Rotovibrational collision-induced absorption by nonpolar gases and mixtures (H2-He pairs): Symmetry of line profiles

Intermolecular-interaction potentials depend on the vibrational coordinates of the molecules involved. We study the effect of this v dependence (as we will call it for brevity) on the symmetry of line shapes of rotovibrational collision-induced-absorption (RVCIA) spectra of collisional complexes such as ${\mathrm{H}}_2$-He or ${\mathrm{H}}_2$-${\mathrm{H}}_2$. If the v dependence is ignored, individual line shapes Γ(ω) of CIA spectra satisfy the widely used ‘‘detailed balance’’ relationship Γ(-ω)=$e^{\mathrm{-}ħ\omega /\mathrm{kT}}$Γ(ω), where ω designates the frequency shift relative to the molecular transition frequency, and T the temperature. However, if one accounts for the v dependence, the symmetry of a computed profile is modified significantly, the more so the higher the temperature and the higher the vibrational overtones one considers. These differing symmetries are described in quantitative terms that are of interest in modeling or analyzing RVCIA spectra. This paper may be considered the second, concluding part of our theoretical study of the influence of the v dependence on RVCIA spectra; the previous part [Phys. Rev. A 36, 4700 (1987)] deals with the influence of the v dependence on the integrated intensities (spectral moments) of RVCIA spectra.