Below-Room-Temperature Water-Vapor Continuum Absorption within the 8- to 12-Micrometer Atmospheric Transmission Window.
AEROSPACE CORP EL SEGUNDO CA CHEMISTRY AND PHYSICS LAB
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Described are the results of a laboratory study of the weak water-continuum absorption within the 8- to 12-micrometer atmospheric transmission window. The highly sensitive photoacoustic detection technique was used in conjunction with a line-tunable CO2 laser to measure water continuum absorption throughout the CO2 laser 10.4-micrometer band. These measurements were performed over a range of atmospheric temperatures and water pressures important to the attenuation of radiation for long-slant atmospheric transmission paths. Water continuum CO2 laser absorption spectra are reported for temperatures between 27 and -10 deg C. Below room temperature, spectra of this type have not been determined previously. The water continuum absorption was found to possess a negative temperature dependence between 27 and -10 deg C that is similar to that observed between 100 and 27 deg C by previous investigators. The temperature, pressure, and wavelength dependences observed for the 8- to 12-micrometer water continuum in this study, and in previous studies reviewed thoroughly here, are best modeled by assuming simultaneous contributions of water-dimer and collisional-broadening mechanisms to the continuum. At room temperature and below, the water dimer mechanism becomes more important than the collisional-broadening mechanism.