Comparison of hyperfine anomalies in the 5 S 1 / 2 and 6 S 1 / 2 levels of 85 Rb and 87 Rb .

We observe a hyperfine anomaly in the measurement of the hyperfine splitting of the 6S1/2 excited level in rubidium. We perform two step spectroscopy using the 5S1/2 → 5P1/2 → 6S1/2 excitation sequence. We measure the splitting of the 6S1/2 level and obtain for the magnetic dipole constants of Rb and Rb A = 239.18(4) MHz and A = 807.66(8) MHz, respectively. The hyperfine anomaly difference of 87δ85 = −0.0036(2) comes from the Bohr Weisskopf effect: a correction to the point interaction between the finite nuclear magnetization and the electrons, and agrees with that obtained in the 5S1/2 ground state.