Multielectron Atoms in External Fields

In Chapter 8 we saw that the quadratic Stark effect on the ground state of hydrogen is a consequence of the nondegenerate nature of this state. The linear Stark effect that is characteristic of the excited states of hydrogen is a direct consequence of the accidental degeneracy. On the other hand, excited states of multielectron atoms are nondegenerate and should therefore exhibit the quadratic Stark effect. In fact, this is true, but in some instances excited states of multielectron atoms behave in a hydrogenic fashion. We therefore make a distinction between hydrogenic states (of multielectron atoms) and states of hydrogen. The existence of hydrogenic states is most often observed in alkali metal atoms when they are subjected to an external electric field. As might be expected from the discussion of Chapter 11, high angular momentum states, those having the smallest quantum defects, are most nearly hydrogenic.