THE ELECTRONIC-STRUCTURE OF SI3 AND GE3 IN THE LOCAL DENSITY FUNCTIONAL APPROXIMATION

The electronic structures of the lowest-lying singlet and triplet states of Si3 and Ge3 and the lowest-lying states of their respective cations have been determined within the local density functional approximation. The geometries were optimized and vibrational frequencies were calculated. The lowest-lying 1A1 (C2v, theta = 82.5-degrees) and 3A2' (D3h) states for Si3 are predicted to be nearly degenerate in energy in agreement with other workers. On the basis of the calculations, the dominant vibrational frequency separations. almost-equal-to 360-370 cm-1, attributed to the lowest-lying states observed in negative ion photoelectron and laser-induced plasma spectroscopy, are associated with the degenerate e' vibrations of the D3h 3A2' state or with a symmetric stretch-bending mode difference frequency combination band of the lowest 1A1 state. The 1A1 (theta = 85.0-degrees) and 3A2' states are separated by a larger energy in Ge3. The 1A1 state lies lowest and the calculated vibrational frequencies for the 1A1 state compare more favorably with the lower state level separations characterizing the observed laser-induced plasma emission spectrum. A B-2(2) ground state is predicted for the Si3+ and Ge3+ ions with bond angles of 72.1-degrees and 75.2-degrees, respectively. The 2A2 state found by previous workers is not a minimum on the potential energy surface.