Time-dependent quantum dynamics of NO(X) + Ar: Rotational excitation pathways and comparison with mixed quantum/classical theory.

We present time-dependent quantum wavepacket simulations of inelastic NO(X) + Ar collisions using the Multi-Configurational Time-Dependent Hartree (MCTDH) method. The computed state-to-state cross sections and opacity functions agree well with time-independent close-coupled calculations. The time-dependent populations and rotational couplings provide a dynamical perspective on the well-known propensity behavior of this near-homonuclear system. In particular, they reveal how the established interplay between parity-conserving Δj = 2 and parity-breaking Δj = 1 coupling pathways develops during the collision to produce the inverse propensity. A particular emphasis is placed on comparison with mixed quantum/classical theory (MQCT), which previously offered a time-resolved, trajectory-based interpretation of the inverted rotational-population pattern. By broadening the time-dependent MQCT rotational populations to reflect the quantum packet's inherent spread in arrival times, we recover the broad sequence of parity-dependent growth seen in MCTDH, although the detailed long-time populations of individual channels differ somewhat between the two methods. Nevertheless, the wavepacket dynamics support the broad mechanistic interpretation obtained from MQCT while providing a rigorous account of the coupling and excitation process.