Theoretical study of the spectroscopy and radiative transition probabilities of Si₂ 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 Sigma(-)(u)-X-3 Sigma(-)(g), K-3 Sigma(-)(u)-X-3 Sigma(-)(g) and D-3 Pi(u)-L-3 Pi(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)Pi(u)-d(1)Sigma(+)(g) band around 4184 cm(-1) corresponds to the second (1)Sigma(+)(g)-b(1)Pi(u) combination in the infrared region. The calculated oscillator strengths of the singlet-singlet transitions (f(1)Pi(g)-e(1)Sigma(-)(u), 2(1)Pi(g)-b(1)Pi(u), b(1)Pi(u)-d(1)Sigma(+)(g) and g(1)Delta(u)-a(1)Delta(g)) are in the order of 10(-3). From a theoretical point of view, the 0-0 sub-band for the f(1)Pi(g)-e(1)Sigma(-)(u) transition, 0-7 for 2(1)Pi(g)-b(1)Pi(u), 0-0 for b(1)Pi(u)-d(1)Sigma(+)(g) and the 0-7 vibronic bands for the g(1)Delta(u)-a(1)Delta(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.