RELATIVISTIC CORRECTION TO THE DIPOLE POLARIZABILITY OF A HYDROGENIC ION

The first relativistic correction of order alpha(2) to the dipole pola rizability of a hydrogenic ion has been investigated by using mean exc itation energy of the ion within the second-order perturbation theory. The density-dependent mean excitation energy Is estimated via Bethe t heory for the stopping cross section for a moving point charge interac ting with the hydrogenic ion. In this approach only the unperturbed Di rac wavefunctions are required to evaluate the appropriate matrix elem ents. The first relativistic correction turns out to be - (13/12) (alp ha Z)(2). This has the correct sign and is within 5% of the exact resu lt which is - (28/27) (alpha Z)(2).