Photodissociation of NO2 in the Region 217−237 nm: Nascent NO Energy Distribution and Mechanism

Abstract

Photodissociation of NO2 in the region of 217−237 nm is investigated by probing the nascent NO generated using a one-laser photofragmentation/fragment-detection technique. By mixing O2 in the sample (20%−100%) and by setting proper detection timing, only mass-resolved excitation spectra (MRES) of photofragmented NO are obtained and examined. The nascent NO spectrum changes depending on the intensity (W/cm2) of the laser beam. For low laser intensity, the dissociation of NO2 produces rotationally and vibrationally cold NO through the NO2 22B2 (B̃) excited state. Excess energy for this photofragmentation is ∼0.4 eV at 226 nm. This excess energy is distributed almost entirely to the kinetic energy of NO(X 2A2) and O(1D) products. The spectrum of photofragmented NO becomes very crowded as the laser intensity is increased because higher rotational and vibrational levels of NO become populated through multiphoton excitation and eventual photofragmentation of NO2 from higher-energy electronic states. The rotational temperature of NO is ca. 200 K for high laser intensity and less than 30 K for low laser intensity. Near 230 nm, a rovibronic spectrum is observed that cannot be attributed to NO even though it is detected in the NO mass channel. The time-of-flight mass spectrum line width for photofragmented NO in this photolysis region increases by a factor of 2 to about 40 ns (estimated NO kinetic energy is 0.40 ± 0.04 eV) for high laser intensity. These observations lead to the conclusion that a higher electronic state of NO2 has been accessed to generate highly excited NO through multiphoton absorption processes for the high laser intensity experiments. Such intermediate states can be Rydberg, ion pair, or other high-energy states of NO2 accessed by multiphoton absorption.