Probing the NO2→NO+O transition state via time resolved unimolecular decomposition

Abstract

Time resolved, subpicosecond resolution measurements of photoinitiated NO2 unimolecular decomposition rates are reported for expansion cooled and room temperature samples. The molecules are excited by 375–402 nm tunable subpicosecond pulses having bandwidths ≥20 cm−1 to levels which are known to be thorough admixtures of the 2 B 2 electronically excited state and the 2 A 1ground electronic state. Subsequent decomposition is probed by a 226 nm subpicosecond pulse that excites laser‐induced fluorescence(LIF) in the NO product. When increasing the amount of excitation over the dissociation threshold, an uneven, ‘‘step‐like’’ increase of the decomposition rate vs energy is observed for expansion cooled samples. The steps are spaced by ∼100 cm−1 and can be assigned ad hoc to bending at the transition state. Relying on experimental estimates for the near threshold density of states, we point out that simple transition state theory predictions give rates that are consistent with these measured values. The rates are sufficiently rapid to question the assumption of rapid intramolecular vibrational redistribution, which is implicit in transition state theories. In contrast to expansion cooled samples, room temperature samples exhibit a smooth variation of the reaction rate vs photon energy. By comparing rates for rotationally cold and room temperature NO2, the ON–O bond is estimated to be ∼40% longer in the transition state than in the parent molecule.