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

Collision-induced molecular energy transfer in the 12 700 cm−1 ʹ4νCHʹ 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 ʹsymmetry-breakingʹ processes are invariant to choice of fluorescence-monitored vibronic band. The prominent (1 0 3 0 0)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 4νCH manifold, offers a mechanistic key to the apparent symmetry breaking.