Prediction of 2S Rydberg energy levels of 6Li and 7Li based on quantum-mechanical calculations performed with explicitly correlated Gaussian functions

Abstract

Accurate variational quantum-mechanical calculations are performed for the nine lowest 2S (1s2ns), n = 2, . . . ,10 states of the lithium atom. The effect of the finite nuclear mass is explicitly included in the calculations allowing for the determination of the isotopic shifts of the energy levels. The wave functions of the states are expanded in terms of all-electron explicitly correlated Gaussian functions and their exponential parameters are variationally optimized with the aid of the analytical energy gradient determined with respect to those parameters. The experimental results for the lower states (n = 3, . . . ,6) and the calculated results for the higher states (n = 7, . . . ,10) fitted with quantum-defect-like formulas are used to predict the energies of 2S 1s2ns states for 7Li and 6Li with n up to 30