Ionization and Metastable Excitation in Low-Energy Collisions of Ground-State Argon Atoms

A low-energy beam of argon atoms, formed by nonresonant charge transfer of Ar+ in H2, has been used to explore the near-threshold behavior for ionizing and exciting collisions between argon atoms. For excess center-of-mass energies below 12 eV, the ionization data are consistent with the empirical relationship σ− ≈ 1.8 × 10−21(E − 15.8)1.3, where E is the center-of-mass energy in electron volts. Target-gas mixtures of argon and acetylene were used in an effort to observe metastable atoms formed in atom–atom collisions by Penning ionization. In this case, beam energies were restricted to values less than 30 eV to avoid direct ionization of the acetylene (Penning) gas by the ground-state atom beam. At 14.5 eV c.m. energy, the cross section for metastable production is shown to be (3.6 ± 2.4) × 10−20 cm2, but this value is dependent upon assumed values of the Penning cross section for Ar* + C2H2 collisions.