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

The effect of rotational excitation on the specific rate constants k(E,J) o f the unimolecular decomposion of NO2 has been investigated. Time-resolved pump- and probe experiments with sub-ps time resolution are reported in whi ch NO2 molecules with well-defined rotational and vibrational energy distri butions were optically excited near and above the dissociation threshold. T he subsequent unimolecular decay of the reacting NO2 molecules was probed b y time-resolved laser-induced fluorescence of the disappearing NO2 via exci tation to Rydberg states. At constant photolysis wavelength, increasing rot ational energy (up to 310 cm(-1)) was found to leave the overall decay rate nearly unaffected. This observation can be rationalized by assuming a comp ensation of the angular momentum and energy dependences of the specific rat e constants when J and E are, changed at the same time. Keeping the total e nergy E nearly constant and changing J independently, the effect of rotatio n on the decay rate can be separated and observed more clearly. From the ex perimental data we conclude that, for sufficiently high J and constant tota l energy, molecules with larger J dissociate more slowly than molecules wit h small J, which is in agreement with predictions from statistical unimolec ular rate theory. (C) 2001 American Institute of Physics.