Comparison of semiclassical line-shape models to rovibrational H2O spectra measured by frequency-stabilized cavity ring-down spectroscopy

A single-mode cavity ring-down spectrometer, which incorporates a stabilized and tuneable comb of resonant frequencies and a continuous-wave external-cavity diode probe laser, was used to study rovibrational absorption line shapes within the 2 nu(1)+nu(3) and 3 nu(3) vibrational bands of water vapor. This spectrometer, which has a noise-equivalent absorption coefficient of 2x10(-9) cm(-1) Hz(-1/2) and frequency resolution of 50 kHz, enables high-precision measurements of line-shape effects and pressure shifting of relatively weak absorption transitions. We investigated the room-temperature pressure dependence over the range from 0.5 Pa to 50 kPa of two H-2 O-16 transitions perturbed by He, N-2, and SF6. Foreign-gas broadening and pressure-shift coefficients were determined for a relatively strong transition at 10 687.36 cm(-1), and for a weaker transition at 10 834.34 cm(-1) the self- and N-2-broadening and pressure-shift parameters were measured. In the low-pressure limit the room-temperature Doppler width was measured within 0.2% of its expected value. Doppler-free saturation effects were also observed with linewidths below 2 MHz. The data were compared to semiclassical line-shape models that considered the influence of Dicke narrowing as well as the speed dependence of pressure broadening and pressure shifting. Taking both of these effects into account gave the best agreement with our observations and allowed us to model the observed asymmetries of experimental profiles. Hard- and soft-collision as well as billiard-ball collision models were considered. These results allowed us to quantify systematic errors in line intensity and in pressure broadening associated with oversimplified models of line shape.