Approach towards the critical charge of some excited states of the Be isoelectronic series

A definitive determination of the nature of the approach to the critical charge is illustrated for the (1s(2)2s(3)s)S-1,S-3 and the (1s(2)2s3p)P-1,P-3 states of the Be isoelectronic series. The features that conclusively indicate that the outermost orbital becomes an infinitely diffuse hydrogenlike orbital upon approaching the critical charge Z(c) = 3 are the asymptotic proportionality of the ionization energy to (Z - 3)(2) and the approach of the corresponding quantum defect to an integer which is equal to the number of orbitals of the same symmetry in the atomic core.