SYMMETRY-BREAKING PERTURBATIONS IN THE NU(2)-NU(3) ROVIBRATIONAL MANIFOLD OF ACETYLENE - SPECTROSCOPIC AND ENERGY-TRANSFER EFFECTS(3)

Time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectro scopy is used to characterize complex rovibrational levels in the high ly perturbed nu(2)+3 nu(3) region (similar to 11 600 cm(-1)) of gas-ph ase acetylene, C2H2. Here, very few of the known rovibrational levels have appreciable Franck-Condon factors linking them to accessible exci ted rovibronic levels, as is needed in the fluorescence-detected IR-UV DR excitation scheme; rovibrational levels that are ''IR-bright'' ten d to be ''UV-dark'' and vice versa. The rovibrational states detectabl e by IR-UV DR in this region are strongly perturbed, such that IR-brig ht (but UV-dark) vibrational basis states are coupled to other states with more favorable Franck-Condon factors. The characterization of the se perturbed rovibrational states (and their associated dynamical prop erties) is facilitated by a novel IR-UV DR technique in which the UV a nd IR laser frequencies are simultaneously scanned in opposite directi ons, with their sum held constant. From the observed IR-UV DR spectra, it is inferred that local perturbations tend to break symmetries and spoil quantum numbers (such as l, J, and possibly 1) that are usually regarded as ''good'' in the C2H2 molecule. The most remarkable case en tails an apparent collision-induced breaking of g/u symmetry that give s rise to rovibrational energy transfer with odd Delta J (rather than the usual even-Delta J situation). This observation is consistent with IR-UV DR kinetic measurements of collision-induced state-to-state ene rgy transfer that are also briefly described. The supposed mechanism r elies on Coriolis coupling to cause strong rovibrational perturbations by basis states with dominant bending character, such that the result ing perturbed state is then susceptible to dynamical breaking of g/u s ymmetry, with odd-Delta J rovibrational transfer a direct consequence. Other possible mechanisms imply that excitation of C2H2 to a particul ar perturbed rovibrational level might cause facile interconversion of the ortho and para nuclear-spin modifications. One such interpretatio n of g/u symmetry-breaking in C2H2 invokes a combination of Coriolis c oupling and nuclear hyperfine interaction, thereby mixing basis states that have a very close accidental coincidence in energy. Another (but energetically unlikely) possibility is that g/u symmetry is spoiled p hotochemically by intramolecular state-mixing involving the vinylidene isomer, thereby destroying the molecule's center of symmetry. (C) 199 6 American Institute of Physics.