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

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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.