Ab initio configuration interaction calculations of the potential curves and lifetimes of the low-lying electronic states of the lead dimer

Abstract

The low-energy electronic spectrum of the lead dimer is described by means of a multireference configuration interaction treatment based on a semicore relativistic effective core potential (RECP) including spin–orbit coupling. The X0+g ground state is found to be a heavy mixture of the ...σ2π2 3Σ−g, the ...σπ2π* 5Πg and ...π4 1Σ+g Λ–S states, underscoring the importance of the spin–orbit interaction in determining the electronic structure of this heavy system. The first excited state has 1g symmetry and is predominantly 3Σ−g but also with a heavy admixture of 5Πg character. The lowest-lying excited state as yet observed (A) seems to be the 2u(I) state, however, with a 0.09 Å smaller computed re value than for X0+g. The B state with an experimental Te value of 12 457 cm−1 appears to be second 0−u state which arises from an avoided crossing between the ...σπ3 3Πu and the ...σ2ππ* 1Σ−u Λ–S states. Another avoided crossing between the lowest two 0+u states is shown to produce the experimental C and F states, which possess the strongest transitions to X0+g of any of the low-lying Pb2 states. The present computed radiative lifetime for the C state is in very good agreement with the measured value of Bondybey and English (1.5 μs). The calculations also find that the F state’s lifetime is only about half as long as for the C state, whereas the experimental results give a smaller ratio of close to 0.1, indicating that nonradiative transitions may also be important for depopulating this state. Eleven other states are found to lie between the F and C states, despite their relatively small Te value difference of 4500 cm−1. Two of these are believed to have been observed in emission processes to the A state, but there is insufficient experimental data to make specific assignments in this case.