Photoionization Study of Ion–Molecule Reactions in Mixtures of Hydrogen and Rare Gases

Abstract

The reactions producing HeH⁺, NeH⁺, and ArH⁺ in mixtures of hydrogen with rare gases were studied by photoionization mass spectrometry. The HeH⁺ and NeH⁺ ions are produced by vibrationally excited H₂⁺ ions; the thresholds for reaction were found to be very near the $\text{υ\hspace{0.17em}=\hspace{0.17em}3}$ and $\text{υ\hspace{0.17em}=\hspace{0.17em}2}$ states of H₂⁺, respectively. Above the vibrational threshold the reaction cross section increases with vibrational quantum number and there is no evidence for a kinetic‐energy threshold for these states. When accelerated to sufficient kinetic energy, H₂⁺ ions in vibrational states below threshold react with a cross section which increases as the deficit in vibrational energy decreases. The rate constant for reaction of those vibrational states that are near or about the reaction threshold and that contribute most to the observed reaction becomes nearly constant at low repeller voltages. The threshold for the reaction producing HeH⁺ can be made consistent with a theoretical value for the dissociation energy $D_0({\mathrm{HeH}}^{+}\text{)\hspace{0.17em}=\hspace{0.17em}1.835}\mathrm{eV}$ . The difference $D_0({\mathrm{NeH}}^{+}{\mathrm{)–D}}_0({\mathrm{HeH}}^{+})$ is found to be 0.25 ± 0.03 eV. This yields $D_0({\mathrm{NeH}}^{+}\text{)\hspace{0.17em}=\hspace{0.17em}2.085}\mathrm{eV}$ . In argon–hydrogen mixtures, the ArH⁺ is formed by an exothermic reaction of H₂⁺ in all vibrational states with Ar and also by reaction of Ar⁺ ions in the ${}^2{P}_{\text{3/2}}$ ground state and (with ∼ 30% larger cross section) of Ar⁺ in the ${}^2{P}_{\text{1/2}}$ excited state with H₂. The cross section has little or no dependence on vibrational energy. Chemi‐ionization processes leading to the formation of ArH⁺ by excited H* and Ar* atoms were observed. The dissociation energy of ArH⁺ was found to be certainly greater than 2.647 eV and, from a tentative interpretation of the chemi‐ionization processes, probably greater than 3.397 eV.