A memory function approach to the shape of pressure broadened molecular bands

Abstract

A unified treatment of the shape of pressure-broadened molecular absorption bands from near resonance to the far wings is achieved by an approach using memory functions. Empirical models for these functions are employed that interpolate between known short and long time behaviour. This treatment includes the following band shaping mechanisms: line interference, finite duration of collision, molecular torques, and detailed balance. Line interference can produce a very sharp decrease in absorption in the periphery of a band, e.g., as in the v 3 band of CO2. Absorption in the far wings, which in this band exhibits a much less rapid decrease with increasing frequency, is attributed to the effect of molecular torques acting during collisions. By using a model memory function, a simple analytical expression is obtained for the far wing absorption. The parameters in this model are related to known molecular quantities. The computed and measured far wing absorption of the v 3 band of CO2 broadened by argon at 296 K and 613 K are compared.