Formation of spectral lines in planetary atmospheres—V. Collision narrowed profiles of quadrupole lines in hydrogen atmospheres

Abstract

The interpretation of a hydrogen quadrupole spectrum is complicated by the collision narrowing phenomenon exhibited by these lines. The profile must be accurately computed for use within any interpretative technique since the spectroscopic features of line profile and center to limb variation of the equivalent width are sensitive to the quadrupole line shape. The Dicke profile, which assumes that the quadrupole line shape is Lorentzian with a half-width that varies inversely as the pressure has been suggested as a simple alternative to the Galatry profile. In this paper, we present comparisons of the line profile, equivalent width and center-to-limb variation of the equivalent width of quadrupole lines for a characteristic scattering and nonscattering hydrogen atmosphere, where the line profile has been computed by the Dicke and Galatry shapes. Our computations show that the results produced by the Dicke profile may be very different from the features predicted by the Galatry profile, which has been shown to give results in good agreement with experiment. We therefore conclude that it is invalid to use the Dicke approximation for computing collision-narrowed profiles of hydrogen quadrupole lines at visible and near infrared spectroscopic wavelengths in any hydrogen atmosphere.