THE KINETICS OF INFRARED-LASER PHOTODISSOCIATION OF NORMAL-BUTYLBENZENE IONS AT LOW-PRESSURE

Molecular ions of n-butylbenzene trapped in the ion cyclotron resonance ion trap were dissociated by cw CO2-laser irradiation at pressures near 1 X 10(-8) Torr where collisions play a negligible role in the kinetics. The dissociation as a function of time after laser onset showed a well-defined induction time tau(ind), followed by exponential decay of a parent ion with first-order rate constant k(diss). Data were collected over a range of laser intensities giving k(diss) values from 0.3 to 10.5 s-1. Computer simulation of the dissociation curves was carried out using a random-walk method to solve a master equation for the kinetics, and assuming infrared radiative relaxation as the mechanism of ion deactivation. The simulations yielded curves also showing well-defined induction times and dissociation rate constants. It was necessary to consider both the induction times and the dissociation rates to constrain the fit of experiment and computer simulation. The resulting fit was good. The thermal picture of low-pressure infrared multiphoton dissociation kinetics developed in previous work was applied to the data, and thermal kinetics were found to give an excellent description of the observed kinetics. The observed activation energies were successfully interpreted in terms of the modified Tolman's theorem.