The photofragmentation of Ar+3

Abstract

The photofragment kinetic energy spectrum of Ar⁺₃ has been recorded in a crossed‐beam apparatus at 532 nm; the only photofragment product observed is Ar⁺. For the purposes of analysis, a spectrum for the photofragmentation of Ar⁺₂ was recorded under similar experimental conditions. In each case, the ions were prepared by the electron impact ionization of a neutral argon cluster beam. The Ar⁺₃ spectrum consists of two quite distinct features, a high‐energy component which closely resembles the result observed for Ar⁺₂, and a second, low‐energy feature, which is peculiar to Ar⁺₃ alone. The two high‐energy wings appear to arise from a very rapid dissociation process where approximately 70% of the excess energy appears as Ar⁺ kinetic energy. A computer simulation of this region of the spectrum gives an anisotropy parameter, β, of 1.1±0.2. The low‐energy, component to the spectrum arises from a two‐step dissociation process, in which a weakly bound atom carries away a relatively large fraction of the available excess energy to leave a quasibound dimer ion. The energetics associated with this latter process can be accounted for using a partitioning scheme proposed by Baer et al. [J. Chem. Phys. 7 6, 5917 (1982)]. A computer simulation of the low‐energy component gives β≂0.0. A detailed discussion of the results in terms of electronic structure, photofragmentation dynamics, and allowed electronic transitions, concludes that Ar⁺₃ takes the form of a stable dimer ion and a weakly bound atom, and that it is most probably nonlinear.