Semiclassical Model of Nonadiabatic Energy Transfer in Atom-Molecule Collisions

The recently developed semiclassical theory of nonadiabatic energy transfer in three-dimensional atom-atom collisions is extended to predict electronic excitation and quenching in atom-molecule collisions. The main assumption of the theory is the critical role of orbiting collisions, which result in multiple crossings of the seam between two potential energy surfaces during a single collision and enhance the net nonadiabatic transition probability. The projectile capture, which results in orbiting collisions, is modeled using the previously developed stochastic model of rotational energy transfer in atom-molecule collisions. The cross sections and rate coefficients predicted by the theory are compared with 3-D quasi-classical trajectory / surface hopping calculations and quantum scattering (wavepacket) calculations for N(2D)+N2↔N(4S)+N2, using the ab initio interaction potential energy surfaces and nonadiabatic coupling, and with the experimental data where available. Additional comparison with the quasi-classical trajectory / surface hopping calculations and experimental data is made for N(2D)+CO↔N(4S)+CO, demonstrating the range of applicability of the present approach. In both cases, the semiclassical theory predictions are in good agreement with the high-fidelity simulations at relatively low collision energies, up to 0.5-1.0 eV, and temperatures, up to T ~ 1000 K. At higher collision energies and temperatures, the theory appears to overpredict the energy transfer rate coefficients. This may be indicative of less efficient projectile capture and shorter lifetime of the atom-diatom transition complex. Approximate analytic expressions for the cross sections and rate coefficients are obtained. This approach may also be used to predict rate coefficients of electronic energy transfer in other atom-molecule collisions, for which the potential energy surfaces are available. The results provide the rate coefficients for the predictive simulations of low-temperature plasmas, and plasmas generated behind hypersonic shock waves.