Rotational energy transfer in D2CO (v4=1): IR–UV double resonance studies of J-changing collisions

Abstract

The technique of time-resolved infrared–ultraviolet double resonance (IRUVDR) spectroscopy is used to characterize the rate and mechanism of state-to-state rotational energy transfer (RET) in D2CO/D2CO collisions. The investigations employ CO2-laser irradiation to prepare a D2CO molecule in the v4=1, (J,Ka) =(18,11) rovibrational level of its X̃ 1A1 electronic ground state. Vapor-phase collisions with other D2CO (v=0) molecules then induce RET, with IRUVDR-monitored quantum-number changes ΔJ for the state-selected molecule ranging between +3 and −7. Kinetic modeling of the resulting experimental data shows that the inelastic cross sections for such J-changing rotational relaxation can be described adequately by simple scaling laws based on the rotational energy change ‖ΔE‖ for the state-selected molecule, with a power-gap fitting law proving marginally superior to an exponential-gap fitting law. The range of ‖ΔJ‖ monitored in these experiments is sufficiently extensive to discredit a simple propensity-rule fitting law, comprising consecutive collision-induced processes with individual changes ‖ΔJ‖ confined to values of 1 or 2. The microscopic rate constants derived reflect the dominance of ΔJ=±1 contributions for J-changing RET in D2CO/D2CO collisions, owing to long-range dipole/dipole interactions. These results elucidate RET in collisions between a pair of dipolar polyatomic (D2CO) molecules at a level of detail usually confined to studies of dipolar diatomic molecules, such as HF. Less detailed IRUVDR results, for RET in self-collisions of HDCO and for D2CO colliding with a variety of foreign-gas molecules, are also presented.