Measurement of Rotational-State Relaxation by Time-Resolved Infrared Double-Resonance Spectroscopy
Many trace species of critical importance in atmospheric processes, notably ozone and methane, can be monitored by remote sensing of their infrared absorption and emission bands. Recovery of accurate column densities and mixing ratios from field measurements requires accurate knowledge of band parameters, viz., assigned line frequencies, line intensities, and pressure-broadened linewidths. The last-named quantities are generally derived from line-broadening models which have been tested against only a limited experimental data base. We have been using time-resolved infrared double-resonance spectroscopy to measure inelastic collision rates, which are a key component of broadening models. A pulsed infrared laser is used to prepare selected rovibrational levels of the species of interest; these are probed by a tunable semiconductor diode laser which provides the resolution necessary for resolution of energy-transfer processes at the state-to-state level. Results obtained for several systems indicate that in many cases the fast-order collision models used to account for rovibrational relaxation and collision broadening are inadequate to describe the inelastic processes in detail, particularly for large J and K changes in collisions.
KeywordsMethane Silane Ozone Coherence Acetylene
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References and Notes
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