Collisional energy transfer of highly vibrationally excited NO2: The role of intramolecular vibronic coupling and the transition dipole coupling mechanism

Abstract

The collisional relaxation of highly vibrationally excited NO2 has been studied for a variety of collision partners (He, Ar, CO, N2, O2, N2O, NO2, CO2, SF6, and toluene) by time-resolved Fourier transform infrared emission spectroscopy. The average energy 〈E〉 of the vibrationally excited NO2 molecules during collisional quenching was obtained from the IR spectra by modeling the ν3 and ν1+ν3 bands, using the known harmonic frequencies and anharmonicity constants. The average amount of energy lost per collision 〈ΔE〉 was determined from the 〈E〉 versus time data. The results show that there is a dramatic increase in the amount of energy transferred for all bath gases at NO2 energies above 10 000–12 000 cm−1, which is near the origin of the NO2 Ã2B2/B̃2B1 states. This threshold in the energy-transfer rate occurs because of strong vibronic coupling between the X̃2A1 and Ã2B2/B̃2B1 electronic states. The increase in vibration-to-vibration (V-V) energy transfer can be understood within the context of the transition dipole coupling model. Vibronic coupling in NO2 produces extensive broadband emission in the IR and near-IR, which enhances 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 of excited NO2 was directly extracted from the IR emission spectra. These 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 vibration-to-translation, rotation (V-T,R) energy transfer for NO2 deactivation. The calculations showed that V-T,R energy transfer is the major relaxation channel for triatomic or smaller collision partners. For larger species like SF6, however, V-V energy transfer is the dominant channel. Vibronic coupling may cause an increase in the V-T,R energy-transfer rate by allowing electronic potential related terms, possibly the electronic transition dipole moment, to contribute to the matrix elements responsible for V-T,R energy transfer.