MRCI Study of the Electronic Structure and Transition Properties of a Tin Dimer

The ground and excited states of Sn₂ are calculated using the multireference configuration interaction method combined with Davidson correction (MRCI+Q). The influence of the spin-orbit coupling (SOC) effect on the electronic structure is also considered by the state interaction method of Breit-Pauli Hamiltonian. In the calculations, the potential energy curves and spectroscopic constants of 23 Λ-S states and 31 Ω states of Sn₂ are obtained. The prominent spectral features in the visible region, new constants, and potential energy curves are discussed. The intensity of weak magnetic and quadrupole transitions in the near IR spectra is also calculated. From a computational point of view, we predict that the weak v'(0-2)-v″(0-5) bands of the magnetic b¹Σ_g,0⁺-X³Σ_g,1(Ms=±1)^- transition may be detected experimentally; the sub-bands (0, 0), (1, 0), and (2, 0) of the a¹Δ_g,2-X³Σ_g,1(Ms=±1)^- transition also may be observed in experiments since they are not overlapped by the strong electric dipole transition in the same IR region. According to the SOC matrix elements and contributions of the 1⁵Π_u0, 1⁵Π_u1 (|Σ| = 0), and 1⁵Π_u1 (|Σ| = 2) states to the predissociation line width of the 1³Σ_u^- -X³Σ_g1^- transition, the broading and other predissociation features of the 1³Σ_u^- state are analyzed. From our calculations, it follows that the strong coupling between the bound 1³Σ_u^- state and the repulsive 1⁵Π_u state causes the predissociation of the 1³Σ_u^- state at the vibrational levels v' ≥ 8. In addition, our results suggest that the previously observed bands of Sn₂ in the visible range of 19000-20000 cm^-1 should be reassigned into the mixing transitions among the X³Σ_g,1^--2³Σ_u,0⁺ and X³Σ_g,0^--2³Σ_u,1⁺ manifold. The results are expected to provide new comprehensive information for better understanding the spectra and dynamics of the electronic excited states of the Sn₂ molecule.