Spin-orbit effects in electric field gradients of alkali metal atoms

Spin-orbit effects in electric field gradients (EFG) of alkali atoms in the P-2(3/2) state are investigated by comparison of correlated (CCSD(T)) four-component Dirac-Fock results with scalar relativistic Douglas-Kroll calculations. Since the Douglas-Kroll method is based on a unitary transformation of the full four-component Dirac Hamiltonian to a two-component form the use of the untransformed EFG operator leads to the well known picture change error. We therefore use the point charge nuclear quadrupole moment method in combination with the Douglas-Kroll transformation which removes picture-change effects since only Coulomb-type operators are to be transformed. We find that spin-orbit effects partially compensate scalar relativistic effects and are only important for the heavier elements Rb and Cs. A comparison to EFGs derived from valence-only (r(-3)) values shows that Sternheimer corrections are roughly proportional to the dipole polarizability of the positively charged atom and can become very large for the heaviest atom studied, Cs, amounting to 46% of the total EFG.