Rotationally specific mode-to-mode vibrational energy transfer in D₂CO/D₂CO collisions. I. Spectroscopic aspects

Time-resolved infrared-ultraviolet double-resonance (IRUVDR) spectroscopy is used to look for rotationally specific channels in collision-induced vibrational energy transfer between the v₆ and v₄ modes of D₂CO. The efficiency of such V-V transfer has been shown in previous work to be enhanced, by a combination of Coriolis coupling and rotor asymmetry. IRUVDR spectra, recorded in pure D₂CO vapor with a range of delay intervals between IR pump and UV probe laser pulses, reveal (J,K _a)-dependent propensities in the resulting v₆ → v₄ transfer arising from D₂CO/D₂CO collisions. At the same time, rotational relaxation within the rovibrational manifold (v₆ = 1) initially prepared by the IR pump laser is found to be more pronounced than the growth of population in the neighboring v₄ = 1 manifold, due to v₆ → v₄ transfer. This trend is shown to be reversed in the case of D₂CO/N₂O collisions, where the effects of rotational relaxation appear to be less pronounced than those of v₆ → v₄ transfer. This work, performed with spectroscopic resolution superior to that in previous investigations, has demonstrated a number of new effects, including the identification of weakly allowed t-type (ΔK_a = 3) features in the IRUVDR spectra. It also provides the spectroscopic background to paper II of this series, which explores the detailed kinetics of (J,K_a)-resolved v₆ → v₄ transfer in D₂CO.