Theory of rovibrational line intensities in allowed and collision-induced absorption spectra of linear molecules

Abstract

A theoretical approach is developed for the Herman-Wallis factors describing the variation of the line intensities caused by rovibrational interaction in a linear molecule. In the case of ${\ensuremath{\nu}}_{2}$ and ${\ensuremath{\nu}}_{3}$ fundamentals and the ${\ensuremath{\nu}}_{1}+{\ensuremath{\nu}}_{3}/2{\ensuremath{\nu}}_{2}+{\ensuremath{\nu}}_{3}$ resonance combination band in a ${\mathrm{CO}}_{2}$ molecule, our theory is capable of generating reliable numerical estimates for the linear Herman-Wallis coefficients without recourse to complicated algebra. On the assumption of nearly free rotating monomers, an analog of the Herman-Wallis factors is introduced for the dipole-forbidden ${\ensuremath{\nu}}_{2}+{\ensuremath{\nu}}_{3}$ band, which manifests itself in ${\mathrm{CO}}_{2}$ collision-induced absorption (CIA). The rovibrational perturbation is appropriate to explain a pronounced crossflow of intensity among rovibrational branches in a virtually rotationally unresolved CIA envelope.