The Hydrogen and Helium Pressure Broadening at Planetary Temperatures of the 183 and 380 GHz Transitions of Water Vapor

The pressure broadening of the 3 1,3-2 2,0 and 4 1,4-3 2,1 transitions of water at 183 and 380 GHz, respectively, has been obtained experimentally in the temperature region between 80 and 600 K using hydrogen (H 2) and helium (He) as broadening gases. This extends for the first time the study of condensable gases in general and water in particular into the temperature regime typical of the atmospheres of the outer planets. Above 250 K the measurements were taken in a conventional equilibrium cell. Low temperature measurements were taken in a collisionally cooled cell which can provide a laboratory environment very similar to that of the atmospheres of the outer planets. For the lines broadened by He, data were found to fit to the usual power law for the entire temperature range studied with resultant temperature exponent n values of 0.49 ± 0.02 and 0.54 ± 0.03, respectively. For the H-broadened lines the data above 150 K were found to fit to the power law with resultant n values of 0.95 ± 0.07 and 0.85 ± 0.05, respectively. Below 150 K the H 2 pressure broadening parameters were measured to have smaller values than predicted by the relation. The results are compared with earlier experimental and theoretical work.