Mixed quantum/classical theory for rotationally and vibrationally inelastic scattering of open-shell molecules and its application to the NH(XΣ) + He collisional system.

We developed the mixed quantum/classical theory (MQCT) for rotationally and vibrationally inelastic scattering of a diatomic molecule in a Σ electronic state with a closed-shell atom. In this approach, the rotational (and vibrational) fine-structure states of the molecule are treated quantum-mechanically, whereas the relative motion of collisional partners is treated classically. This theory is benchmarked against full quantum (close-coupling) calculations for the NH(XΣ) + He system. Good agreement is found in all cases, and MQCT reproduces all main features of energy dependence of the cross section, except narrow scattering resonances at very low energies. Our method recovers propensity rules that govern values of close-coupling cross sections. Particularly, MQCT correctly predicts that the magnitudes of F-conserving transitions are significantly larger than those of F-changing ones. This new development makes MQCT a viable candidate for obtaining fine-structure resolved rotational rate coefficients at higher temperatures and for complex polyatomic systems where the standard full quantum treatment is computationally infeasible.