Approximate determination of absolute unimolecular rate constants using the field ionization kinetic method

A method is described which allows the determination of absolute unimolecular rate constants from ion lifetime measurements using the field ionization technique. For this purpose the molecules are heated to ∼ 800 K prior to ionization by a high electric field. The internal energy distribution of the molecular ions generated in this way is mainly determined by the thermal energy distribution which can be calculated with fair accuracy. The additional small amount of energy transferred during the ionization process can be estimated by comparing the experimental and calculated temperature dependence of the molecular ion. If ions energized in this manner undergo a unimolecular decomposition, the rate of formation of a given fragment can be determined in the time range 10⁻¹¹-10⁻⁵ s after ionization. Knowledge of the internal energy distribution of the precursor and the rate of formation allows the determination of the rate constant as a function of the internal energy. The accuracy of this method is limited by uncertainty in the determination of the energy distribution and the potential distribution in the source. Approximate rate constants are reported for methyl loss from ionized t-butylbenzene and diethylether in the range 10⁴-10⁹ s⁻¹. Reasonable agreement with RRKM calculations and—as far as available—with photo-electron-photoion coincidence measurements is achieved. It is demonstrated that the method allows the determination of very steep k(E) functions, which do not lend themselves to other methods.