Mixed quantum/classical theory for rotational energy exchange in symmetric-top-rotor + linear-rotor collisions and a case study of the ND + D system.

The extension of mixed quantum/classical theory (MQCT) to describe collisional energy transfer is developed for a symmetric-top-rotor + linear-rotor system and is applied to ND + D. State-to-state transition cross sections are computed in a broad energy range for all possible processes: when both ND and D molecules are excited or both are quenched, when one is excited while the other is quenched and , when the ND state changes its parity while D is excited or quenched, and when ND is excited or quenched while D remains in the same state, ground or excited. In all these processes the results of MQCT are found to approximately satisfy the principle of microscopic reversibility. For a set of sixteen state-to-state transitions available from the literature for a collision energy of 800 cm the values of cross sections predicted by MQCT are within 8% of accurate full-quantum results. A useful time-dependent insight is obtained by monitoring the evolution of state populations along MQCT trajectories. It is shown that, if before the collision, D is in its ground state, the excitation of ND rotational states proceeds through a two-step mechanism in which the kinetic energy of molecule-molecule collision is first used to excite D and only then is transferred to the excited rotational states of ND. It is found that both potential coupling and Coriolis coupling play important roles in ND + D collisions.