Relativistic many-body investigation of the hyperfine interaction in ground-state rubidium

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Abstract
The hyperfine structure of the rubidium atom in the ground state has been studied by means of the relativistic many-body perturbation procedure. Relativistic effects are found to have an important influence on direct, exchange-core-polarization (ECP), and correlation effects, the enhancement over nonrelativistic results being, respectively, 20, 19, and 15%. Correlation effects are found to be significantly larger in magnitude as compared to ECP effects, the ratio of the two to the direct contribution being 0.32 and 0.19, respectively. This feature is opposite in nature to that in lithium and sodium, but represents a continuation of the trend found in going from lithium to sodium, and indicates that the relative importance of correlation will be even more pronounced in the heavier alkali metals, cesium and francium. The net theoretical value for the hyperfine constant was obtained as 3460±50 MHz, in satisfactory agreement with the experimental result of 3417 MHz. The physical reasons for the observed trends in the contributions from different mechanisms will be discussed, including some detailed features such as the relative contributions from different core shells to the ECP and correlation effects.