Q-branch spectral line shapes of D2 in foreign gases

Abstract

The collisional Dicke narrowing and broadening of spectral lines with foreign gas perturbers has been studied by measuring the third-order nonlinear susceptibility of isolated Q-branch (0-1) D2 lines. Perturber gas mass has been varied by employing dilute (5%) binary mixtures of D2 in H2, He, N2, or Ar, and densities at 296 K were varied from 0.25 to 5.7 amagat. We have used these data to test the validity of soft and hard collision models for describing the spectral line shape and the linewidth density dependence. At high densities, the data are well described by Lorentzian line shapes and a Dicke diffusion expression for the density dependence of the lindwidth. At lower densities, the line shape functions were fit to the data while fixing both the pressure broadening parameters to their high density values and the narrowing parameters to the values predicted from diffusion theory. For the light perturbing gases (H2, He), only the soft collision model fit the line shape and gave agreement with the observed linewidth. For the heavy perturber gases (N2, Ar), neither model described the line shape or the linewidth dependence, the disagreement being greater for Ar.