X-ray Absorption Spectroscopy

A range of X-ray Absorption Spectroscopy techniques are available at Diamond, including X-ray Absorption Near-Edge Structure (XANES), Extended X-ray Absorption Fine Structure (EXAFS), Resonant Inelastic X-ray Scattering (RIXS) and X-ray Emission Spectroscopy (XES).

At characteristic wavelengths the X-ray absorption of an element changes dramatically, these are called absorption edges. Near the absorption edge, the spectra may contain fine structure that reveals the electronic and geometrical environment of the absorbing atom. This technique is XANES; further from the edge EXAFS reveals the local atomic environment to the element. Both can be used to can follow reactions on timescales down to the millisecond.

Benefits of Synchrotron Techniques

The high intensity of synchrotron X-rays make it possible to detect elements present in very low concentrations. The tuneability of synchrotron radiation allows XAS spectra of virtually all the elements to be collected in short time periods. Combining the high brightness with the small spot size available makes it possible to examine samples with very high spatial resolution.

Applications

X-ray absorption spectroscopy is important in gaining structural understanding of a range of materials, including biomaterials, novel materials with special electronic properties such as superconductivity, dilute species in fluids, and complex inhomogeneous materials. It can provide information on bio-remediation processes, study minute minerals returned from space missions and be used to understand chemical reactions such as heterogeneous catalysis and hydrothermal synthesis of industrial materials.

Beamlines

I18 - Microfocus Spectroscopy
I20 - Versatile X-ray spectroscopy
B18 - Core EXAFS

The I20 Scanning end station, with the 64-element monolithic germanium fluorescence detector, three OKEN ion chambers and sample stages - all mounted on a motorized heavy-duty 2-axis experimental table.

Xspress4 

The new Digital Pulse Processor, Xspress4, developed at Diamond Light Source [1] employs a novel event cross-talk correction algorithm to allow a significant increase in detector count rate of the 64-element Ge detector for the equivalent MCA spectral performance of Xspress2. The improved resolution attained with Xspress4 allows more dilute and complex samples to be measured in fluorescence mode as we can operate the 64-element fluorescence detector to the Output Count Rate (OCR) of 600 to 800 KHz per element, instead of apporx. 250 kHz with Xspress2.

X-ray Absorption spectrum collected from 43ppm yttrium standard sample (GSD11) using the Xspress4 system on Beamline I20: merger of 4 consecutive XAS spectra together taken with Xspress4 and Xspress 2; MCA spectra taken with  Xspress4, showing that the fluorescence line of interest Y Ka is well resolved with Xspress4, thanks to the improved MCA resolution at approx. 630 kHz OCR per element.

Above shows a) a merge of 10 spectra XAS spectra and b) extracted EXFS signal taken using the 64-element Ge detector with the Xspress4 pulse processor system. The spectra were collected from a contaminated sediment containing 80ppm Hg in the in-house LN2 cryostat with the sample-detector distance adjusted to give the OCR of apporx. 600 kHz (Sam Shaw et al., University of Manchester).  

[1] G. Dennis, W. Helsby, D. Omar, I .Horswell, N .Tartoni, S. Hayama, I. Mikulska, and S .Diaz-Moreno. SRI2018 proceedings.