Field shift crossed-second-order effects in lNs configurations

In l N s configurations the coupling of the s electron causes each parent term of the l N core to split into two Russell-Saunders terms. Far-configuration mixing ('crossed-second-order effects') leads to different electron charge densities at the nucleus for these two terms and thus to different field isotope shifts. With the Hartree-Fock computer code of Froese Fischer (1976), the difference in the non-relativistic electron charge densities of those pairs of terms belonging to the parent with the highest multiplicity and the highest L value, was calculated for the configurations np N (n+1)s (n=2, 3, 4, 5, 6), nd N (n+1)s (n=3, 4, 5), and nf N (n+2)s (n=4, 5). In the d series the crossed-second-order effect is generally an order of magnitude larger than in the p and f series with the same principal quantum number. The crossed-second-order effect increases with n and is the same for isoelectronic sequences. There is good agreement between theoretical and experimental field shift differences.