Infrared water vapor continuum absorption: equilibria of ions and neutral water clusters

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Abstract
The temperature dependence of (Cs0)λ, the self-broadening coefficient for the water vapor continuum absorption at wavelength λ, can be modeled by the equilibrium ion product of water which is tabulated widely in physics texts and handbooks. A theoretical basis for this modeling is developed from water cluster theory, and it is shown that experimental values of (Cs0)λ could be seriously in error, especially at high temperatures, if the saturation ratio (fractional RH) of water vapor is not taken into account by the experimenter for reasons other than normalization of the coefficient for partial pressure of the sample. An explanation is suggested for the departure of (Cs0)λ from the usual negative dependence on increasing water vapor temperature in some experiments. Figures are given showing equilibrium sizes and populations of neutral water clusters and ions in water vapor as functions of humidity and temperature, based in part on data of Bignell and other workers.