Atom-molecule collisions; the role of nuclear dynamics

Experimental evidence indicates that the physics of molecular collisions is characterised by an underlying simplicity when viewed from the perspective of the dynamics of the nuclei in a particle representation of the species involved. We review this evidence and describe a simple, transparent kinema tic theory of inelastic collisions from which the quantum state-resolved ou tcome of atom-molecule and molecule-molecule collisions are predicted. The principal mechanism of change is linear-to-angular momentum conversion via a torque-arm of molecular dimension constrained to operate within boundarie s set by the quantisation of molecular eigenstates and by overall energy co nservation. The mechanism is unchanged throughout the wide variety of proce sses molecules undergo but is modified in a process- and system-specific ma nner by boundary condition changes and this gives the wide variety of outco mes seen experimentally. The mechanism and boundary conditions may be repre sented in velocity (momentum)Dangular momentum diagrams that illustrate viv idly the interplay of momentum and energetic factors. Rapid, accurate calcu lation routines based on these principles reproduce rotational and vibratio nal distributions observed in experiment over the wide range of inelastic p rocesses that diatomic molecules undergo as well as in atom-diatom reactive encounters.