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

Collision-induced molecular energy transfer in the 12 700 cm(-1) '4 nu(CH)' rovibrational manifold of acetylene is studied by infrared-ultraviolet dou ble-resonance spectroscopy. As in previous work, there is evidence of (form ally forbidden) odd-numbered changes in rotational quantum number J. Such ' symmetry-breaking' processes are invariant to choice of fluorescence-monito red vibronic band. The prominent (10300)(0) J = 12 rovibrational state appe ars to be a likely gateway and is rationalised in terms of a combination of quasi-resonant Coriolis and Stark mixing. An unusual collision-induced qua si-continuous background, underlying the discrete rovibrational states of t he 4 nu(CH) manifold, offers a mechanistic key to the apparent symmetry bre aking. (C) 2000 Elsevier Science B.V. All rights reserved.