Measurement of the rate coefficients for the bimolecular and termolecular ion–molecule reactions of He2+ with selected atomic and molecular species

Abstract

This work reports the measurement of bimolecular and termolecular charge transfer reactions of He₂⁺ into nonassociative product channels. In this study ion destruction frequencies have been experimentally determined from the selectively excited fluorescence of N₂⁺ in high pressure afterglows of mixed gases excited by intense electron beam discharges. Data have been obtained as functions of helium pressure over the range from 400 to 1500 torr and as functions of the partial pressure of reactant from 50 to 200 mtorr. From these data pressure‐dependent rate coefficients have been extracted and subsequently resolved into contributions from bimolecular and termolecular components for reactions of He₂⁺ with Kr, H₂, O₂, NO, HBr, HCl, H₂O, N₂O, NO₂, C₃H₈, NH₃, and CCl₂F₂, selected because of their widely varying values of polarizability and dipole moment. The bimolecular components have been found to agree with the NOAA flowing afterglow results, where available, and to compare favorably with theoretical values obtained from Langevin, ADO, and locked dipole approximations. The sensitivity of the method has been sufficient to detect termolecular components as small as 2×10⁻³⁰ cm⁶ sec⁻¹ and values were found to range widely from 9×10⁻³⁰ cm⁶ sec⁻¹ for H₂ to 140×10⁻³⁰ cm⁶ sec⁻¹ for HBr. A termolecular analog to the Langevin limit has been constructed which explains these values in terms of the rates at which third‐body encounters change glancing collisions into inwardly spiraling trajectories. The data reported are in general agreement with this model and indicate that the reaction probabilities are near unity in both bimolecular and termolecular channels. The size of these termolecular rates suggests the general importance of three‐body ion–molecule reactions in higher pressure plasmas such as those found in e‐beam lasers.