New measurements of the water vapor continuum in the region from 0.3 to 2.7 THz

Abstract

We present a spectroscopic study of the water vapor continuum absorption in the far-IR region from 10 to 90 cm −1 (0.3–2.7 THz). The experimental technique combines a temperature-stabilized multipass absorption cell, a polarizing (Martin–Puplett) interferometric spectrometer, and a liquid-He-cooled bolometer detector. The contributions to the absorbance resulting from the structureless H 2O–H 2O and H 2O–N 2 continua have been measured in the temperature range from 293 to 333 K with spectral resolution of 0.04–0.12 cm −1. The resonant water vapor spectrum was modeled using the HITRAN04 database and a Van Vleck–Weisskopf lineshape function with a 100 cm −1 far-wing cut-off. Within experimental uncertainty, both the H 2O–H 2O and H 2O–N 2 continua demonstrate nearly quadratic dependencies of absorbance on frequency with, however, some deviation near the 2.5 THz window. The absorption coefficients of 3.83 and 0.185 (dB/km)/(kPa THz) 2 were measured for self- and foreign-gas continuum, respectively. The corresponding temperature exponents were found to be 8.8 and 5.7. The theoretically predicted foreign continuum is presented and a reasonable agreement with experiment is obtained.