Quantum scattering study of collisional energy transfer in He+NO₂: The importance of the vibronic mixing

We present the results of a quantum scattering study of collisional energy transfer in the title reaction, considering energies up to 14000 cm^-1 above the NO₂ zero point energy. The collisions are described using the VCC-IOS (vibrational-coupled-channel infinite-order-sudden) quantum scattering method, with two coupled potential surfaces and as many as 329 total states in the basis expansion. The intramolecular potentials describe the X²A′(²A₁) and A²A′(²B₂) states of NO₂, and their coupling through a conical intersection. The intermolecular potential is based on empirical sums of atom-atom potentials. We find that vibronic mixing between the X and A states of NO₂ strongly enhances collisional energy transfer and, as a result, there is a noticeable change in the energy transfer efficiency as vibronic energy in NO₂ is increased above the threshold for A state excitation. This change in efficiency occurs even if the same intermolecular potential is used for both NO₂ electronic states. Many transitions are enhanced because the energy gaps between vibronically mixed and unmixed states are smaller. Energy transfer is also enhanced between states which are both vibronically mixed, whenever a significant component of each state involves the same zeroth order level of the excited electronic state. The calculated results are in good agreement with recent measurements.