Molecular ion formation in decaying plasmas produced in pure argon and krypton

Abstract

The formation of molecular ions Ar2+ and Kr2+ in decaying plasmas of the respective rare gases is studied at pressures between 0.2 and 3 Torr at room temperature. Measurements of the decay of He2+ in He are made to test the experimental technique used. The reaction rate coefficient for the process Ar+(2P3/2)+2 Ar→Ar2++Ar is found to be (2.35±0.2)×10−31 cm6 s−1. The corresponding reaction rate coefficient in krypton is found to be (2.2±0.2)×10−31 cm6 s−1. Using a recently proposed reaction model [Helm and Varney (1978)] for the excited fine structure state (2P1/2), the pressure dependence of the reaction rate coefficient describing the destruction Ar+(2P1/2)+2Ar→products is calculated. The pressure dependence and magnitude of the coefficient found are in agreement with the experimental findings of Liu and Conway (1975) for the above process. The process could not be studied experimentally in our system due to the low density of Ar+(2P1/2) in our discharge. The low density is inferred indirectly and the process responsible for the fast removal of the excited ion species in the argon plasma is regarded to be the superelastic collision process with slow plasma electrons Ar+(2P1/2)+e→Ar+(2P3/2)+e+0.18 eV for which a rate coefficient ≳10−7 cm3 s−1 is estimated. At late times in the afterglow a significant source of ionization is observed in argon. This source is consistent with ionizing collisions of metastable argon atoms. Its importance increased with pressure since the diffusive loss of metastable atoms becomes insignificant as the pressure is raised.