1.1.1, 21-June-2004)

X-ray Absorption Measurements

XAS measures the energy dependence of the x-ray absorption coefficient µ(E) at and above the absorption edge of a selected element. µ(E) can be measured in two ways:

Transmission: The absorption is measured directly by measuring what is transmitted through the sample:

Fluorescence: The re-filling the deep core hole is detected. Typically the fluorescent x-ray is measured, but sometimes emitted electrons are measured. Either way,

X-ray Absorption Measurements: Experimental Design

Important items for an XAFS measurement:

Monochromatic x-rays: Need x-rays with a small energy spread or bandwidth: ∆E ≈ 1eV at 10keV.

Linear Detectors: The XAFS χ(k) ∼ 10 −2 or smaller, so we need a lot of photons and detectors that are very linear in x-ray intensity (ion chambers). These usually means using a synchrotron source.

Well-aligned Beam: The x-ray beam hitting the detectors has to be the same beam hitting the sample.

Homogeneous sample: For transmission measurements, we need a sample that is of uniform and appropriate sample thickness of ∼2 absorption lengths. It should be free from pinholes. If a powder, the grains should be very fine-grained (absorption length) and uniform.

Counting Statistics: For good data µ(E) should have a noise level of about

. That means we need to collect at least 106 photons.

Transmission: Fluxes at synchrotrons are > 108 photons/sec.

Count rate is not much of an issue.

Fluorescence: May be a concern, especially when concentrations are very low.

An X-ray Beamline: Synchrotron and Monochromator

The synchrotron produces white radiation – x-rays of all energies. A monochromator selects a single energy.

A monochromator is typically a pair of parallel, highly perfect silicon crystals that use Bragg diffraction nλ = n hc

to select an energy E by selecting angle θ. The lattice spacing d is set by the “cut” of the crystal: Si(1), Si(220), and Si(311) are common.

Important characteristics of monochromators are: Angle/Energy Range: sets what edges can be accessed.

Angle/Energy Calibration: how accurate and reproducible is the energy?

Harmonics: At angle θ, energy E, 2E, 3E, etc can be passed (some cuts don’t pass 2E). These harmonics needed to be removed from the “monochromatic” beam before the experiment.

It is important to remove the harmonics (energies of 2E, 3E,) from the x-ray

Monochromator: Harmonic Rejection beam before it gets to the experiment.

Two ways of to reject harmonics: Detuning: The angular width of a Bragg reflection is finite (a few µrad), and decreases with increasing energy: the harmonics have a narrower angular width than the fundamental energy.

Making the two crystals slightly non-parallel rejects most of the harmonics, an preserves most of the fundamental.

A piezo-electric crystal on the second crystal adjusts the parallelness, and so intensity, and harmonic content. Rule of thumb:

Adjust the piezo until the total intensity is about half the maximum intensity.

Harmonic Rejection Mirror: An x-ray mirror will not pass energies above a critical energy set by the pitch of the mirror (in mrad). This works very effectively for harmonic rejection. Use one or both of these methods!

Monochromator: Energy Calibration and Reproducibility

The absolute monochromator angle is usually not known to great precision (relative changes are much more precise).

We need to calibrate the energy for a particular edge. Typically, a metal foil is used and an arbitrary position on the edge (say, maximum of dµ/dE) is set to the tabulated edge energy.

Many monochromators drift in angle from scan to scan (or over time). It’s good to measure a energy reference sample, such as a metal foil. The energy reference can be measured periodically or, in some cases, at the same time as the sample:

Either of these approaches can be used to calibrate the edge energy during a normal XAFS scan.

An XAFS Beamline End Station A typical XAFS station for Transmission and Fluorescence XAFS:

Slits: To define beam size (just out of the picture).

I0 Ion Chamber: To measure of the incident x-ray intensity I Ion Chamber: To measure of the transmitted x-ray intensity

Fluorescence Detector: To measure fluorescence signal. Motorized Sample Stage: To move sample into beam.

Ion Chamber: measure x-ray intensities

X-rays enter a chamber filled with an inert gas (e.g., N ). A gas molecules absorbs an x-ray, ejecting an electron and a charged ion, which ionizes more gas molecules. For most gases used, each x-ray of energy E generates

(a 10keV x-ray generates 312 electrons!). A voltage applied across the chamber sweeps out the electrons, giving a current. Each absorbed x-ray generates electrons, and the total current is amplified to a voltage (I-to-V amplifier):

for E in eV, V in Volts, and G the amplifier gain in Volts/Amps. The x-ray flux absorbed in the chamber is then:

Typical values:

Ion Chambers: ...continued

(Parte 1 de 5)