Water cluster interpretation of IR absorption spectra in the 8–14-μm wavelength region

Abstract

Recently various investigators have suggested that the anomalous atmospheric absorption spectra in the window wavelength region of 8–14 μm is due to the presence of the water dimer or other water clusters. This suggestion has been made on the grounds that the IR absorptivity has a dependency on the squared value of the partial water vapor pressure or nonlinear vapor pressure and negative temperature dependence, and that the effect of foreign broadening is negligible compared with that of self-broadening. Here we present a theoretical study of the IR continuous absorption by the water dimer in the path containing pure water vapor. In this paper we report our computed results of the intermolecular normal vibrational frequencies, concentrations, and absorption coefficients of the water dimer in the gas phase, focusing our primary attention to the IR absorption in the 8–14-μm wavelength region. Our analysis indicates that the dimer may be an important contributor to the IR absorption in the above mentioned spectral range, and that this region corresponds to the wing of the vibrational transition band associated with an intermolecular librational motion of the water dimer.