What is X-ray absorption spectroscopy and how could I utilize it?
X-ray absorption spectroscopy (XAS) is a well-established and powerful method to obtain information on structural and chemical properties of matter by studying attenuation of X-rays.
In the XAS analysis, the spectra is typically divided into two parts: XANES and EXAFS.
XANES (X-ray absorption near-edge structure) is the area that covers the vicinity of the absorption edge. XANES part of the spectrum probes the lowest unoccupied states of the absorbing ion, and thus can be used to probe e.g. the chemical environment, coordination and oxidation state of the ion. The XANES region is affected by multiple factors which can make the data difficult to interpret usually requiring a combination of experimental standard measurements and computer simulations.
The extended X-ray absorption fine structure (EXAFS) refers to the region well above the absorption threshold. The absorbed X-ray photon ionizes a photoelectron, which propagates as a quantum mechanical wave. The wave is scattered by the surrounding atoms, leading to interference which is seen as oscillations in the EXAFS region. These oscillations are basically the Fourier transfer of the electron density of the local environment and thus carry information about the distances, coordination number and types of the surrounding ions. Compared to XANES, the analysis of the EXAFS data is rather straightforward.
Why laboratory based instruments?
Since the mid-1970s, XAS experimentation has mostly taken place at the high brilliance synchrotron light sources. However, the demand for synchrotron beamtime has since increased dramatically which severely limits its availability to the user community. Also the beam time application process, the transportation of samples, equipment and scientists, and the related byrocracy are resource consuming activities.
To tackle these problems, laboratory-based XAS instruments offer an appealing alternative to the synchtrotron experiments. Although not as bright as synchrotron light sources, a laboratory instrument equipped with a conventional X-ray tube and spherically bent crystal monochromator provides such a high photon flux that a high quality absorption spectrum of an optimally prepared sample can be obtained in a timescale of hours.
In addition to being a competitive alternative, the laboratory instrument can also be truly complementary to the synchrotron in cases where the system needs to be studied over an extremely long times or the samples are prohibited from the synchtrotrons for safety reasons. Easy access to the instrument is also essential in the education of the next-generation spectroscopists.
We have built a Johann-type scanning monochromator X-ray absorption spectrometer based on strip-bent spherically bent crystal analysers with 0.5 meter bending radius. The X-ray source is a conventional 1.5 kW silver anode tube which provides smooth brehmstrahlung over ~3-20 keV region, covering most 3d transition metal K edges and lanthanide and 5d transition metal L edges. The instrument is equipped with the helium chamber, which reduces the background noice and allows the efficient usage of the low energy range of the tube output. The measurement room is equipped with the ventilated gas bottle cabinet and gas sensors which allow conducting in situ chemical reaction studies safely.
Comparison of Co K edge XANES spectrum of metallic cobalt measured with our instrument and synchrotron (Honkanen et al. 2019).
Comparison of a) Ni K edge spectrum, b) EXAFS signal, and c) its Fourier transform measured with our instrument and synchrotron (Honkanen et al. 2019)
HelXAS can be used with multiple types of detectors, including fluorescence and image detectors. In addition to the standard transmission mode, we can measure absorption spectra indirectly via fluorescence, allowing the XAS studies of samples that are not possible to measure in transmission, such as functional battery cells and thin films on thick substrates.
Imaging sensor in conjunction with the tunable monochromatic beam allows us to map the element distribution in the sample using the absorption edge as the contrast mechanism.