Journal of Chemical Physics, Vol.106, No.9, 3592-3606, 1997
The Nu(CC)+3-Nu(CH) Rovibrational Manifold of Acetylene .1. Collision-Induced State-to-State Transfer Kinetics
Infrared-ultraviolet double resonance (IR-UV DR) spectroscopy is used to measure collision-induced, rotationally resolved state-to-state energy transfer in the nu(CC)+3 nu(CH) vibrational manifold of gas-phase acetylene, C2H2. Attention focuses on three sets of vibrational eigenstates spectroscopically labeled (0 1 3 0 0)(I)(0), (0 1 3 0 0)(II)(0), 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-W pulse delay with the IR PUMP and UV PROBE lasers tuned to particular spectroscopic features. There is a clear propensity for even-numbered changes Delta J 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-Delta J 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)(I)(0) or (0 1 3 0 0)(II)(0) 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-Delta J channels of RET and V-V transfer. It is remarkable that the kinetics of symmetry-breaking odd-hd V-V transfer between the (0 1 3 0 0)(I)(0), 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.
Keywords:ROTATIONAL ENERGY-TRANSFER;VIBRATIONALLY EXCITED ACETYLENE;ULTRAVIOLET DOUBLE-RESONANCE;ANGULAR-MOMENTUM MODEL;GAS-PHASE ACETYLENE;CO/D2 CO COLLISIONS;INFRARED-ULTRAVIOLET;TRANSFER RATES;PARTIAL DEPERTURBATION;RELAXATION