Transition Probabilities in Molecular Collisions: Computational Studies of Rotational Excitation

Exact numerical solutions are presented for the coupled-state formulation of a rigid-rotor–structureless-atom collision, and are compared with qualitative ideas and semiquantitative decoupling approximations for the system. The advantages of using an adiabatic basis for the internal states of the system are demonstrated, in distorted-wave-type computations and in accounting for the direct part and the resonance energies in slow inelastic collisions, in the absence of curve crossing. The diabatic regime is also considered, and a diabatic basis is used for a curve-crossing problem. For these slow, head-on collisions, the Franck–Condon factors are not exponentially small, and significant (>10−1) transition probabilities are possible.