Calculation of Far Wings of Allowed Spectra: The Water Continuum

Abstract

A far-wing line shape theory based on the binary collision and quasistatic approximations that is applicable for both the low- and high-frequency wings of allowed vibration-rotational lines has been developed. This theory has been applied in order to calculate the frequency and temperature dependence of the continuous absorption coefficient for frequencies up to 10 000 cm-1 for pure H2O and for H2O-N2 mixtures. The calculations are made assuming an interaction potential consisting of an isotropic Lennard-Jones part and the leading long-range anisotropic part, and utilizing the measured line strengths and transition frequencies. The results compare well with existing data, both in magnitude and in temperature dependence. This leads us to the conclusion that although dimer and collision-induced absorptions are present, the primary mechanism responsible for the observed water continuum is the far-wing absorption of allowed lines. Recent progress on near-wing corrections to the theory and validations with recent laboratory measurements are discussed briefly.