Vibrational predissociation of van der Waals molecules: An internal collision, angular momentum model

Abstract

We describe an “internal collision” model of vibrational predissociation (vpd) in triatomic van der Waals (vdW) molecules based on the angular momentum (AM) model of collision-induced vibration–rotation transfer. The probability of vpd is related to the probability of disposing the vibrational energy into rotational and orbital AM. In T-shaped species, two internal collision configurations are likely to dominate namely, the turning points of excursions by the weakly bound species relative to the diatomic. These two geometries result in a bimodal distribution of final rotational states. Velocity-AM diagrams demonstrate why halogen and hydride vdW molecules have very different properties and illustrate the physics of quantitative calculations that reproduce experimental distributions in a wide range of vdW molecules. We introduce an analogy between a metastable dissociative state and the optical resonator and define a quality factor (Qjl) that relates vpd lifetime to stored energy and to ease of generating rotational and orbital AM by dissociation. Data on vdW molecules of OH are analyzed using the Qjl concept and the accelerated dissociation on forming the vdW complex with an efficient energy acceptor is likened to the formation of a particularly low-Q molecular resonator.