Rovibrational states of the 1A1 ground electronic state of Si 3

Abstract

The rovibrational spectrum of the ground electronic state (1A1) of Si3 has been calculated using QCISD(T)_FC electronic model, with a large contracted basis set. The three-dimensional ab initio discrete potential energy surface was fitted with a 6th order power series expansion, which incorporated a Dunham expansion variable. The χ2 for the fit was 6·7 x 10-4 au. The force field was embedded in the Eckart–Watson Hamiltonian in order to obtain variationally the low-lying vibrational and rovibrational wavefunctions and eigenenergies. The differences between the anharmonic and harmonic calculations for the fundamental modes v 1(a1),v 2(a1),v 3(b2) are 11, 2 and 20 cm-1respectively. Although the vibrational separations observed in photodetachment studies of Si3 - are 337 ± 10 cm-1 501 ± 10 cm-1, which are in close agreement with the calculated values of 343 cm-11 (two-quanta excitation in the bend mode) and 498 cm-1 (one-quantum excitation in the asymmetric stretch mode), respectively, it would be anticipated that the Franck–Condon factor would be zero for the latter transition, and so it would not be observed in this experiment.