Appearance Potentials by the Retarding Potential-Difference Method for Secondary Ions Produced by Excited—Neutral, Excited Ion—Neutral, and Ion—Neutral Reactions

Abstract

A time‐of‐flight mass spectrometer with a retarding potential‐difference ion source has been used to measure appearance potentials and ionization efficiency curves for secondary ions. Reaction types studied include excited neutral‐molecule, charge‐transfer, attachment and branching ion‐molecule reactions. The systems investigated were argon, krypton, methane, acetylene, 1–3 butadiene, ethyl iodide, and methane—krypton mixtures. Rare‐gas molecular ions from excited neutral‐molecule reactions were observed for Ar₂⁺ with appearance potentials at 14.94±0.02, 15.2±0.1, and 15.8±0.1 eV and for Kr₂⁺ at 13.20±0.02, 13.8±0.1, and 14.0±0.1 eV. A charge‐transfer reaction for Kr–CH₄ produced CH₃⁺ at 14.6±0.1 eV corresponding to Kr⁺(²P_½). The activated complex formed by C₄H₆⁺+C₄H₆ from butadiene produces six different secondary ions. Ionization efficiency curves of the six secondary ions showed the same structure in the energy range to 2 eV above onset. Similar results were observed for two competitive reactions formed by C₂H₂⁺+C₂H₂ from acetylene, for energies below 14.5±0.2 eV. At higher energies, the ratio of secondary C₄H₃⁺/C₄H₂⁺ changed by a factor of two thereby demonstrating the influence of reactions of excited C₂H₂^*+. The predominant excitation level for the excited reaction was 17.0±0.2 eV. Ionization efficiency curves of the primary and secondary ions in the energy range near onset for all systems were essentially identical except for the collision complex C₄H₁₀I₂⁺ which was slightly different from that of the primary C₂H₅I⁺ ion. The technique greatly improves the reliability of correlating primary ions with secondary ions and provides detail not otherwise measurable.