The hyperfine field at the nucleus of singly ionized radium has been investigated using the relativistic linked-cluster many-body-perturbation-theory procedure, including the effects of distributed charge and magnetization over the nucleus. The total hyperfine field of 1239 T, when combined with the experimentally observed hyperfine constant for ' Ra+, yields a nuclear moment of 0.607(12)pz, in excellent agreement with the experimentally observed moment of 0.6133(18)p& from Zeeman measurements.Our investigation leads to exchange core-polarization and correlation contributions of 14% and 13%, respectively, of the direct contribution of the 7s valence electron, these ratios being smaller than the corresponding ratios for the isoelectronic atom francium, in keeping with the expected trends for other isoelectronic alkaline-earth-metal ions and alkali-metal atoms.Comparison is made with the results of other calculations of the nuclear moment of "Ra. Possible reasons for the success of the semiempirical Goudsmit-Fermi-Segre formula in predicting the hyperfine field in this ion and related systems will be discussed.

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To enhance the current understanding of mechanisms contributing to magnetic hyperfine interactions in excited states of atomic systems, in particular, alkali-metal atom systems, the hyperfine fields in the excited 5 2 S 1/2 -8 2 S 1/2 states of potassium and 8 2 S 1/2 -12 2 S 1/2 states of francium atoms have been studied using the relativistic linked-cluster many-body perturbation procedure. The net theoretical values of the hyperfine fields for the excited states studied are in excellent agreement with available experimental data for both atoms. There is a significant decrease in importance of the correlation contribution in going from the ground state to the excited states, the correlation contributions as ratios of the direct contribution decreasing rapidly as one moves to the higher excited states. However, the contribution from the exchange core polarization ͑ECP͒ effect is nearly a constant fraction of the direct effect for all the excited states considered. Physical explanations are offered for the observed trends in the contributions from the different mechanisms. A comparison is made of the different contributing effects to the hyperfine fields in potassium and francium to those in the related system, rubidium, studied earlier. Extrapolating from our results to the highly excited states of alkali-metal atoms, referred to as the Rydberg states, it is concluded that in addition to the direct contribution from the excited valence electron to the hyperfine fields, a significant contribution is expected from the ECP effect arising from the influence of exchange interactions between electrons in the valence and core states.

Using relativistic many-body perturbation theory, we have investigated the valence, exchange core polarization, and correlation contributions to the magnetic hyperfine constants in the ground states of the lithiumlike ions corrections have also been investigated. Both the exchange core polarization and the correlation contributions as fractions of the valence-electron contribution decrease rapidly as one goes to more highly charged ions, the decrease being more drastic for the correlation effect. The radiative effect, on the other hand, increases very rapidly with increasing charge, becoming of the same order of magnitude as the correlation effect in O'+, F +, and Ne'+. For Bi' + the radiative effect is larger than the correlation contribution, being about 0.3% of the hyperfine field from the valence 2s electron. The significance of these results and trends will be discussed, and comparisons will be made between the net hyperfine fields obtained in the present work and available experimental results as well as with the results of earlier calculations.

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