GROUND-STATE AND EXCITED-STATE PROPERTIES OF NEUTRAL AND ANIONIC SELENIUM DIMERS AND TRIMERS

Ab initio molecular-orbital methods using a newly constructed large, f lexible basis set of the atomic-natural-orbital type and extensive tre atments of electron correlation were used to accurately predict the gr ound-state potential-energy curves of Se-2 and Se-2(-) In addition, th e potential curves of low-lying electronically excited states of the S e-2(-) anion, which are coupled to the ground state through the electr onic dipole operator, are determined. Calibration of the accuracy expe cted for the employed theoretical models is achieved by calculations o n electronically excited states of neutral Se-2, for which accurate ex perimental data are available. Besides reporting accurate predictions for spectroscopic constants, electron affinities, and transition matri x elements, particular emphasis is laid on the use of the computed dim er potential-energy curves for interpreting the recently investigated peculiar optical properties of diatomic selenium species embedded in s olid host matrices. Further, the equilibrium geometries and relative s tabilities of the C-2v and D-3h forms of the neutral Se-3 cluster, the electron affinity of Se-3 and the low-lying electronic states of Se-3 (-) have been investigated theoretically.