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 H₂O–H₂O and H₂O–N₂ 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 H₂O–H₂O and H₂O–N₂ 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.

Podobedov V.B., Plusquellic D.F., Siegrist K.M., Fraser G.T., Ma Q., Tipping R.H. Continuum and magnetic dipole absorption of the water vapor-oxygen mixtures from 0.3 to 3.6 THz. J. Molec. Spectrosc., 2008. V.251, 203-209, doi:10.1016/j.jms.2008.02.021.

The measurement of absorbance in water vapor-oxygen mixtures is reported for the far-IR region from 10 to 120 cm^-1 (0.3–3.6 THz). The experiments were performed in a temperature-stabilized multipass absorption cell coupled to a far-infrared Fourier transform spectrometer with a liquid-He-cooled bolometer detector. The absorbance components due to both the H₂O–O₂ continuum and the oxygen magnetic dipole discrete lines have been measured in the temperature range from 294 to 333 K with a spectral resolution of 0.03 to 0.07 cm^{-1. In the range up to 2.5 THz, the H}₂O–O₂ continuum demonstrates a nearly quadratic dependence of absorbance on frequency, while in the 3.43 THz window deviation from this dependence was detected. The series of rotational lines associated with magnetic dipole transitions was measured in pure oxygen. In the mixture of 1.36 kPa of water vapor and 79.2 kPa of oxygen, a comparable contribution from continuum and discrete lines associated with magnetic dipole transitions of O₂ was observed. The absorption coefficient of 0.066 (dB/km)/(kPa THz)² and its temperature exponent of 4.7 were measured for the H₂O–O₂ continuum. Experimental continua data compared to theoretically predicted values exhibit good agreement. The modeling of the resonant water vapor spectrum was performed using a Van Vleck–Weisskopf lineshape with a 215 cm^-1 far-wing cut-off and the HITRAN2004 database.