Laser-photoacoustic spectroscopy of water-vapor continuum and line absorption in the 8 to 14 μm atmospheric window

Abstract

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 CO 2-laser wavelengths. We have assigned four weak absorption lines which occur at the CO 2-laser emissions 10P(40), 10R(20), 9P(38) and 9R(36) to pure rotational transitions of H 2O, 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 N 2 or synthetic air. The observed changes are explained by UV-photodissociation of H 2O 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.