Nanosecond time-resolved IR emission from molecules excited in a supersonic jet: intramolecular dynamics of NO2 near dissociation.

IR emission from NO2 cooled in a supersonic jet and excited to a single, B 2B1 state rovibronic level at 22 994.92 cm(-1) above the ground-state zero point was detected with 10(-8)-s time resolution. The IR emission together with the laser-induced fluorescence decay measurement allows the deduction of the relaxation dynamics near the dissociation of NO2. Following the excitation this single rovibronic B 2B1 level decays on 1.0-s time scale primarily through electronic radiation. Collisions induce internal conversion with a rate constant of 3.0 x 10(7) Torr(-1) s(-1) to the mixed AX states. Collisions further induce internal conversion of the AX mixed states into highly vibrationally excited levels in the X states with a rate constant at least one order of magnitude slower. This mechanism results in the observation of a double-exponential decay in the laser-induced fluorescence and a rise in the IR emission intensity corresponding to the fast decay in the fluorescence intensity. The IR emission rate of the highly vibrationally excited X-state levels is estimated to be about one order of magnitude larger than the isoenergetic AX mixed states and much larger than the B 2B1 level, both with much less vibrational excitation.