Measurement of the rate coefficients for the bimolecular and termolecular de-excitation reactions of He(2 3S) with selected atomic and molecular species

Abstract

This work reports the measurement of second and third order excitation transfer reactions of metastable 2 ³S helium atoms with 15 reactants selected because of their widely varying values of polarizability and dipole moment. In this study He(2 ³S) destruction frequencies have been experimentally determined from measurements of the optical absorption of the 3889 Å, 2 ³S →3³P, helium transition 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 800–2000 Torr and as functions of the partial pressure of reactant from 14 to 200 mTorr. From this 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 0.3×10⁻³⁰ cm⁶ sec⁻¹ and values were found to range from 0.9×10⁻³⁰ cm⁶ sec⁻¹ for Kr to 15×10⁻³⁰ cm⁶ sec⁻¹ for CCl₃F. A classical approximation to the three‐body capture rate has been derived which explains these values in therms of the rates at which third body encounters change glacing collisions into inwardly spiraling orbits. The data reported here are in agreement with this model and indicate that the reaction probabilities are generally approximated by values of 0.25, 0.55, and 1.0 for monatomic, diatomic, and polyatomic reactants, respectively. The sizes of these termolecular rates suggests their general importance in excitation transfer reactions in plasmas at atmospheric pressures.