An <i>ab initio</i> study of the ground states and some excited states of BeRG, Be+RG, and Be+2RG van der Waals complexes (RG=He, Ne)

Abstract

By ab initio methods, we have characterized the potential curves of the unusual, doubly excited valence van der Waals states, Be(2pπ+12pπ−1 3PJ)⋅RG[3Σ−], where RG=He, Ne. Similar to the Mg(3pπ+13pπ−1 3PJ)⋅RG[3Σ−] states (RG=He, Ne, Ar, Kr) which were characterized experimentally and theoretically earlier, these Be(2pπ2pπ 3PJ)⋅RG[3Σ−] states are much more strongly bound than their singly excited Be(2s2pπ 3PJ)⋅RG[3Π] analogs, and even much more strongly bound than the analogous Be+(2s 2S)⋅RG[2Σ+] ground-state ions. This is attributed to the lack of a large Be(2s) electron cloud with density along the internuclear axis, so that quadrupole/induced-dipole and dispersion attraction forces can proceed to much smaller internuclear distances before repulsion sets in. The BeHe[3Σ−] state is also almost five times more bound than the BeNe[3Σ−] state, despite the fact that the polarizability of the He atom is only one-half that of the Ne atom. This is again attributed to minimization of repulsive forces, since strong Be(2pπ)/RG(npπ) exchange repulsion is completely absent when RG=He. The fact that the bond strengths of the Be(2pπ2pπ)⋅He[3Σ−] and Be+(2pπ)⋅He[2Π] states are quite similar, and only about 20% less than the bond strength of the free Be++⋅He[1Σ+] ion, is consistent with this interpretation. All of the strongly bound Be⋅RG neutral and ionic states calculated here have bond strengths greater than, and bond lengths smaller than, their Mg⋅RG analogs. This is because the Be electron clouds are all smaller than their Mg analogs, so that, again, all attractive forces can proceed to smaller distances before repulsion sets in.