THE NU(CC)-NU(CH) ROVIBRATIONAL MANIFOLD OF ACETYLENE .1. COLLISION-INDUCED STATE-TO-STATE TRANSFER KINETICS(3)

Infrared-ultraviolet double resonance (IR-UV DR) spectroscopy is used to measure collision-induced, rotationally resolved state-to-state ene rgy transfer in the nu(CC)+3 nu(CH) vibrational manifold of gas-phase acetylene, C2H2. Attention focuses on three sets of vibrational eigens tates 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 1 1 585-11 600 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 cha nnels is measured by scanning the IR-W pulse delay with the IR PUMP an d UV PROBE lasers tuned to particular spectroscopic features. There is a clear propensity for even-numbered changes Delta J of rotational qu antum number in the observed RET and V-V transfer, consistent with con servation of ortho or para nuclear-spin symmetry in the state-selected C2H2 molecule, but there are some notable exceptions as unusual symme try-breaking processes result in odd-Delta J V-V transfer. Anomalies o f this type have been investigated in a preceding paper [A. P. Milce a nd B. J. Orr, J. Chem. Phys. 104, 6423 (1996)]. A detailed IR-UV DR st udy 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 prep ares molecules in various J-states of (0 4 0 3 3)(+)(0), while V-V tra nsfer 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 empi rical exponential-gap fitting laws are used to describe even-Delta J c hannels 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 b y the same model. The dynamical implications of these results are disc ussed. (C) 1997 American institute of Physics.