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+.