This is a comprehensive study of water-vapor line and continuum absorption in the 8–14μm atmospheric window by laser-photoacoustic spectroscopy. The characteristics of laser-photoacoustic spectroscopy and detectors are discussed with results on continuum and line absorption at selected CO2-laser wavelengths.
We have assigned four weak absorption lines which occur at the CO2-laser emissions 10P(40), 10R(20), 9P(38) and 9R(36) to pure rotational transitions of H2O, and have determined the dependence of the continuum water-vapor absorption over the temperature range +70°C and −20°C. The measured negative temperature coefficient of the continuum is consistent with both monomer and dimer models, yet not with predictions of larger water clusters.
Experiments with supersaturated water vapor indicate that for S ⩾ 1 collision broadening of distant strong lines as well as water dimer absorption contribute to the continuum. However, the dimer absorption is an order of magnitude too small to cause a significant contribution at ambient atmospheric conditions.
We have investigated the effect of UV-radiation on the 8–14μm absorption of water vapor, buffered either with N2 or synthetic air. The observed changes are explained by UV-photodissociation of H2O molecules and by ozone production. There is no evidence in favor of a cluster model.
Finally, we compared our measured spectra with LOWTRAN 6 and HITRAN models. The LOWTRAN yields a stronger negative temperature dependence than observed while the HITRAN does not predict the observed continuum absorption.
Recently various investigators have suggested that the anomalous atmospheric absorption spectra in the window wavelength region of 8-14 mm 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-,um 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.
