On the sensitivity of hard X-ray spectroscopies to the chemical state of Br

Abstract

The sensitivity of the 1s X-ray emission and high-energy-resolution fluorescence-detected X-ray absorption spectroscopies (XES and HERFD-XAS) to resolve the variations of the chemical state (electronic structure and local coordination) of Br has been investigated for a selected set of compounds including NaBrO 3 , NH 4 Br and C 2 H 4 Br 2 (1,2-dibromoethane). For the Br K-edge XAS, employing the HERFD mode significantly increases the energy resolution, which demonstrates that with a crystal spectrometer used as detector the absorption technique becomes a more powerful analytical tool. In the case of XES, the experimental results as well as the density functional theory (DFT) modeling both show that the chemical sensitivity of the main 1s diagram emission lines (Kα 1,2 and Kβ 1,3 ) is rather limited. However, the valence-to-core (Kβ 2 ) region of XES displays significant shape and intensity variations, as expected for transitions having the same final states as those of photoemission spectroscopy. The spectra are in good agreement with the molecular orbital description delivered by DFT calculations. Calculations for an extended series of Br compounds confirm that valence-to-core XES can serve as a probe for chemical analysis, and, being a hard X-ray photon-in/photon-out technique, it is particularly well-suited for in situ investigations of molecular transformations, even on the ultrafast time scales down to femtosecond time resolution.

Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy

“…In particular, the Br K-edge has been reported to be very sensitive to and undergo significant edge shifts depending on its environment, even without a change of the oxidation state. [72][73][74] The appearance of the 1s-4p resonance at 13.475 keV and the estimated oxidation shift of ~5 eV both point to a full positive charge localizing at the Br centres, by analogy with case of photoexcited aqueous Br -. 68 Charge localization may be caused by small hole polaron formation either at a defect or at a regular site of the lattice.…”

“…The analysis of the spectra in Ref. 71 was carried out by simulating the density of states in order to reproduce the spectra. It was concluded that the pre-edge feature at 13.032 keV is dominated by the Pb p-orbitals, but it arises in the L 3 -edge spectrum due to their strong mixing with Pb d-orbitals.…”

“…the type of ligand and the bond distance between the atom of interest and its neighbors. [72][73][74] Photoinduced structural changes will therefore have a direct impact on the edge, even in the absence of an oxidation state change. 75,76 Since the above-edge transitions probe the unoccupied density of states above the Fermi level, a reduction of intensity right above the edge reflects the filling of unoccupied states, similar to the band filling found in optical studies of semi-conductors.…”

“…Turning now to the Pb L3-edge spectrum, it is known to be rather featureless as found in the case of various lead oxides with different oxidation states and our steady-state spectrum, shown in Figure 3, reflects the same aspect. 69,74 This is due to the very short core-hole lifetimes, which broaden all transitions, making their assignment difficult.…”

New insights on Br speciation in volcanic glasses and structural controls on halogen degassing

“…Especially, 'site-selective' crystal analyser spectrometers (Proux et al, 2017) can be used to improve the signal-to-background ratio and reveal spectral features that were invisible to conventional XANES. This high-energy resolution fluorescence detection (HERFD)-XAS has been successfully applied to better detect absorbing atoms (Co, Br, lanthanides) present in different oxidation states or different sites, in metallic oxides, minerals and even hightemperature supercritical fluids (Bordage et al, 2013;Hamalainen et al, 1991;Proux et al, 2017;Vitova et al, 2013). In the present study, HERFD-XAS has been used to investigate Br speciation in natural volcanic glasses.…”