Abstract
The quick-EXAFS (QEXAFS) method adds time resolution to X-ray absorption spectroscopy (XAS) and allows dynamic structural changes to be followed. A completely new QEXAFS setup consisting of monochromator, detectors and data acquisition system is presented, as installed at the SuperXAS bending-magnet beamline at the Swiss Light Source (Paul Scherrer Institute, Switzerland). The monochromator uses Si(111) and Si(311) channel-cut crystals mounted on one crystal stage, and remote exchange allows an energy range from 4.0 keV to 32 keV to be covered. The spectral scan range can be electronically adjusted up to several keV to cover multiple absorption edges in one scan. The determination of the Bragg angle close to the position of the crystals allows high-accuracy measurements. Absorption spectra can be acquired with fast gridded ionization chambers at oscillation frequencies of up to 50 Hz resulting in a time resolution of 10 ms, using both scan directions of each oscillation period. The carefully developed low-noise detector system yields high-quality absorption data. The unique setup allows both state-of-the-art QEXAFS and stable step-scan operation without the need to exchange whole monochromators. The long-term stability of the Bragg angle was investigated and absorption spectra of reference materials as well as of a fast chemical reaction demonstrate the overall capabilities of the new setup.
Highlights
Quick-extended X-ray absorption fine structure (EXAFS) (QEXAFS) enables time-resolved X-ray absorption fine structure (XAFS) spectroscopy down to the millisecond time scale (Frahm, 1988, 1989; Stotzel et al, 2010a; Fonda et al, 2012; Nonaka et al, 2012)
A completely new QEXAFS setup consisting of monochromator, detectors and data acquisition system is presented, as installed at the SuperXAS bending-magnet beamline at the Swiss Light Source (Paul Scherrer Institute, Switzerland)
Absorption spectra can be acquired with fast gridded ionization chambers at oscillation frequencies of up to 50 Hz resulting in a time resolution of 10 ms, using both scan directions of each oscillation period
Summary
Quick-EXAFS (QEXAFS) enables time-resolved X-ray absorption fine structure (XAFS) spectroscopy down to the millisecond time scale (Frahm, 1988, 1989; Stotzel et al, 2010a; Fonda et al, 2012; Nonaka et al, 2012). QEXAFS is basically a straightforward evolution of conventional step-scanning measurements and is directly applicable to the same samples, the same sample environments and the same detection schemes This includes de-excitation processes such as fluorescence or total electron yield, which are important for dilute samples or surfaces. Measured spectra are free of self-absorption effects and exhibit a high energy resolution Using this method, the sample is irradiated with a fixed monochromatic photon energy tuned below the absorption edge of interest. The monochromator presented here allows remote adjustment of the oscillation frequency and spectral range It is equipped with two different channel-cut crystals, Si(111) and Si(311), to extend the accessible X-ray energy range. The current setup enables acquisition rates of up to 100 spectra sÀ1, reaching a time resolution of 10 ms
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