Observation of a Speed-Dependent Collisional Inhomogeneity inH2Vibrational Line Profiles

Abstract

We report observations of inhomogeneous broadening of the vibrational line profiles of a gas in the "impact" density regime. In measurements up to 27 amagat (where the spectra are dominated by collision broadening), non-Lorentzian, asymmetric features are observed in Raman $Q$-branch transitions of ${\mathrm{H}}_2$ dilute in a heavy perturber gas. We compare these measurements with an inhomogeneous line-profile model based on collisional shifts that strongly depend on the (thermal) speed of the ${\mathrm{H}}_2$ radiator. Quantitative agreement is obtained only when speed-changing collisions are considered, which result in spectral line narrowing.