Mass-Spectrometric Study of Ions in Xe, Kr, Ar, Ne at Pressures up to 40 torr: Termolecular Formation of the Rare-Gas Molecular Ions. Bond Dissociation Energy of Ar2+ and Ne2+

Abstract

The positive ions obtained from alpha‐particle irradiation of the rare gases Xe, Kr, Ar, Ne were studied in the pressure range 5–40 torr. In all cases both the rare‐gas molecular ion R₂⁺ and the atomic ion R⁺ could be observed. The R₂⁺/R⁺ ratio could be decreased by the addition of gases which react with R₂⁺ and R⁺ through charge transfer. This technique led to a determination of the rate constant for the termolecular reaction R⁺+2R→R₂⁺+R. The rate constant for argon is 6×10⁻³² cm⁶ atom⁻²·sec⁻¹. The ratio of Hornbeck—Molnar ionization to direct ionization in Ar was found equal to 0.4. Xe₂⁺ is a major ion in Kr (at 20 torr) containing 10 ppm xenon. The kinetics of the efficient formation of Xe₂⁺ is examined. It is concluded that Xe₂⁺ is formed by Kr₂⁺+Xe→KrXe⁺+Kr followed by KrXe⁺+Xe→Xe₂⁺+Kr. The rate constants for these two reactions are estimated at 10⁻¹⁰ cm³ molecule⁻¹·sec⁻¹. Both reactions must be exothermic. A schematic representation of the energies of formation of the Kr, Xe, homo‐ and heteronuclear ions is given. Changes in the charge‐transfer spectrum of ethylene with change of the concentration ratio of R₂⁺ and R⁺ are used to derive the ethylene charge‐transfer spectrum due to R⁺ and R₂⁺. The charge‐transfer spectrum with R⁺ is in agreement with that obtained from experiments using low pressures and near‐thermal velocity R⁺. The charge‐transfer spectrum due to R₂⁺ is used in conjunction with the breakdown graph of ethylene to obtain the recombination energy R.E. of R₂⁺. The bond dissociation energy of R₂⁺ is then estimated from D(R⁺—R)=I.P.(R)—R.E.(R₂⁺). Values between 1.5–2 eV are obtained for the dissociation energies of Ar₂⁺ and Ne₂⁺.