The Continuum of Water Vapor Mixed with Methane: Absolute Absorption at 239 GHz and Linewidth Calculations.

In the aim of achieving a better understanding of the molecular mechanisms involved in the absorption continuum of the water-vapor rotational spectrum, various H(2)O-X mixtures are investigated in an atmospheric window in the mmw range. In the present study, the X mixing gas is methane. Absolute absorption rates have been measured at 239 GHz. The dependence on pressure as well as temperature has been obtained. The experimental data are compared with models using conventional line profiles. As these models require the knowledge of the collisional half-widths of H(2)O broadened by CH(4), theoretical calculations using the complex Robert-Bonamy formalism have been carried out. A "continuum effect" is observed when comparing the experimental absorption with the models, as well as for the magnitude of the absorption discrepancy and for the strong temperature dependence of this absorption. The effect is compared for various X mixing gases to the collisional broadening efficiency in the impact approximation. Copyright 2000 Academic Press.