The νCC+3νCH rovibrational manifold of acetylene. I. Collision-induced state-to-state transfer kinetics

Angela P. Milce, Brian J. Orr

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    Infrared-ultraviolet double resonance (IR-UV DR) spectroscopy is used to measure collision-induced, rotationally resolved state-to-state energy transfer in the νCC+3νCH vibrational manifold of gas-phase acetylene, C2H2. Attention focuses on three sets of vibrational eigenstates spectroscopically labeled (0 1 3 0 0)0 I, (0 1 3 0 0)0 II, and (0 4 0 3 3)0 +, with vibrational term energies in the region 11 585-11 600 cm-1. IR-UV DR spectroscopy identifies the channels of J-changing rotational energy transfer (RET) and inter-mode vibrational (V-V) transfer. The second-order state-to-state kinetics of these channels is measured by scanning the IR-UV pulse delay with the IR PUMP and UV PROBE lasers tuned to particular spectroscopic features. There is a clear propensity for even-numbered changes ΔJ of rotational quantum number in the observed RET and V-V transfer, consistent with conservation of ortho or para nuclear-spin symmetry in the state-selected C2H2 molecule, but there are some notable exceptions as unusual symmetry-breaking processes result in odd-ΔJ V-V transfer. Anomalies of this type have been investigated in a preceding paper [A. P. Milce and B. J. Orr, J. Chem. Phys. 104, 6423 (1996)]. A detailed IR-UV DR study is made with the UV PROBE laser monitoring the (0 4 0 3 3)0 +, J=12 rovibrational level; RET is measured when the IR PUMP laser prepares molecules in various J-states of (0 4 0 3 3)0 +, while V-V transfer is monitored when initial J-states of (0 1 3 0 0)0 I or (0 1 3 0 0)0 II are prepared by the IR PUMP. The corresponding IR-UV DR kinetic curves are fit to a detailed rate-equation model in which empirical exponential-gap fitting laws are used to describe even-ΔJ channels of RET and V-V transfer. It is remarkable that the kinetics of symmetry-breaking odd-ΔJ V-V transfer between the (0 1 3 0 0)0 I, J=5 and (0 4 0 3 3)0 +, J=12 rovibrational levels is well fit by the same model. The dynamical implications of these results are discussed.

    Original languageEnglish
    Pages (from-to)3592-3606
    Number of pages15
    JournalJournal of Chemical Physics
    Issue number9
    Publication statusPublished - 1 Mar 1997


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