Rovibrational energy transfer in the 4ν_CH manifold of acetylene, viewed by IR-UV double-resonance spectroscopy. 4. Collision-induced quasi-continuous background effects

The 4ν_CH rovibrational manifold around 12 700 cm^-1 in the electronic ground state, X̃, of acetylene (C₂H ₂) 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" (ν₁ + 3ν₃) 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-ΔJ features and unexpected odd-ΔJ features. This paper examines an unusual collision-induced quasi-continuous background (CIQCB) effect that is apparently ubiquitous, accompanying regular even-ΔJ rovibrational energy transfer and accounting for much of the observed collision-induced odd-ΔJ 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.