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.
The Journal covers the entire field of infrared physics and technology: theory, experiment, devices and instrumentation. Its core topics can be summarized as the generation, propagation and detection, of infrared radiation; the associated optics, materials and devices; and its use in all fields of science, industry and medicine. Infrared techniques occur in many different fields, notably spectroscopy and interferometry; material characterization and processing; atmospheric physics, astronomy and space research. Scientific aspects include lasers, quantum optics, quantum electronics and semiconductor physics. Some important applications are medical diagnostics and treatment, industrial inspection and environmental monitoring. A fuller though not exhaustive list of topics would include:
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