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 the electronic excitation and quenching in atom-molecule collisions. The two main assumptions of the present model are (i) the critical role of orbiting collisions, which result in multiple crossings of the seam between the potential energy surfaces in a resonance state of the atom-diatom complex, and (ii) a statistical distribution of the atom-diatom complex over the resonance vibrational states. The cross sections and rate coefficients predicted by the model are compared with quasi-classical trajectory/surface hopping calculations and quantum wavepacket scattering for N(2D) + N2 ↔ N(4S) + N2, N(2D) + CO ↔ N(4S) + CO, and C(1D) + N2 ↔ C(3P) + N2, using the ab initio potential energy surfaces and nonadiabatic couplings, and with the experimental data where available. The quenching rate coefficients predicted by the semiclassical model are in good overall agreement with the quasiclassical trajectory calculations and quantum scattering. The semiclassical excitation rate coefficient for N + N2 is in good agreement with that obtained from quasiclassical trajectory calculations with Landau-Zener surface hopping (QCT-LZ) using the detailed balance, over a wide range of temperatures. Both the semiclassical and QCT-LZ excitation rates at high temperatures are significantly higher compared to those predicted by QCT with coherent switches/decay of mixing with zero-point energy correction and by quantum scattering. The approximate analytic expressions for the semiclassical transition probabilities and rate coefficients have been obtained.