Absorption of electromagnetic radiation by pure water vapor at frequencies near 1.5 THz

Abstract

The coefficient of absorption of pure water vapor is measured in the window of its relative transparency at wavelengths λ ≈ 196−205μm. The measurements were carried out using an echelette vacuum spectrometer with a spectral resolution of about 0.8 cm −1 at a temperature of 304 K in the pressure interval from 12.5to 28.5Torr. The data obtained in combination with the results of previous spectrometric measurements of absorption in a mixture of water vapor and dry air performed in this window (at the atmospheric pressure of the mixture) indicate that the component of the absorption coefficient of the atmospheric water vapor, which depends quadratically on its partial pressure, is 26% smaller than the corresponding component of the absorption coefficient of pure water vapor. Unlike in the long-wave submillimeter transparency windows, where the discrepancy of the above components was first found, in the short-wave window centered at λ=200μm the percentage difference in their values is approximately two times less.