Theoretical study of the spectroscopy and radiative transition probabilities of Si from visible to infrared.

High level calculations on the electronic states of a silicon dimer (Si) have been carried out by employing a multi-reference configuration interaction plus Davidson correction (MRCI + Q) approach with the aug-cc-pVQZ basis set. The scalar relativistic correction is taken into consideration by the second-order Douglas-Kroll-Hess approximation. In the present work, the transition properties (oscillator strength, Einstein spontaneous emission coefficient and radiative lifetime) of the singlet-singlet, triplet-triplet, and quintet-quintet transitions of Si are discussed. We emphasize the triplet-triplet emission bands HΣ-u-XΣ-g, KΣ-u-XΣ-g and DΠ-LΠ which are dominant for 0-11 (18 822 cm), 0-0 (30 672 cm), and 0-0 (28 881 cm) transitions, respectively. In addition, the strong experimentally observed bΠ-dΣ+g band around 4184 cm corresponds to the second Σ+g-bΠ combination in the infrared region. The calculated oscillator strengths of the singlet-singlet transitions (fΠ-eΣ-u, 2Π-bΠ, bΠ-dΣ+g and gΔ-aΔ) are in the order of 10. From a theoretical point of view, the 0-0 sub-band for the fΠ-eΣ-u transition, 0-7 for 2Π-bΠ, 0-0 for bΠ-dΣ+g and the 0-7 vibronic bands for the gΔ-aΔ transition may be observed experimentally. It is expected that the present results could provide theoretical support for a deeper understanding of the experimental Si spectra providing further applications in astrophysics.