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

The rovibrationaI spectrum of the ground electronic state ((1)A(1)) of Si-3 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 chi(2) 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 upsilon(1)(a(1)), upsilon(2)(a(1)), upsilon(3)(b(2)) are 11, 2 and 20 cm(-1) respectively. Although the vibrational separations observed in photodetachment studies of Si-3(-) are 337+/-10 cm(-1) and 501 +/- 10 cm(-1), which are in close agreement with the calculated values of 343 cm(-1) (two-quanta excitation in the bend mode) and 495 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.