What is X-ray Absorption Spectroscopy?
X-ray absorption spectroscopy (XAS) is an analytical technique that relies on the variable absorption of x-rays by matter to probe its chemical state and local structure. A beam of x-rays of nearly one energy is put onto a sample, and the change in x-ray absorption is measured as the energy of the beam is varied.
For each element of the periodic table, every electron shell in an atom of that element has a unique energy required to eject it from the atom. In an XAS measurement, the energy of the incident x-rays is first tuned just below the energy of a particular electron shell, then the energy is gradually increased until eventually the x-ray has enough energy to eject that electron and is absorbed.
Due to quantum mechanical effects, the absorption does not increase all at once, but rather has a detectable turn-on and more complicated structure that can tell us about the chemical state of the absorbing atom.
The now free electron (i.e. no longer bound to the atom) is called a photoelectron. This photoelectron has a “wave function” that can scatter off nearby atoms creating oscillations of more/less absorption. Think of it as ripples in a pond. This wave interference can tell us about the local structure around the absorbing atom.
One of the most useful properties of XAS measurements is that the energies of the electronic transitions of different elements are well separated in energy compared to the resolution of XAS spectrometers. This means that by tuning the energy of a particular transition, an element-specific measurement of the sample can be performed.
The plot to the left shows the XAS for 3d transition metals and the well separated energy ranges.
The plot below shows only the energy range for Fe XAS. The difference in oxidation state of the samples can clearly been seen by the shift in the energy position of the absorption edge.
Elements of a XAS scan
Pre-edge: X-ray energy below the electron shell absorption energy. This region can sometimes include features from the excitation of a core electron into an unoccupied, but still bound, state.
Near-Edge (XANES) region: X-ray absorption starts to occur at electron binding energy. Changes or shifts in edge energies occur for different oxidation states of the same element. See the plot for Fe above, where the Fe2O3 edge is shifted to higher energy than for Fe metal foil.
Extended XAFS (EXAFS) region: Oscillations in absorption extend far beyond the binding energy. These oscillations are caused by the specific local structure around the absorbing element.
When should I use laboratory XAS?
Use XAS when you need:
By tuning to the energy of an atomic absorption edge, XAS probes only the desired element, regardless of what else is in the sample.
The absorption length of XAS in the energy range 5-12 keV is on the order of several microns, giving a bulk average measurement of properties.
Oxidation state sensitivity
The XANES (x-ray absorption near-edge spectroscopy) region of the spectrum gives strong sensitivity to the oxidation state of the element being measured.
Ambient or special sample environment setups
XAS does not require samples to be in vacuum. Standard measurements are conducted at ambient temperature/pressure. Sample chambers for heating or cooling samples or for air-sensitive samples are possible.
Because of the penetrating power of x-rays at these energies, many in-situ measurements are possible from operando battery measurements to in-situ catalyst reaction cells and more.
No long-range order required
XAS provides sensitivity to the local structure of the element being probed, regardless of long range order (e.g. works on non-crystalline or nano-sized materials).