Rovibrational energy transfer in the 4vCH manifold of acetylene, viewed by IR-UV double resonance spectroscopy: kinetics of a collision-induced quasi-continuous background

The 4νCH rovibrational manifold at ~12 700 cm⁻¹ in the electronic ground state of acetylene (C₂H₂) has been studied by time-resolved IR-UV double resonance spectroscopy. An IR laser pulse prepares rotational J-states, associated with the "IR-bright" (ν₁ + 3ν₃) vibrational combination level, and subsequent collision-induced state-to-state energy transfer is probed by pulsed UV laser-induced fluorescence. Anharmonic, ℓ-resonance, and Coriolis couplings affect J-states of interest, resulting in a congested rovibrational manifold with complex intramolecular dynamics. These experiments reveal collision-induced rovibrational satellites, comprising regular even-ΔJ features and odd-ΔJ features that are unexpected in view of the ortho/para nuclear-spin symmetry of C₂H₂. An unusual collision-induced quasi-continuous background (CIQCB) effect is apparently ubiquitous, accompanying regular even-ΔJ rovibrational energy transfer and accounting for much of the observed collision-induced odd-ΔJ satellite structure. This CIQCB phenomenon is examined in terms of a congested IR-dark rovibrational manifold populated by collisional transfer from the nearby IR-bright (ν₁ + 3ν₃) submanifold. A kinetic master-equation model provides a satisfactory phenomenological fit to the processes of interest.