Rovibrational Energy Transfer in the 4ν_CH Manifold of Acetylene, Viewed by IR−UV Double Resonance Spectroscopy. 5. Detailed Kinetic Model

Abstract

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 (C2H2) in its electronic ground state X. We consider the 4nuCH rovibrational manifold of C2H2 at approximately 12,700 cm(-1), where the principal source of IR-brightness is the (nu1+3nu3) or (1 0 3 0 0)0 Sigma+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-DeltaJ rotational energy transfer (10(-3)<P<0.1), in addition to regular even-DeltaJ transfer (P approximately 0.3 for typical |DeltaJ|=2 transfer); particular rovibrational "gateway" channels, such as via (nu1+3nu3) Sigma+u J=12 (with P as high as approximately 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-DeltaJ 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.

Rotational and vibrational energy transfer in vibrationally excited acetylene at energies near 6560 cm−1

“…25,31,35 The rate constants for removal of population from a single ro-vibrational level are large, typically indicative of transfer occurring at more than 1.5 times the Lennard-Jones collision frequency. Rotational energy transfer (RET) dominates and usually accounts a) Author to whom correspondence should be addressed.…”