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, $B̃B12$ state rovibronic level at $22994.92cm−1$ 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 $B̃B12$ level decays on $1.0-s$ time scale primarily through electronic radiation. Collisions induce internal conversion with a rate constant of $3.0×107Torr−1s−1$ to the mixed $Ã∕X̃$ states. Collisions further induce internal conversion of the $Ã∕X̃$ mixed states into highly vibrationally excited levels in the $X̃$ 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 $X̃$-state levels is estimated to be about one order of magnitude larger than the isoenergetic $Ã∕X̃$ mixed states and much larger than the $B̃B12$ level, both with much less vibrational excitation.