The Sigray QuantumLeap X-Ray Absorption Spectroscopy System – Chemical Specification and Electronic Structure with X-Ray Absorption Spectroscopy. We answer your questions in a series of articles about the QuantumLeap.
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Why use Ge Crystals instead of Si for XANES?
Ge crystals allow for higher sample throughput than Si, due to their wider Darwin width (the FWHM of the total reflective profile of the monochromator crystal).
Despite this preference, other crystals are available upon request if higher resolution is preferred, for example.
Why use HOPG Crystals instead of HAPG for EXAFS?
As for XANES, HOPG crystals are utilised due to providing the optimum balance of throughput and energy resolution.
Despite this preference, other crystals are available upon request if higher resolution is preferred, for example.
Why use Off-Rowland circle geometry for XANES?
This geometry allows for parallel detection of multiple XAS spectra simultaneously as opposed to scanning through individual energies which reduces data acquisition times. As such, full spectra acquisition of high concentration samples takes only a few minutes. Micro-XANES acquisition at 100µm spot size analysis is also possible with this geometry.
Why are the X-rays not monochromatized in XAS before reaching the sample as in other techniques e.g. XPS?
The use of Johannsson geometry, which utilises a single crystal downstream of the sample to diffract the wavelength(s) of interest toward the detector, causes monochromatization downstream.
The von Hamos geometry is a multi-wavelength detection technique which disperses x-rays with respect to their wavelength along a spatially resolved detector and so, depending on the position of the x-rays detected, their energies can be determined.
