Strengths of Forty-two Lines in the ν_1 and ν_3 Bands of Water Vapor*

The strengths of 12 lines in the ν1 band and 30 lines in the ν3 band of H2O were measured by a curve-of-growth method. The experimental strengths of these infrared lines were compared to strengths calculated from the asymmetric rigid-rotator model of the H2O molecule. In general, the ratio of experimental strength to rigid-rotator strength, varied from less than unity in the R branch to greater than unity in the P branch with intermediate values in the Q branches for both bands, ν1 and ν3. This variation was larger in the ν1 band by a factor of about 3. Superposed on the gradual variation of the ratio, experimental strength/rigid-rotator strength, were erratic changes, in an extreme case resulting in a ratio of 257, due to accidental perturbations of the wavefunctions involved in these transitions. The experimental line strengths of the ν1 band were in good agreement with a recent calculation which includes the effect of centrifugal stretching on the line strengths. A corollary result of the comparison between experiment and the new theory was that the band strength of the ν1 fundamental is 1.33×104 cm−1/g cm−2 at 287.7°K.