Specific rate constants k(E,J) for the dissociation of NO2. I. Time-resolved study of rotational dependencies

Abstract

The effect of rotational excitation on the specific rate constants k(E,J) of the unimolecular decomposion of NO2 has been investigated. Time-resolved pump- and probe experiments with sub-ps time resolution are reported in which NO2 molecules with well-defined rotational and vibrational energy distributions were optically excited near and above the dissociation threshold. The subsequent unimolecular decay of the reacting NO2 molecules was probed by time-resolved laser-induced fluorescence of the disappearing NO2 via excitation to Rydberg states. At constant photolysis wavelength, increasing rotational energy (up to 310 cm−1) was found to leave the overall decay rate nearly unaffected. This observation can be rationalized by assuming a compensation of the angular momentum and energy dependences of the specific rate constants when J and E are changed at the same time. Keeping the total energy E nearly constant and changing J independently, the effect of rotation on the decay rate can be separated and observed more clearly. From the experimental data we conclude that, for sufficiently high J and constant total energy, molecules with larger J dissociate more slowly than molecules with small J, which is in agreement with predictions from statistical unimolecular rate theory.