Effect of velocity on saturation behavior of HF molecular beams

Infrared absorption of a HF molecular beam is measured by a bolometric detection system. With ∼50-80 mW of laser power available from a single-mode stabilized chemical laser, HF vibrotational transitions in the 2.5μm spectral region are shown to be almost completely saturated. The observed saturation behavior is well characterized by using a rate equation approach. In one particular limit, it is shown that the factor governing the extent of saturation is the time required for equilibration between spontaneous and stimulated emission processes, relative to the transit time of the molecules through the laser beam. Measurements for different HF beam velocities substantiate this dependence. Saturation of the transitions greatly simplifies determination of rotational state distributions and affords the highest sensitivity available for the present apparatus, equivalent to ∼5 ⨯ 10⁴ HF molecules/s per quantum state.