Dispersion studies of the 22 GHz water vapor line shape: I. The Lorentzian behavior

Abstract

The line shape of the self-broadened rotational (5 −1 → 6 −5) transition of water vapor at v 0 = 22.235 GHz has been investigated on the basis of resonance dispersion information. Detailed information on absolute intensity, width, shift, and wing response was obtained without molecular effect modulation. A differential refractometer with one dual-mode cavity sensor was operated in the pressure range 5 × 10 −4 to 20 torr at 27°C. The dispersion was measured at fixed frequencies ( v 0±250 MHz) with slowly varying vapor pressure. In this first part of the study the results obtained for pressures above 0.1 torr are presented. For pressures higher than 2 torr, a Lorentzian line shape was confirmed with a constant maximum dispersion of ±0.387 × 10 −6. Down to 0.1 torr, the line center displayed Lorentzian behavior with the pressure-linear width of 17.99 MHz/torr and a violet shift of 1.38 MHz/torr, but the wing dispersion dropped more rapidly with decreasing pressure while the maximum dispersion increased. The Lorentzian line strength was determined and found to be in good agreement with the theoretical value as determined by the transition probability for the asymmetric rotor. The experimental strength yielded a maximum linear power absorption coefficient of 7.21 × 10 −6 cm −1.