Double-electron Excitation Channels At the Br K-edge of Hbr and Br2 Rid C-3277-2009

The x-ray-absorption spectra of gaseous HBr and Br2 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 KN2,3, KN1, KM4,5, and KM2,3 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 chi2 single-scattering signals and the chi4 triple-scattering contribution in the case of Br2. 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