Sideband modeling in molecular crystals N2 and CO2

Abstract

Vibron-phonon excitation bands, phonon sidebands to the zero phonon line, mirror the one-phonon density of states g(ω) calculated in the harmonic approximation. The origin of bands in the sideband is investigated, e.g., phonon contributions from librational and translational modes or phonons at special points of the Brillouin zone. The temperature-dependent structure of the sideband (frequency shifts, line broadening) is due to anharmonic processes which modify g(ω): Temperature-dependent frequency shifts of maxima in the sideband are shown to depend mainly on the volume effect, whereas line broadening is due to phonon–phonon interactions, which are simulated in the lattice dynamics calculations of g(ω) by a special modeling procedure. Compared to the CO2 solid, effects are more pronounced in the N2 crystal due to the presence of strong mechanical and electrical anharmonicities. The latter give rise to multiphonon contributions in the vibron-phonon excitation process. Intensity changes with temperature can be explained by the thermal weighting of the one-phonon density of states.