Picosecond photofragment spectroscopy. I. Microcanonical state-to-state rates of the reaction NCNO→CN+NO

Abstract

This paper, the first in a series of three papers, gives a detailed account of our studies on picosecond photofragment spectroscopy. The unimolecular reaction NCNO→CN+NO is examined in detail here. Microcanonical state-to-state rates are measured in molecular beams at different energies in the reagent NCNO using pump–probe techniques: one picosecond pulse initiates the reaction from an initial (v,J) state and a second pulse, delayed in time, monitors the CN radical product in a specific rovibrational state, or the reagent NCNO (transient absorption). The threshold energy for reaction is determined to be 17 083 cm−1 (bond energy=48.8 kcal/mol). Measured rates are found to be sharply dependent on the total energy of the reagent, but independent of the rotational quantum state of product CN. Results of transient absorption measurements are used to argue that the ground state potential energy surface dominates the reaction in the range of excess energies studied. The energy dependence of the rates, kMC(E), is compared with that predicted by statistical theories. Both standard RRKM (tight transition state) and phase space theory (loose transition state) fail to reproduce the data over the full range of energies studied, even though nascent product state distributions are known to be in accord with PST at these energies. Furthermore, kMC(E) is not a strictly monotonically increasing function of energy but exhibits some structure which cannot be explained by simple statistical theories. We advance some explanations for this structure and deviations from statistical theories.