Neon—Hydrogen Ion—Molecule Reactions

Abstract

Ion—molecule reactions producing NeH⁺ in a Ne–H₂ mixture subjected to electron bombardment in the ion source of a mass spectrometer have been studied. Ionization‐efficiency curves show that NeH⁺ is primarily formed by reaction of H₂⁺ and Ne. The calculated rate constant is in good agreement with theory when only H₂⁺ ions in the second or higher vibrational levels are considered as reactant. The energetic requirements of reaction appear to be satisfied exclusively by internal energy of H₂⁺. At higher ion‐source pressures NeH⁺ is formed in a third‐order process which was found to be first order in hydrogen and second order in neon pressure. The ionization‐efficiency curve of the third‐order NeH⁺ closely resembles a Ne^* metastable excitation curve. Results suggest a Ne^*–H₂ interaction producing a highly excited hydrogen molecule which subsequently ionizes to H₂⁺ and then reacts with Ne producing NeH⁺. The H₂⁺–He ion molecule reaction served as a sensitive probe to show that Ne^*–H₂ collisions produce H₂⁺ in vibrationally excited states with v≥5. A momentum‐transfer cross section calculated for the metastable atom—molecule interaction is in reasonable agreement with the measured cross section.