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

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/D ₂CO collisions. The investigations employ C0₂-laser irradiation to prepare a D₂CO molecule in the v₄ = 1, (J,K_a) = (18,11) rovibrational level of its X̃¹A ₁ electronic ground state. Vapor-phase collisions with other D ₂CO (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 (D ₂CO) 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.