Hyperfine splitting of the $6s$ level in Rb and its hyperfine anomaly

We present a hyperfine splitting measurement on the $6s$ level in $^{85}$Rb and $^{87}$Rb. The source of atoms is a 30 cm long cell with the natural abundance of the two isotopes. The cell operates at room temperature in a controlled magnetic environment. Two step excitation through the $5P_{1/2}$ level with lasers at 795 nm and 1.3 $\mu$m allows us to study the hyperfine separation of the $6s$ level. Use of AM sidebands in the 1.3 $\mu$m laser produces in-situ calibration of the scan. Detection through changes in the absorption at 795 nm shows resonances with good signal to noise ratio to permit a preliminary measurement of the splittings to better than 0.4 MHz. This resolution allows quantitative extraction of the hyperfine anomaly, a manifestation of the space distribution of the nuclear magnetization, in the first excited state of the $s$ manifold. We compare our results with ab initio calculations to test the quality of their wave-functions at the nucleus.