The statistical theory proposed by Rosenkranz to calculate the continuous absorption by water molecules in the high-frequency (infrared) wing of the pure rotational band is reviewed and extended. In the review there is a discussion, in particular, of the approximations that are made, including those that are necessary and which limit the applicability of the theory to other spectral regions, and those that are made for calculational convenience. Then, several extensions to the theory are discussed, including increasing the number of rotational states used to calculate the band-average relaxation parameter, modifying the definition of this parameter to account for near-wing effects, and eliminating the boxcar approximation. This last modification, effected by using asymmetric-top functions instead of symmetric-top functions to calculate matrix elements of the density operator and to diagonalize the dipole-dipole interaction, results in significant enhancement of the relaxation parameter. This improvement, in turn, allows one to eliminate an inconsistency in the original formulation of Rosenkranz while obtaining substantially the same final results. The implications of the present results for the calculation of the absorption in the high-frequency wing of the V 2 fundamental vibration-rotational band of H 2 0 are discussed briefly.
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The continuum absorption by H2O has several characteristics that are common throughout the windows in the infrared and millimeter-wave regions. Values of the continuum absorption coefficient calculated on the basis of simple, widely used line shapes may differ greatly from observed values in the windows between strong absorption lines. The temperature dependence of this absorption is also not predictable from present day understanding of line shapes or of dieters, which may also contribute. The shapes of self-broadened H2O lines are quite different from those of N2-broadened lines, and the difference increases with increasing distance from the centers of the lines. Data obtained from laboratory samples and from atmospheric paths are presented to compare the various windows in the infrared and millimeter regions.
