Infrared HCN Lineshapes as a Test of Galatry and Speed-Dependent Voigt Profiles

Absorption lineshapes of the ν₂ infrared band of HCN in collision with He, Ne, Ar, Kr, Xe, N₂, HCN, and CH₃Br have been considered. Observed lineshapes differ significantly from the usual Voigt profile and are characteristic of line narrowing processes. The Galatry profile fairly reproduces lineshapes observed in the low-pressure regime, but leads to unexpected nonlinear behavior of optical diffusion rates in the high-pressure regime and strongly fails with the heaviest perturbers. On the other hand, fair results are obtained for all experimental conditions by using a speed-dependent Voigt profile that includes a pressure-dependent narrowing parameter. These observations are discussed by consideration of the polarization correlation functions related to considered line profiles and of the optical and kinetic radii involved in collision processes. It is shown that observed line narrowings primarily result from the dependence of relaxation rates on molecular speeds and that diffusion processes show up with the two lighter perturbers only, He and, to a small extent, Ne. Finally, it can be claimed that, for atmospheric applications, deviations from the Voigt profile must be taken into account by a speed-dependent Galatry profile that reduces to a speed-dependent Voigt profile in most cases.