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 σ_PI and σ_AI of Ar atoms by He(2 ³S) and He(2 ¹S) metastable atoms, and of their sum σ_T = σ_PI + σ_AI, 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 σ_AI/σ_T and σ_T(singlet)/σ_T(triplet) are reported along with the relative magnitude of the cross sections σ_AI, σ_PI, σ_T. 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 σ_AI/σ_T, 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 σ_T(singlet)/σ_T (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 σ_AI and σ_PI 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 σ_T and partial ionization cross sections σ_AI and σ_PI in good agreement with the data. Information on both the coupling width and the He + Ar⁺ interaction potential well are obtained.