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 14 000 cm(-1) ab ove the NO2 zero point energy. The collisions are described using the VCC-I OS (vibrational-coupled-channel infinite-order-sudden) quantum scattering m ethod, with two coupled potential surfaces and as many as 329 total states in the basis expansion. The intramolecular potentials describe the X (2)A'( (2)A(1)) and A (2)A'(B-2(2)) states of NO2, and their coupling through a co nical 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 resul t, there is a noticeable change in the energy transfer efficiency as vibron ic energy in NO2 is increased above the threshold for A state excitation. T his change in efficiency occurs even if the same intermolecular potential i s used for both NO2 electronic states. Many transitions are enhanced becaus e the energy gaps between vibronically mixed and unmixed states are smaller . Energy transfer is also enhanced between states which are both vibronical ly 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. (C) 2000 American Institute of Physics. [S0021-9606(00)00313-5].