Collisional properties of the OH molecule

The disposition of the rovibrational levels in a diatomic molecule has a ma jor influence on the outcome of inelastic collisions involving that molecul e. In the case of the hydrides of moderately heavy elements, unusual collis ional properties are anticipated in view of conflict between the demands of momentum and energy in these species. This arises because hydride rotation al and vibrational quanta are generally large yet the species may be quite heavy and thus carry substantial momentum. This leads to competition betwee n the momentum based mechanism for elastic transfer and constraints which r esult from energy conservation. We illustrate these principles in investiga ting rotational, vibration-rotation, and quasiresonant vibration-rotation t ransfer, as well as vibrational predissociation of OH-containing van der Wa als molecules. Collisional transfer is (almost) invariably constrained by e nergy conservation in this species and the impact of this on the linear-to- angular momentum mechanism is strongly evident in the collisional behavior of the OH molecule. Molecular collision partners may accept vibrational ene rgy from OH without generating angular momentum, resulting in more efficien t deactivation of vibrationally excited OH. Recent observation of emission from very high N levels of (X)(2)Pi OH in the nightglow appears to represen t only the second recorded example of quasiresonant vibration-rotation tran sfer.