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 D_3h, C_2v, 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.