Speed Dependence of Pressure Broadening in Molecular Rotational Spectra Using a Novel Technique

Abstract

The line shape of a mm-wave rotational transition of ${\mathrm{CH}}_3\mathrm{I}$ in an excited vibrational state is observed for molecules in different velocity classes. The collisional self-broadening is measured as a function of relative speed by a novel experimental approach and clear evidence of speed dependence is observed. This result confirms the failure of an oversimplified model, which however is still widely used. We discuss it in detail, and we also show that the rotational temperature of the perturbing molecules has a large influence on the pressure broadening coefficient.