Energy Distribution and Fragmentation Processes Resulting from Electron Impact on Propane and n-Butane

Abstract

The internal energy distribution has been determined for propane and n-butane ions produced by electron impact. The method involves the use of differential ionization efficiency curves and depends on the validity of the linear threshold law for simple ionization. The experimental distributions are in fair agreement with theoretical calculations of Lennard-Jones and Hall and indicate that the major fraction of the parent ions are formed by simple removal of a valence electron. Only a minor fraction of the parent ions are apparently formed by removal of one valence electron together with excitation of one or more remaining electrons. Breakdown curves for propane and n-butane have been determined experimentally and, except for the energy scales, are in fair qualitative agreement with the predictions of the statistical theory of mass spectra. The discrepancy between theoretical and experimental energy scales is ascribed at least in part to inadequacy of the theoretical rate equations used in the statistical theory. There is no evidence for gross failure of the assumptions of the theory regarding energy randomization although the number of crossings of potential surfaces of the molecular ion is shown to be very much smaller than originally assumed by Rosenstock et al.