Mixed Quantum/Classical Theory for Fine-Structure Inelastic Transitions in the Scattering of 2Π Molecule with Structureless Target

Mixed Quantum/Classical Theory (MQCT) offers an efficient alternative for computing molecular scattering cross sections, scaling favorably with the size of the basis as compared to the exact close-coupling methods. In this work, the MQCT framework is extended to the rotationally inelastic scattering of 2Π molecules with structureless targets, using the Ar + NO (X2Π) system as a benchmark. The time-dependent equations of motion are solved for independent MQCT trajectories by treating the translational motion classically and the electronic and rotational motions quantum mechanically. This formalism naturally incorporates electronic nonadiabatic couplings, enabling calculations of state-to-state cross sections across all possible transitions, including pure rotational excitation, transitions between spin–orbit manifolds and between the individual states of Λ-doublet components. For validation purposes, these are compared against exact full-quantum calculations using HIBRIDON package. Good quantitative agreement is observed, with complicated quantum interference effects successfully reproduced, specifically capturing major features of the Λ-doublet propensity.