Polarization Model for the Excited States of Neutral Helium

The polarization model for the excited states of He I is reconsidered in the framework of modern perturbation theory. The central potential exerted on the optical electron by the ionic core is obtained in an unified way, and its first three terms — Coulomb, dipole, and quadrupole — are shown to account quantitatively for the binding energy of the excited levels ($n,l$) with $n>~3$ and $l>~2$. The usual dipole approximation is then established on firm grounds and appears to be sufficient for practical uses of the polarization model.