Ground- and excited-state properties of neutral and anionic selenium dimers and trimers

Ab initio molecular-orbital methods using a newly constructed large, flexible basis set of the atomic-natural-orbital type and extensive treatments of electron correlation were used to accurately predict the ground-state potential-energy curves of Se₂ and Se₂^0ex0ex-. In addition, the potential curves of low-lying electronically excited states of the Se₂^0ex0ex- anion, which are coupled to the ground state through the electronic dipole operator, are determined. Calibration of the accuracy expected for the employed theoretical models is achieved by calculations on electronically excited states of neutral Se₂, for which accurate experimental data are available. Besides reporting accurate predictions for spectroscopic constants, electron affinities, and transition matrix elements, particular emphasis is laid on the use of the computed dimer potential-energy curves for interpreting the recently investigated peculiar optical properties of diatomic selenium species embedded in solid host matrices. Further, the equilibrium geometries and relative stabilities of the C₂ᵥ and D₃ₕ forms of the neutral Se₃ cluster, the electron affinity of Se₃, and the low-lying electronic states of Se₃^- have been investigated theoretically.