15min:

MARK A. PAYNE, ANGELA P. MILCE, MICHAEL J. FROST AND BRIAN J. ORR, School of Chemistry and Centre for Lasers and Applications, Macquarie University, Sydney, NSW, Australia 2109.

Time-resolved fluorescence-detected infrared-ultraviolet optical double resonance (IR-UV DR) experiments have been performed with a Raman-shifted dye laser preparing gas-phase C₂H₂ molecules in the "4_CH" region followed by laser-induced fluorescence (LIF) probing in the S₁ <- S₀ vibronic band system. Unusual symmetry-breaking energy transfer has been observed, induced (at least in part) by collisions. This takes the form of odd-numbered changes of the rotational quantum number J, despite the fact that intramolecular transfer between the ortho and para nuclear-spin modifications of such a molecule is usually forbidden.

The selection rules for IR-UV DR spectra determine that the final vibronic levels should be of gerade symmetry. UV spectra are obtained by IR excitation of particular rotational levels of the (1 0 3 0 0)⁰ vibrational eigenstate that terminate in ungerade vibronic levels. Odd-numbered changes of J are also observed in these spectra.

We have verified that the mechanism is an intramolecular process. One possibility is collision-induced dynamical symmetry breaking involving transitions between "+" and "-" local-mode states which spoil the g / u symmetry. We also consider whether a / s nuclear-spin interchange symmetry can be involved.