Symmetry-breaking collisional energy transfer in the 4 νcH rovibrational manifold of acetylene: Spectroscopic evidence of a quasi-continuum of background states

Collision-induced molecular energy transfer in the 12700 cm^-1 '4ν_CH' rovibrational manifold of acetylene is studied by infrared-ultraviolet double-resonance spectroscopy. As in previous work, there is evidence of (formally forbidden) odd-numbered changes in rotational quantum number J. Such 'symmetry-breaking' processes are invariant to choice of fluorescence-monitored vibronic band. The prominent (10300)⁰ J=12 rovibrational state appears to be a likely gateway and is rationalised in terms of a combination of quasi-resonant Coriolis and Stark mixing. An unusual collision-induced quasi-continuous background, underlying the discrete rovibrational states of the 4ν_CH manifold, offers a mechanistic key to the apparent symmetry breaking.