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 (ΔKa = 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,Ka)-resolved v6 → v4 transfer in D2CO.