Symmetry-breaking perturbations in the ν2+3ν3 rovibrational manifold of acetylene: Spectroscopic and energy-transfer effects

Abstract

Time-resolved infrared–ultraviolet double resonance (IR-UV DR) spectroscopy is used to characterize complex rovibrational levels in the highly perturbed ν2+3ν3 region (∼11 600 cm−1) of gas-phase acetylene, C2H2. Here, very few of the known rovibrational levels have appreciable Franck–Condon factors linking them to accessible excited rovibronic levels, as is needed in the fluorescence-detected IR-UV DR excitation scheme; rovibrational levels that are ‘‘IR-bright’’ tend to be ‘‘UV-dark’’ and vice versa. The rovibrational states detectable by IR-UV DR in this region are strongly perturbed, such that IR-bright (but UV-dark) vibrational basis states are coupled to other states with more favorable Franck–Condon factors. The characterization of these perturbed rovibrational states (and their associated dynamical properties) is facilitated by a novel IR-UV DR technique in which the UV and IR laser frequencies are simultaneously scanned in opposite directions, 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 I) that are usually regarded as ‘‘good’’ in the C2H2 molecule. The most remarkable case entails an apparent collision-induced breaking of g/u symmetry that gives rise to rovibrational energy transfer with odd ΔJ (rather than the usual even-ΔJ situation). This observation is consistent with IR-UV DR kinetic measurements of collision-induced state-to-state energy transfer that are also briefly described. The supposed mechanism relies on Coriolis coupling to cause strong rovibrational perturbations by basis states with dominant bending character, such that the resulting perturbed state is then susceptible to dynamical breaking of g/u symmetry, with odd-ΔJ rovibrational transfer a direct consequence. Other possible mechanisms imply that excitation of C2H2 to a particular perturbed rovibrational level might cause facile interconversion of the ortho and para nuclear-spin modifications. One such interpretation of g/u symmetry-breaking in C2H2 invokes a combination of Coriolis coupling 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 photochemically by intramolecular state-mixing involving the vinylidene isomer, thereby destroying the molecule’s center of symmetry.