Calculation of the isotope shift in Na

The specific mass shift (SMS) dominates the isotope shift for a light atom like Na. It arises from a correlation between the electronic momenta and is very sensitive to pair correlation effects in the wavefunction. We here extend earlier work on the SMS of Li and K and evaluate the second-order pair correlation contributions for the 3s, 3p and 3d levels of Na. For the resonance line, 3s→3p, the inclusion of the lowest-order correlation effects changes the theoretical result from 16% to about 80% of the experimental value. We also demonstrate that for scalar operators, like the SMS and also the field shift, diagrams involving only single excitations are included in a restricted Hartree-Fock calculation if the valence electron is included in the potential. The analogy is illustrated by numerical results both for the specific mass shifts and the field shifts for Na and K. (orig.)