DYNAMICAL ANGULAR-MOMENTUM MODELS FOR ROTATIONAL TRANSFER IN POLYATOMIC-MOLECULES

We propose a model for collision-induced rotational transfer (RT) in polyatomic molecules based on the angular momentum (AM) sphere, a classical representation of the dynamical motion of the rotational AM vector in the molecular frame. The model develops further that proposed by us [AlWahabi et al., J. Chem. Soc., Faraday Trans. 85, 1003 (1989)] in which RT probabilities are related to the AM gap linking initial and final Nkakc states. The AM sphere representation embodies the full internal motion of the molecule via its effect on the inertial axes and the trajectory of the individual rotational state vectors. In this representation there is no unique AM gap for a particular transition between states of nominally well-defined Nkakc and here we propose and test several models for obtaining the distance in AM space between initial and final trajectories. Models are evaluated from their ability to fit data on NH2-H collisions. We find that even the simplest approximations, such as shortest distance in AM space, give good fits to data sets but the best fits are obtained when both AM trajectory and molecular geometry are averaged over. © 1995 American Institute of Physics.