Rovibrational Energy Transfer in the 4ν_CHManifold of Acetylene, Viewed by IR−UV Double-Resonance Spectroscopy. 4. Collision-Induced Quasi-Continuous Background Effects

Abstract

The 4nu(CH) rovibrational manifold around 12 700 cm(-1) in the electronic ground state, X, of acetylene (C2H2) is monitored by time-resolved infrared-ultraviolet double-resonance (IR-UV DR) spectroscopy. An IR laser pulse initially prepares rotational J states, associated with the "IR-bright" (nu1 + 3nu3) or (1 0 3 0 0)0 vibrational combination level, and subsequent collision-induced state-to-state energy transfer is probed by UV laser-induced fluorescence. Anharmonic, l-resonance, and Coriolis couplings affect the J states of interest, resulting in a congested rovibrational manifold that exhibits complex intramolecular dynamics. In preceding papers in this series, we have described three complementary forms of the IR-UV DR experiment (IR-scanned, UV-scanned, and kinetic) on collision-induced rovibrational satellites, comprising both regular even-DeltaJ features and unexpected odd-DeltaJ features. This paper examines an unusual collision-induced quasi-continuous background (CIQCB) effect that is apparently ubiquitous, accompanying regular even-DeltaJ rovibrational energy transfer and accounting for much of the observed collision-induced odd-DeltaJ satellite structure; certain IR-bright (1 0 3 0 0)0 rovibrational states (e.g., J = 12) are particularly prominent in this regard. We examine the mechanism of this CIQCB phenomenon in terms of a congested IR-dark rovibrational manifold that is populated by collisional transfer from the nearly isoenergetic IR-bright (1 0 3 0 0)0 submanifold.

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

“…The time dependence of this non-resonant background was consistent with the population of a dense manifold of IRdark/UV-bright states by CET. This phenomenon, described by Payne et al 30 as a collision-induced quasi-continuous background (CIQCB) was also seen in the present study following excitation of the IR-bright (1,0,1,0 0 ,0 0 ) state.…”

“…The (1,0,1,0 0 ,0 0 ) level is only 6560 cm −1 above theXstate zero-point level, so the density of nearby ro-vibrational states was far lower than would be present for the vibrational states examined by Payne et al 30 However, UV probe transitions near 39 700 cm −1 were used, accessing levels of theÃ-state that are around 46 000 cm −1 aboveX (0,0,0,0 0 ,0 0 ). It is feasible that the CIQCB effects seen here resulted from probing congested manifolds within theÃ-state.…”

“…The alternative was that the spectral congestion was associated with a smaller number of ground state levels that have good Franck-Condon access to a dense manifold ofÃ-state levels. While the data did not provide a means for discriminating between these two models, Payne et al 30 suggested that the former was more probable. Note that the probe laser transitions used in their study were at energies near 30 800 cm −1 , such that the region of theÃ-state accessed was ∼43 500 cm −1 above the ground state zero-point level.…”

“…In considering the origin of the CIQCB in their studies of vibrational levels near 12 700 cm −1 , Payne et al 30 proposed two possible mechanisms. The first idea was that vibrational relaxation occurred to a manifold of nearby ro-vibrational levels that was dense enough to yield a UV excitation spectrum that could not be resolved by their probe laser.…”

“…Payne et al 30 encountered complications associated with the relative detection efficiencies in their studies of vibrational levels at energies near 12 700 cm −1 (members of the [4,20,0] polyad). Following excitation of levels that were predominantly C-H stretch modes, they observed UV probe spectra that included an effectively continuous background signal.…”