Integrated Absorptances of Spectral Line Groups in the 2.7-μ Bands of Hot Water Vapor, Including Effects of Centrifugal Distortion

Integrated absorptances of two groups of H2O lines, centered at 3504 cm−1 (principal transition, 11−11←12−12) and 3990 cm−1 (principal transition, 14−14←13−13), were calculated and measured. By matching the calculated and experimental integrated absorptances, we determined the band strength Sν of the asymmetric stretch vibration (ν3) and a vibration—rotation interaction parameter c, where S = Sr (1—cm), and S and Sr are the true and rigid rotator line strengths, respectively. The calculated and measured integrated absorptances at 1273°K agreed to within ±10% when we took Sν as 243±24 cm−1/atm·cm at 273°K and c as 0.0433. Using these parameters, the experimental integrated absorptances for the two groups of lines were successfully predicted to within an average of ±7% for temperatures between 637° and 1630°K. The value of c determined provides the first quantitative estimate for the intensity perturbation due to the vibration—rotation interaction in the ν3 fundamental of H2O. It predicts that, neglecting the population, a P-branch line and R-branch line with the same J and τ will not have approximately equal intensities (as the rigid rotator predicts), but that for J ≈ 10 the P-branch line will have three times the intensity of the corresponding R-branch line.