The Multiple Origins of Broadening in Near-edge X-ray Spectra of Nitrogen Compounds
John T. Vinson, Terrence J Jach, W. T. Elam, J. D. Denlinger
A comparison of the N Kα emission spectrum of crystalline ammonium nitrate to molecular orbital calculations reveals a unique observation: the peak associated with the σNO recombination is broadened so extensively as to virtually disappear. We have determined the band structure using DFT and calculated radiative transitions from the nitrogen 1s level using a Bethe-Salpeter based method that takes into account electron-hole interactions. A calculation of the self- energy using the GW method reveals an unexpected many-body effect: a large imaginary component of the self-energy associated with the σNO states. Contrasting ammonium nitrate and ammonium chloride x-ray emission spectra shows that the remaining valence band contains no additional anomalies in the self-energy. However, the absorption and emission spectra of both crystals demonstrate a large, state-dependent sensitivity to molecular dynamics. We demonstrate that thermally activated and even zero-point motion of non-activated phonon modes are responsible for most of the density of states effects observed in the spectra. The standard computational approach, using average atomic positions and uniform broadening to account for lifetime and phonon effects, is unsatisfactory.
, , T.
, Elam, W.
and Denlinger, J.
The Multiple Origins of Broadening in Near-edge X-ray Spectra of Nitrogen Compounds, Physical Review B, [online], https://doi.org/10.1103/PhysRevB.90.205207
(Accessed August 10, 2022)