Theoretical study of the spectroscopy and radiative transition probabilities of Si2 from visible to infrared

Abstract

High level ab initio calculations on the electronic states of a silicon dimer (Si₂) 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 Si₂ are discussed. We emphasize the triplet–triplet emission bands H³Σ⁻_u–X³Σ⁻_g, K³Σ⁻_u–X³Σ⁻_g and D³Π_u–L³Π_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 b¹Π_u–d¹Σ⁺_g band around 4184 cm⁻¹ corresponds to the second ¹Σ⁺_g–b¹Π_u combination in the infrared region. The calculated oscillator strengths of the singlet–singlet transitions (f¹Π_g–e¹Σ⁻_u, 2¹Π_g–b¹Π_u, b¹Π_u–d¹Σ⁺_g and g¹Δ_u–a¹Δ_g) are in the order of 10⁻³. From a theoretical point of view, the 0–0 sub-band for the f¹Π_g–e¹Σ⁻_u transition, 0–7 for 2¹Π_g–b¹Π_u, 0–0 for b¹Π_u–d¹Σ⁺_g and the 0–7 vibronic bands for the g¹Δ_u–a¹Δ_g transition may be observed experimentally. It is expected that the present results could provide theoretical support for a deeper understanding of the experimental Si₂ spectra providing further applications in astrophysics.