Velocity dependence of the Penning and associative ionization cross sections of Ar atoms by He(2 3S) and He(2 1S) atoms

Abstract

The velocity dependence of both the Penning and associative ionization cross sections σPI and σAI of Ar atoms by He(2 3S) and He(2 1S) metastable atoms, and of their sum σT = σPI + σAI, is measured in the velocity range 1200–4500 m/sec (30–400 meV) in a crossed-beam experiment by a time-of-flight technique. Characteristic cross-section ratios σAI/σT and σT(singlet)/σT(triplet) are reported along with the relative magnitude of the cross sections σAI, σPI, σT. The measurements clearly show a difference in the reaction mechanisms involving He(2 3S) and He(2 1S). This difference appears not only in the ratio σAI/σT, whose magnitude reaches 19% and 38% for He(2 3S) and He(2 1S), respectively, at the velocity v = 1200 m/sec (30 meV), and in the ratio σT(singlet)/σT (triplet), which increases from 0.5 to 3.8 for velocity decreasing from 4500 m/sec to 1200 m/sec, but also in the different shapes of the σAI and σPI cross sections for He(2 3S) and He(2 1S). The experimental results for He(2 3S) are also compared to theoretical calculations made in the classical impact parameter formulation. The model gives a total ionization cross section σT and partial ionization cross sections σAI and σPI in good agreement with the data. Information on both the coupling width and the He + Ar+ interaction potential well are obtained.