Measurement of the rate coefficients for the bimolecular and termolecular ion–molecule reactions of Ar+2 with selected atomic and molecular species

This work reports the measurement of second and third order ion–molecule reactions of diatomic argon ions with ten reactants selected because of their widely varying values of polarizability and dipole moment. In this study Ar+2 destruction frequencies have been experimentally determined from measurements of optical absorption at 280 and 290 nm in high pressure afterglows of mixed gases excited by intense electron beam discharges. Data have been obtained as functions of argon pressure over the range from 1000 to 2500 Torr and as functions of the partial pressure of reactant from 14 to 100 mTorr. From these data pressure dependent rate coefficients have been extracted and subsequently resolved into contributions from second order, bimolecular and third order, termolecular, components. The bimolecular components have been found to agree with tabulated values appearing in the literature. 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 from less than 2×10−30 cm6 sec−1 for Kr and H2 to 44×10−30 cm6 sec−1 for NO. A classical approximation to the three-body capture rate has been evaluated that explains these values in terms of the rates at which third-body encounters change glancing collisions into inwardly spiraling orbits. The data reported here are in agreement with this model and indicate that the reaction probabilities are generally high. The sizes of these termolecular rates suggest their general importance in charge transfer reactions in plasmas at atmospheric pressures.