POTENTIAL-ENERGY CURVES OF M(NP(2)P)CENTER-DOT-RG((2)-UPI) EXCITED-STATES AND M+CENTER-DOT-RG GROUND-STATES (M=LI, NA - RG=HE, NE)

It has been established for some time that the bond energies for any given valence p pi excited state of Group 1 ((2)p) and Group 2 ((1,3)p) metal-atom/rare-gas (M.RG) van der Waals complexes tend to increase with the polarizability of the RG atom. It is also known that the binding energies of the corresponding M(+).RG ground state ions are generally greater than those of the neutral M(p pi).RG excited states with the same RG atom. However, there are two stark exceptions to these trends, both involving Group 1 metal atoms and the rare gas He; Li((2)p P-2).He((II)-I-2) and Na(3p P-2).He((II)-I-2), which are the focus of the present study. We have conducted ab initio calculations of the potential energy curves of M(np P-2).RG((II)-I-2) and M(+).RG states, where M=Li, Na and RG=He, Ne. We find that the unusual behavior of the p pi Group 1 metal atom states is due to (i) the lack of M(p pi)-RG(p pi) repulsive orbital overlap in the He case, and (ii) substantial additional attraction due to correlated motion of the RG atom's electrons and the diffuse M(np pi) electron which is absent in the M(+)RG cases.