COLLISIONAL ENERGY-TRANSFER OF HIGHLY VIBRATIONALLY EXCITED NO2 - THEROLE OF INTRAMOLECULAR VIBRONIC COUPLING AND THE TRANSITION DIPOLE COUPLING MECHANISM

The collisional relaxation of highly vibrationally excited NO2 has bee n studied for a variety of collision partners !He, Ar, CO, N-2, O-2, N 2O, NO2, CO2, SF6, and toluene) by time-resolved Fourier transform inf rared emission spectroscopy. The average energy [E] of the vibrational ly excited NO2 molecules during collisional quenching was obtained fro m the IR spectra by modeling the upsilon(3) and upsilon(1) + upsilon(3 ) bands, using the known harmonic frequencies and anharmonicity consta nts. The average amount of energy lost per collision [Delta E] was det ermined from the [E] versus time data. The results show that there is a dramatic increase in the amount of energy transferred for all bath g ases at NO2 energies above 10 000-12 000 cm(-1), which is near the ori gin of the NO2 (A) over tilde(2)B(2)/(B) over tilde(2)B(1) states, Thi s threshold in the energy-transfer rate occurs because of strong vibro nic coupling between the (X) over tilde(2)A(1) and (A) over tilde(2)B( 2)/(B) over tilde(2)B(1) electronic states. The increase in vibration- to-vibration (V-V! energy transfer can be understood within the contex t of the transition dipole coupling model. Vibronic coupling in NO2 pr oduces extensive broadband emission in the IR and near-IR, which enhan ces the V-V energy-transfer rate by relaxing the resonance conditions in dipole coupling. The V-V energy-transfer probability was calculated using the dipole coupling model, where the transition dipole moment o f excited NO2 was directly extracted from the IR emission spectra. The se calculations successfully reproduced the observed threshold in the V-V energy transfer probability. The transition dipole coupling model was also used to estimate the relative contribution of V-V versus vibr ation-to-translation, rotation (V-T,R) energy transfer for NO2 deactiv ation. The calculations showed that V-T,R energy transfer is the major relaxation channel for triatomic or smaller collision partners. For l arger species like SF6, however, V-V energy transfer is the dominant c hannel. Vibronic coupling may cause an increase in the V-T,R energy-tr ansfer rate by allowing electronic potential related terms, possibly t he electronic transition dipole moment, to contribute to the matrix el ements responsible for V-T,R energy transfer. (C) 1997 American Instit ute of Physics.