Theoretical study of the spectroscopy and radiative transition probabilities of Si 2 from visible to infrared.

High level ab initio calculations on the electronic states of a silicon dimer (Si 2 ) 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 2 are discussed. We emphasize the triplet-triplet emission bands H 3 Σ-u-X 3 Σ-g, K 3 Σ-u-X 3 Σ-g and D 3 Π u -L 3 Π g which are dominant for 0-11 (18 822 cm -1 ), 0-0 (30 672 cm -1 ), and 0-0 (28 881 cm -1 ) transitions, respectively. In addition, the strong experimentally observed b 1 Π u -d 1 Σ+g band around 4184 cm -1 corresponds to the second 1 Σ+g-b 1 Π u combination in the infrared region. The calculated oscillator strengths of the singlet-singlet transitions (f 1 Π g -e 1 Σ-u, 2 1 Π g -b 1 Π u , b 1 Π u -d 1 Σ+g and g 1 Δ u -a 1 Δ g ) are in the order of 10 -3 . From a theoretical point of view, the 0-0 sub-band for the f 1 Π g -e 1 Σ-u transition, 0-7 for 2 1 Π g -b 1 Π u , 0-0 for b 1 Π u -d 1 Σ+g and the 0-7 vibronic bands for the g 1 Δ u -a 1 Δ g transition may be observed experimentally. It is expected that the present results could provide theoretical support for a deeper understanding of the experimental Si 2 spectra providing further applications in astrophysics.