Collision-induced rovibrational energy transfer in small polyatomic molecules: the role of intramolecular perturbations

ABSTRACT

Various forms of time-resolved optical double-resonance spectroscopy facilitate rotationally resolved measurements of collision-induced intramolecular vibration-to-vibration (V–V) energy-transfer processes, which take a gas-phase polyatomic molecule from one distinct rovibrational energy level to another. Of longstanding mechanistic interest are questions concerning the extent to which such V–V energy transfer (ET) may be influenced by intramolecular perturbations – notably Fermi resonance (and other anharmonic mixing effects) and Coriolis coupling – within polyatomic molecular rovibrational manifolds of interest. It is evident that quantum-mechanical interference effects can arise, either inhibiting or enhancing the probability of collision-induced ET in perturbed rovibrational manifolds of certain small gas-phase polyatomic molecules, notably CO₂, D₂CO and C₂H₂. This article focuses on a blend of high-resolution rovibrational spectroscopy (characterising initial and final molecular levels and their intramolecular perturbations) and collision dynamics (with colliding molecules defined in terms of isolated-molecule spectroscopic basis states). It aims to offer fresh insights and to consider some apparent mechanistic anomalies (e.g. collision-induced quasi-continuous background effects in the 4ν_CH rovibrational manifold of C₂H₂). Various reported experiments and related theoretical treatments are critically re-examined, in order to pose and address mechanistic questions some of which still challenge detailed understanding.

GRAPHICAL ABSTRACT

KEYWORDS:

• Rovibrational spectroscopy
• molecular collisions
• energy transfer
• intramolecular perturbations

Acknowledgements

This paper is dedicated to Michel Herman, with whom I have enjoyed many cordial scientific and social interactions since we first met at NRCC Ottawa in 1976; the comprehensive research by Michel and his co-workers on the spectroscopic subtleties of acetylene has been truly inspirational. In fact, this article has benefited from recent consultations with Michel and with Jean Vander Auwera. I am also greatly indebted to the many students and colleagues who have contributed to so much productive research in my own laboratory, as well as to the network of co-researchers who have inspired our work over the years. Many, if not all, are cited in the rather copious list of references. Finally, I acknowledge a succession of research grants, notably those from the Australian Research Council, that have sustained our research in this area.

Disclosure statement

No potential conflict of interest was reported by the author.