MECHANISMS FOR SUPERCOLLISIONS

The mechanism of collisional energy transfer in collisions between a h ighly excited polyatomic and a monatomic bath gas is investigated for benzene-rare-gas systems by carrying out both vibrational close-coupli ng, infinite-order sudden quantum-scattering computations and classica l trajectory calculations with a high degree of initial internal excit ation; the quantum calculations involved up to two vibrational modes. It is found in the quantum-scattering calculations that if one of the vibrational modes is of low frequency (and particularly if it is an ou t-of-plane motion), then the cross-section for transferring large amou nts of energy is particularly large, and involves multi-quantum transi tions. Although the quantum simulations have far fewer modes (and henc e involve a far lower density of states) than in an actual system, thi s suggests that low-frequency/out-of-plane modes are prominent in tran sferring significant amounts of energy (and perhaps in 'supercollision s'), since a microcanonical ensemble in an actual system at high inter nal energy will contain a large proportion of states with high excitat ion in low-frequency modes. Both the quantum and trajectory results ar e consistent with a supercollision mechanism which is a head-on collis ion between a bath-gas atom and a rapidly moving substrate atom involv ed in a large-amplitude motion such as occurs with a highly excited lo w-frequency out-of-plane vibration.