Ionization-Efficiency Curves for Molecular and Fragment Ions from Methane and the Methyl Halides

Abstract

The retarding potential difference method has been used to measure ionization efficiency curves of CH3X+ (X=H, F, Cl, Br, I), CH3+, and X+ from methane and the methyl halides in a time-of-flight mass spectrometer. Discontinuities between linear segments of the ionization efficiency curves are interpreted as appearance potentials (A.P.) of electronic states or processes. For all CH3+, A.P.'s occur at 1.1, 1.7, 2.7, and about 3.4 eV above onset in the ion pair region. Empirical knowledge of this structure makes it possible to identify the A.P. for the ion—neutral pair, CH3+—X, in their ground states. Combining these data and the A.P.'s for X+/CH3X gives the ionization potential for CH3 and the bond dissociation energies, D(CH3—X).