Structure in Ionization-Efficiency Curves near Threshold from Alkanes and Alkyl Halides

Abstract

Appearance potentials have been measured by the retarding potential difference method on a time-of-flight mass spectrometer for: CH2+ from CH4, CH3Cl, CH3Br, and CH3I; CH2X+ from CH3F, CH3Cl, CH3Br and CH3I; C2H6+ and CH3+ from C2H6; C2H5+ and C2H5X+ from C2H5X and C3H7+; and C3H7X+ from C3H7X where X is Cl, Br, or I. Substantially all of the structure in the ionization-efficiency curves of fragment hydrocarbon ions can be accounted for by excitation potentials which are the same for all source molecules or by differences of appearance potentials between ion-pair and ion—neutral processes which match known electron affinities. In all instances the appearance potential of a fragment ion at onset corresponded to an ion-pairing process.