Visible photoabsorption by noble-gas trimer ions

A study of the electronic structure and photoabsorption characteristics of noble-gas trimer ions has been carried out using quantum mechanical methods. This study includes detailed density functional calculations of the potential energy surfaces for a prototype system, Ar+3, an analysis of the spectroscopic properties of the ground state of this ion, and prediction of the strong absorption bands. Studies in D3h, C2v, and D∞h symmetries were carried out. The lowest state of Ar+3 corresponds to the degenerate 2E′ symmetry with an indicated dissociation energy of 0.17 eV relative to Ar+2 (A 2Σ+u)+Ar (1S0). This trimer ion exhibits a small Jahn-Teller distortion which does not significantly alter our conclusions concerning the ground-state stability or absorption characteristics. A strong and very broad photoabsorption band (λ=480–670 nm) is predicted for the 2E′→2A′1 transition of this trimer ion. This suggests that such species may be important in the analysis of loss mechanisms in visible excimer lasers operating under high-pressure conditions with mixtures containing the noble gases.