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Abstract: X-ray absorption spectroscopy of gases is seldom used for structural analysis, except in as much as the extended X-ray absorption fine-structure (EXAFS) signal is used to monitor some specific molecular process. Gas spectroscopy, however, has been the main tool in the systematic study of higher-order photoabsorption processes, multielectron photoexcitations (MEEs), which were introduced with an experiment at the K edge of argon. Along with noble gases, where MEEs are studied in the pure form, free of the structural signal, measurements have been extended to monatomic metal vapours, although at the cost of considerable experimental difficulties. Several types of absorption cells have been devised, among them the ceramic double cell and the heat-pipe cell for experiments in the low-energy region. In a high-temperature oven with considerable technical refinement, edge profiles of nonvolatile metals were measured up to 2500 K. Edge profiles and MEEs, which are strongly dependent on electron correlation, provide sensitive tests of the theory of atomic and molecular structure.
Keywords: X-ray absorption spectroscopy, absorption cells, gas and vapour samples, multielectron photoexcitation, effects of electron correlation
Published in RUNG: 05.09.2024; Views: 617; Downloads: 4
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Abstract: Photoelectric absorption is characterized by a smooth power-law decrease of the cross section with photon energy. Absorption edges reveal rich structure, which continues into the high-energy side. The quasiperiodic signal, superposed onto the smooth basis, due to scattering of the photoelectron on the neighbours of the target atom provides the basis for the structural (XAFS) analysis of the material. Irregular tiny resonances and edges that appear over the same general range as XAFS are recognized as intra-atomic effects: multielectron excitations (MEE) owing to correlated motion in the electronic cloud. The systematic study of MEE began on noble gases and metallic vapours, both of which are gases of free atoms. With some extremely strong MEE, mostly coexcitations of the subvalence d and f electrons, the structural XAFS analysis may be compromised; hence, there is a need to independently determine the MEE signal, the atomic absorption background (AAB) for the analyzed element, and remove it prior to analysis. In view of the scarcity of elements which can practically be prepared in a free-atom gas state, several approaches to approximate the AAB have been developed: analysis of disordered compounds, where the weak and simple XAFS signal can be modelled and removed, and correlation analysis of the absorption spectra of several independent samples, where the AAB is extracted in an iterative procedure.
Keywords: photoexcitation, XAFS, multielectron excitations, atomic absorption background
Published in RUNG: 15.12.2021; Views: 2437; Downloads: 0
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