Long-range additive and nonadditive interactions for the excited Li(2 ²S)-Li(2 ²P)-Li+(1 ¹S) system

Using degenerate perturbation theory, the long-range interactions for a three-body hybrid atom-ion system composed of one ground S state Li atom, one excited P state Li atom and one ground S state Li⁺ ion are evaluated with highly accurate variationally-generated wave functions in Hylleraas coordinates. The detailed long-range potential curves are plotted for the equilateral triangle and for the linear congurations. We demonstrate that the nonadditive interactions that are induced by the degeneracy and enhanced by the induction effect in the three-body system play a signicant role and should be accounted for in constructing precise potential energy surfaces. We explore the competition for M = 0 quantum states between the attractive term C₄ / r⁴ and repulsive term C₆ / r⁶ when the internuclear distances r becomes smaller.