Symmetry-breaking rotational energy transfer in acetylene molecules, induced by collisional and/or infrared fields

Summary form only given. Time-resolved fluorescence-detected infrared-ultraviolet double resonance (IR-UV DR) techniques have been used to investigate the spectroscopic and dynamical behaviour of acetylene (C/sub 2/H/sub 2/) molecules in two rovibrational manifolds: /spl nu//sub CC/+3/spl nu//sub CH/ at /spl sim/11600 cm/sup -1/ and 4/spl nu//sub CH/ at /spl sim/12700 cm/sup -1/. The high symmetry and structural simplicity of the C/sub 2/H/sub 2/ molecule suggest that its behaviour at these ´chemically insignificant´ levels of excitation should be regular and predictable. However, our IR-UV DR spectroscopic experiments in these regions reveal numerous puzzling anomalies. In an extension of the earlier IR-UV DR experiments, we have now completed a comprehensive spectroscopic, kinetic and modelling study of the 12700 cm/sup -1/ 4/spl nu//sub CH/ manifold of C/sub 2/H/sub 2/. This exhibits a marked ´gateway´ role for one IR-bright rovibrational level: V/sub 1/=1, V/sub 3/=3, J=12. Remarkably, collisional and/or radiative fields are understood to couple to a quasi-continuous background of rovibrational states implicated in odd-/spl Delta/J symmetry-breaking energy transfer.