Theoretical calculation of infrared band transitions of the Pb 2 molecule

Intensity of infrared band transitions from the singlet excited b1Σg+ and a 1 Δ g states to the triplet ground X3Σg- state of the Pb 2 molecule has been studied utilizing high-level ab initio multi-reference configuration interaction (MRCI) method taking into account core-valence correlation (CV), the Davidson correction (+Q) and spin–orbit coupling (SOC) effect. Intensity of the magnetic dipole transition b1Σg,0++ - X3Σg,1- is calculated accounting the spin (S) and orbital (L) angular momentum. The former is determined by the zero-field splitting of the ground X3Σg- multiplet and the SOC-induced mixing coefficient between b1Σg,0++ and X3Σg,0+- states; the latter is determined by the admixtures of the intermediate 3 Π g and 1 Π g states in the framework of SOC perturbation theory. Furthermore, the intensity of magnetic dipole a1Δg,2 - X3Σg,1- transition only needs to take into account the orbital angular momenta interactions with magnetic wave, which arise from the a 1 Δ g - 1 Π g and X3Σg- - 3 Π g magnetic transitions. The weak a, b - X3Σg,1- vibronic bands induced by magnetic dipole transition moments are easily overlapped by the stronger a  − A 3 Π u,1,2 and b  − A 3 Π u,1 vibronic bands of electric-dipole nature; therefore, the intensity of related a, b - A bands is also calculated. At the same time, it is concluded that the vibronic progressions (0, 1)-(0 ∼ 12) for the b - X3Σg,1- transition and the (0, 1)−(0 ∼ 11) series for the a - X3Σg,1- intercombination could be observed experimentally. The electro-quadrupole a1Δg,2 - X3Σg,0+- transition can be greatly enhanced, since it “borrows intensity” from the a1Δg,2 - b1Σg,0++ quadrupole transition. Thus, we calculate, predict, and interpret series of the weak forbidden bands in the Pb 2 infrared spectroscopy, which will extend our understanding of SOC effects in the IV series dimers of periodic system.