Absolute absorption rates of pure water vapor and mixtures of water vapor and nitrogen have been measured in the atmospheric window at 239 GHz. The dependence on pressure as well as temperature has been obtained. The experimental data are compared with several theoretical or empirical models, and satisfactory agreement is obtained with the models involving a continuum; in the case of pure water vapor, the continuum contribution based upon recent theoretical developments gives good results. The temperature dependence is stronger than that proposed in a commonly used atmospheric transmission model.
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A theory is presented for the calculation of the continuous absorption of water molecules in the millimeter spectral region. The theory is based on a generalization of Fano's theory in which the spectral density, the Fourier transform of the dipole-moment correlation function, is calculated for a system consisting of a pair of molecules. The internal states are written in terms of the line space of the system, and the resolvent operator is obtained using the well-known Lanczos algorithm. For the interaction between two water molecules, we include only ~he lead~ng dipole-dipole term of the long-range anisotropic potential, and model the isotropic mteractlOn, used to calculate the statistical weight within the quasi-static approximation, by a Lennard-Jones potential. Using reasonable values for the two Lennard-Jones potential parameters, and the known rotational constants and permanent dipole moment of a water molecule, we calculate the absorption coefficient for frequencies up to 450 GHz for temperatures between 282 and 315 K. The present results are in good agreement with an empirical model for the water continuum based on combined laboratory and atmospheric measurements. We conclude from our results that, contrary to some previous assertions, the strong negative temperature dependence as well as the magnitude of the continuum absorption, at least for the millimeter spectral region, can be explained in terms of the far-wings of allowed rotational transitions.
