Theoretical calculation of infrared band transitions of the Pb2 molecule

Intensity of infrared band transitions from the singlet excited b1 sigma+g and a1 Delta g states to the triplet ground X3 sigma-g state of the Pb2 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 b1 sigma+g,0+- X3 sigma-g,1 is calculated accounting the spin (S) and orbital (L) angular momentum. The former is determined by the zero-field splitting of the ground X3 sigma-g multiplet and the SOC-induced mixing coefficient between b1 sigma+g,0+ and X3 sigma-g,0+ states; the latter is determined by the admixtures of the intermediate 3 pi g and 1 pi g states in the framework of SOC perturbation theory. Furthermore, the intensity of magnetic dipole a1 Delta g,2-X3 sigma-g,1 transition only needs to take into account the orbital angular momenta interactions with magnetic wave, which arise from the a1 Delta g-1 pi g and X3 sigma-g-3 pi g magnetic transitions. The weak a, b-X3 sigma-g,1 vibronic bands induced by magnetic dipole transition moments are easily overlapped by the stronger a - A3 pi u,1,2 and b - A3 pi 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-X3 sigma-g,1 transition and the (0, 1)-(0 - 11) series for the a-X3 sigma-g,1 intercombination could be observed experimentally. The electro-quadrupole a1 Delta g,2-X3 sigma-g,0+transition can be greatly enhanced, since it "borrows intensity" from the a1 Delta g,2- b1 sigma+g,0+ quadrupole transition. Thus, we calculate, predict, and interpret series of the weak forbidden bands in the Pb2 infrared spectroscopy, which will extend our understanding of SOC effects in the IV series dimers of periodic system.