We present experimental and theoretical studies of medium infrared absorption by pure water vapor. Measurements have been made in the 1900–2600 cm-1 and 3900–4600 cm-1 regions, for temperatures and pressures in the 500–900 K and 0–70 atm ranges, respectively. They are consistent with available data and enable the determination of continuum absorption parameters. It is shown that calculations with line shapes derived from the impact approximation are very inaccurate. Models accounting for the finite durations of collisions and line-mixing through wave-number dependent effective broadening parameters are introduced. The latter have been determined using two different approaches, which are (i) empirical determinations from fits of experimental data and (ii) direct predictions from first principles using a statistical approach. Effective broadening parameters obtained using these two different approaches are in satisfactory agreement for both the temperature and wavenumber dependencies. These data are tested by calculations of continua in various spectral regions and the agreement with measured values is satisfactory. The remaining discrepancies probably result from the influence of the internal structures of the absorption bands considered and thus from the influence of line-mixing. Nevertheless, accurate predictions are obtained in wide temperature and spectral ranges when the total absorption at elevated density is considered. This agreement, which is due to the relatively weak continuum absorption and large contributions of nearby lines, makes the present models suitable for most practical applications involving elevated densities.
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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.
