RELATIVISTIC MANY-BODY INVESTIGATION OF HYPERFINE INTERACTIONS IN EXCITED S-STATE OF ALKALI-METALS - FRANCIUM AND POTASSIUM

To enhance the current understanding of mechanisms contributing to mag netic hyperfine interactions in excited states of atomic systems, in p articular, alkali-metal atom systems, the hyperfine fields in the exci ted 5 S-2(1/2)-8 S-2(1/2) states of potassium and 8 S-2(1/2) - 12 S-2( 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 stales studied are in e xcellent agreement with available experimental data for both atoms. Th ere is a significant decrease in importance of the correlation contrib ution in going from the ground state to the excited states, the correl ation contributions as ratios of the direct contribution decreasing ra pidly as one moves to the higher excited states. However, the contribu tion from the exchange core polarization (ECP) effect is nearly a cons tant fraction of the direct effect for all the excited states consider ed. Physical explanations are offered for the observed trends in the c ontributions from the different mechanisms. A comparison is made of th e 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 alkal i-metal atoms, referred to as the Rydberg states, it is concluded that in addition to the direct contribution from the excited valence elect ron to the hyperfine fields, a significant contribution is expected fr om the ECP effect arising from the influence of exchange interactions between electrons in the valence and core stales.