Rare-Gas Molecule-Ion Formation by Mass Spectrometry. Kinetics of Ar2+, Ne2+, and He2+ Formation by Second- and Third-Order Processes

Abstract

The ions Ar₂⁺, Ne₂⁺, and He₂⁺ are formed in a mass spectrometer operated at high pressures (up to 300 μ) by the bimolecular excited atom reaction R^*+R=R₂⁺+e, and Ar₂⁺ and Ne₂⁺ are formed by the termolecular ion‐molecule reaction R⁺+2R=R₂⁺+R. In helium, while He₂⁺ is formed by a third‐order process, there is doubt that an ionic reaction is involved. Ratios of the rates (cross‐sections) for excitation leading to R₂⁺ formation and ionization are found to be 5.5×10^—2, 1.0×10^—2, and 6.4×10^—2 for argon, neon, and helium. The measurements were made at nominal electron voltages (EV) of 15, 20, and 22 v, respectively, which correspond to maximum excitation. Values of 1.4×10^—2 and 0.45×10^—2 were also obtained at EV=70 v for argon and neon. The experiments yield only the product of the bimolecular rate constant and the lifetime of the R^* reactant atom (k_rτ_u), and the values of this product obtained at EV=15, 20, and 22 v are for argon, neon, and helium 3.6×10^—16, 11.6×10^—16, and 0.58×10^—16 cc/molecule. At EV=70 v values of 2.7×10^—16 and 1.1×10^—16 cc molecule are obtained for argon and neon. Speculations are given concerning the magnitude of the lifetimes of R^*, and it is concluded that values of about 10^—8 sec must be considered as possible. The corresponding cross sections for the reaction forming R₂⁺ lie in the range 1000–10 000×10^—16 cm². The rate constants for the formation of Ar₂⁺ and Ne₂⁺ by the three‐body process are, respectively, 2.1×10^—28 and 2.0×10^—29 cc²/molecule²sec. It is recognized that values as large as these are in disagreement with values inferred from pulsed‐discharge ion‐drift velocity experiments. A possible mode of reconciliation is suggested; namely, at sufficiently high pressures collisional decomposition of R₂⁺ ions formed by the termolecular process occur in the drift‐velocity experiments.