Spectroscopy of atoms and molecules in gases: Corrections to the Doppler-recoil shift

We analyze the excitation by an electromagnetic wave of a two-level molecule (or atom) within a rarefied gas. This process differs from the excitation of an isolated molecule because there is significant interaction between the molecule and the surrounding gas particles as they are all polarized by the electromagnetic wave. These interactions lead to corrections to the recoil and Doppler shifts of the molecule's transition frequency. We propose an experiment using saturation spectroscopy to observe sodium molecular transitions near the atomic $D$ lines which could easily detect the correction to the first-order Doppler shift caused by the interaction between the sodium molecules and the surrounding atomic vapor.