Rovibrational energy transfer in the 4v_CH manifold of acetylene, viewed by IR - UV double resonance spectroscopy. 5. Detailed kinetic model

Time-resolved infrared-ultraviolet double resonance '(IR - UV DR) spectroscopy provides a distinctive way to examine collision-induced state-to-state energy transfer between rotational J-levels in vibrational manifolds of small polyatomic molecules, such as acetylene (C₂H ₂) in its electronic ground state X. We consider the 4v_CH rovibrational manifold of C₂H₂ at ∼12 700 cm ^-1, where the principal source of IR-brightness is the (v₁ + 3v₃) or (1 0 3 0 0)⁰ ∑_u⁺ vibrational combination level. In this highly congested manifold, anharmonic, l-resonance, and Coriolis couplings affect the J-levels of interest, implicating them in a complicated variety of intramolecular dynamics. Previous papers of this series have reported several seemingly anomalous J-resolved phenomena induced by collisions in C₂H₂ gas at room temperature with pressures and IR-UV pump-probe delay intervals corresponding to remarkably high Lennard-Jones collisional efficiencies P. odd-ΔJ rotational energy transfer (10^-3 < P < 0.1), in addition to regular even-ΔJ transfer (P ≈ 0.3 for typical |ΔJ| = 2 transfer); particular rovibrational "gateway" channels, such as via (v₁ + 3v ₃) ∑_u⁺ J = 12 (with P as high as ∼0.1); an apparently ubiquitous collision-induced quasi-continuous background (10 ^-3 < P < 0.1) that accounts for much of the observed collision-induced odd-ΔJ satellite structure. These phenomena have been characterized by means of systematic IR - UV DR kinetic measurements, with IR pump and UV probe wavelengths and sample pressure fixed while the IR - UV pump - probe delay is scanned. In this paper, a detailed master-equation model is constructed to provide a satisfactory phenomenological fit to the IR - UV DR kinetic data, thereby offering mechanistic insight. This model includes collision-induced energy transfer between discrete rovibrational levels in an IR-bright manifold V and a quasi-continuous bath B, mediated by a J-specific gateway manifold G.