Effect of Unimolecular Decay Kinetics on the Interpretation of Appearance Potentials

Abstract

The interpretation of appearance potential data on diatomic molecules should take account of possible effects caused by predissociation, emission of light and autoionization. In the case of complex polyatomic molecules, the kinetics of predissociation and the internal thermal energy of the molecules become especially important. The intensities of the parent, fragment, and metastable ions produced by photoionization of n-propylamine, n-propanol, and methyl ethyl ketone are studied as a function of photon energy. The excess kinetic energies of the fragment ions are found to be negligibly small. The data are interpreted in terms of Rosenstock's quasi-equilibrium theory of unimolecular decomposition and indicate that the theory is qualitatively correct for the dissociative processes investigated. However, the theory is shown to be quantitatively inadequate at least in the energy range near threshold. In this region the rate constant for dissociation varies much more rapidly with energy than the theory predicts. Some of the assumptions of the theory are examined and compared to deductions from the data. The meaning of appearance potential data is examined in the light of these results. The effects of both the kinetics of dissociation and of internal thermal energy on ionization efficiency curves are significant. Most of the methods used to determine appearance potentials tend to minimize these effects and there is probably some cancellation of errors. A new method for the determination of appearance potentials is described. Experimental methods which can yield more detailed information concerning dissociation processes of complex molecular ions are suggested.