Time dependent thermal lensing measurements of V-T energy transfer from highly excited NO₂

The time dependent thermal lensing technique has been used to measure the vibrational relaxation of NO₂ (initially excited at 21 631 cm ^-1) by Ar, Kr, and Xe. The energy transfer analysis was carried out in terms of 〈〈ΔE〉〉, the bulk average energy transferred per collision. This quantity was found to have a very strong dependence on vibrational energy, with a marked increase at energies greater than about 10 000 cm^-1, where several electronic excited states (²B₂, ²B₁, and ²A ₂) mix with the ground state (²A₁). This effect may be due to large amplitude vibrational motions associated with the coupled electronic states. Even at low energies, deactivation is faster than in other triatomic systems, probably because NO₂ is an open shell molecule and electronic curve crossings provide efficient pathways for vibrational deactivation. The V-T rate constant for deactivation of NO₂(010) by argon is estimated to be (5.1 ± 1.0) × 10^-14 cm ³ s^-1. Results obtained for NO₂*-NO ₂ collisions gave 〈〈ΔE〉〉 values in good agreement with literature results from fluorescence quenching experiments, indicating that V-T may be more important than V-V energy transfer in the quenching process.