Double-electron excitation channels at the BrKedge of HBr andBr2

Abstract

The x-ray-absorption spectra of gaseous HBr and ${\mathrm{Br}}_2$ at the Br K edge have been studied in order to evidence the contribution of double-electron excitation channels to the atomic-absorption background. Evident features due to the simultaneous excitations of 1s4p, 1s4s, 1s3d, and 1s3p electrons, named ${\mathit{KN}}_{2,3}$, ${\mathit{KN}}_1$, ${\mathit{KM}}_{4,5}$, and ${\mathit{KM}}_{2,3}$ channels, respectively, have been identified in both spectra. The data analysis takes proper account of the structural signal of the neighboring atom in the molecules calculated including the ${\mathrm{\chi }}_2$ single-scattering signals and the ${\mathrm{\chi }}_4$ triple-scattering contribution in the case of ${\mathrm{Br}}_2$. Comparisons of energy positions with self-consistent atomic calculations are in excellent agreement with the observed energy onsets. An empirical model function for the double-electron-channel cross section was found to reproduce accurately the observed shapes, the threshold region included. The proper account of both double-electron channels and structural signals allows the correct explanation of the experimental spectra. The fit of the experimental spectra provided a determination of the structural parameters in excellent agreement with the known molecular values. These results are of general importance for structural studies at the Br K edge.