Origins of extreme broadening mechanisms in near-edge x-ray spectra of nitrogen compounds

Abstract

We demonstrate the observation of many-body lifetime effects in valence-band x-ray emission. A comparison of the N $K\ensuremath{\alpha}$ emission of crystalline ammonium nitrate to molecular-orbital calculations revealed an unexpected, extreme broadening of the NO $\ensuremath{\sigma}$ recombination---so extensively as to virtually disappear. GW calculations establish that this disappearance is due to a large imaginary component of the self-energy associated with the NO $\ensuremath{\sigma}$ orbitals. Building upon density-functional theory, we have calculated radiative transitions from the nitrogen $1s$ level of ammonium nitrate and ammonium chloride using a Bethe-Salpeter method to include electron-hole interactions. The absorption and emission spectra of both crystals evince large, orbital-dependent sensitivity to molecular dynamics. We demonstrate that many-body effects as well as thermal and zero-point motion are vital for understanding observed spectra. A computational approach using average atomic positions and uniform broadening to account for lifetime and phonon effects is unsatisfactory.