X-ray Absorption Spectroscopy (XAS)

XAS is a technique that measures X-ray absorption coefficient μ(E) as a function of the incident X-ray energy. The information is element- and orbital-specific, and reports on the local atomic and electronic structure of the element of interest.

X-ray Absorption Near-Edge Structure (XANES) measures the variation of the absorption factor in an energy range from about 50 eV before the absorption edge to about 100 eV after the edge. It provides detailed information on the coordination geometry and oxidation state of the studied atom. The characteristic “fingerprint” of the coordination around the atom is very important in speciation of different ligands to the studied atom. The pre-edge peak provides electronic structural information from transitions to unoccupied valence orbitals.

Extended X-ray Absorption Fine Structure (EXAFS) is the major analytical technique to provide local structural information about the absorber atom, with regard to the type of neighbouring ligands, their bonding distances to the absorber, and the coordination numbers. For structural and functional studies of metalloproteins, the technique is a valuable complement to crystallography with regard to preservation of higher valence (the lower X-ray dose used), and the freedom in sample preparation (no need for crystals).

 

Time-resolved studies

An important aim of the Balder project is to perform time-resolved XAS measurements in-situ, as well as in-operando studies of catalytic materials including biomaterials. It will be facilitated by the installation of an advanced gas-mixing system, and by scanning the monochromator rapidly and repetitively over an absorption edge. The time scale of XANES is anticipated to be in the order of 10–100 ms, and of EXAFS in one second by using the monochromator’s direct drive and recording data on the fly.

 

X-ray Emission Spectroscopy (XES)

XES is a photon-in/photon-out spectroscopic technique where the incoming X-rays from the monochomator are based on the perfect crystal Bragg optics and the emitted X-rays are similarly detected by the spectrometer crystals.

In Non-Resonant XES, the occupied electronic state is probed. The process of X-ray emission is second order: the atom is excited non-resonantly, a core hole (1s) is created, and X-rays are emitted when the core hole is filled, giving rise to fluorescence. Depending on the origin of the filling electron (2p, 3p, valence), different emission lines appear (Kα, Kβ and valence-to-core). Electronic information of the atom of interest is available indirectly or directly from the emission lines, to learn about the oxidation state (Kα, Kβ), spin state (Kβ) and ligand identity (valence-to-core).

In Resonant XES, the excitation energy is tuned to a bound state (scanned over the absorption edge) and the filling of the 1s core hole can be either: 1) from 2p, 3p or valence orbitals, and the detected emission lines are changing with the excitation energy (RXES), or 2) the photo-excited electron itself decays to fill the 1s core hole, so-called Resonant Inelastic X-ray Scattering (RIXS). This way, you can measure the L-edge type spectra of transition metals with hard X-rays. You typically monitor the energy transfer (incoming X-ray minus emitted X-ray energy) versus the incoming energy. The 2D RXES/RIXS plane contains plenty of electronic information about your system that can be analysed in cuts with sharp spectral features; a diagonal cut of the RIXS is the HERFD XANES (fixed emission energy).

For more information and references, please see: Pieter Glatzel’s introduction to XAS/XES (http://www.pieter-glatzel.de/XASXES.html)

 

X-ray Emission Detected Absorption Spectra

By using the emission spectrometer as the fluorescence detector, the spectral resolution is significantly improved (High-Energy Resolution Fluorescence Detected (HERFD) XAS) by reducing the background and partly suppressing the core hole lifetime broadening. The main benefit of the improved spectral resolution will be gained in the analysis of pre-edge XANES. The selectivity of the spectrometer also gives an opportunity to measure EXAFS beyond unwanted absorption edges present in the sample (for example Fe limiting the Mn EXAFS) and without interfering diffraction peaks in polycrystalline samples.

 

The sample

The sample to be measured can contain the absorber atom of interest in any phase: solid, crystal, liquid, frozen or gas. XAS/XES is a bulk technique, in other words, you see the spectrum of the average absorber atom.