Pressure-Induced Molecular Absorption in Stellar Atmospheres

Abstract

Pressure-induced absorption arises in complexes of two or more inert atoms or molecules, due to dipole moments induced during the collisional interaction. The term “pressure-induced” still prevails in the astrophysical literature, yet “collision-induced” absorption (CIA), or “interaction-induced” absorption seems more appropriate and is commonly used elsewhere. Ordinary absorption processes in the infrared arise from individual, polar molecules interacting with electromagnetic radiation. As a consequence, the intensity of the allowed lines increases linearly with density. CIA, on the other hand, is most striking in gases composed of nonpolar, infrared-inactive molecules. Induced spectral lines are observed at rovibrational frequencies which are dipole-forbidden in single (i.e. non-interacting) molecules. Dipole transitions may, however, beinducedin the interacting pair. The new symmetry of the electronic cloud of a collisional complex may be very different from those of the isolated molecules and thus commonly allows for a transient dipole, which then interacts with radiation. Collision-induced absorption increases quadratically in the low density limit, thus reflecting the two-body origin of the basic absorption process. At higher gas densities, ternary interactions become significant and cubic and higher-order contributions to the observable absorption are then commonly seen.