Comparison of hyperfine anomalies in the 5S1/2 and 6S1/2 levels of 85Rb and 87Rb

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 85Rb and 87Rb A=239.18(4)MHz and A=807.66(8)MHz, respectively. The hyperfine anomaly difference of δ8587=−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.