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?1+?3 and 3?3 vibrational bands of water vapor. This spectrometer, which has a noise-equivalent-absorption coefficient of 2x10-9cm-1Hz-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 0.5 Pa to 50 kPa of two H216O transitions perturbed by He, N2 and SF6. Foreign-gas broadening and pressure-shift coefficients were determined for a relatively strong transition at 10687.36 cm-1, and for a weaker transition at 10834.34 cm-1 the self- and N2 - 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 line-widths below 2 MHz. The data were compared to semi-classical 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 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.
Citation: Physical Review A (Atomic, Molecular and Optical Physics)
Issue: No. 1
Pub Type: Journals
cavity ring-down, line intensity, lineshape, pressure broadening, spectroscopy, water vapor