Measurement of the hyperfine splitting of the 6S${}_{1\mathrm{/}2}$ level in rubidium

We present a measurement of the hyperfine splitting of the 6S${}_{1\mathrm{/}2}$ excited level of rubidium using two photon absorption spectroscopy in a glass cell. The values we obtain for the magnetic dipole constant A are 239.18(03) MHz and 807.66(08) MHz for ${}^{85}$Rb and ${}^{87}$Rb, respectively. The combination of the magnetic moments of the two isotopes and our measurements show a hyperfine anomaly in this atomic excited state. The observed hyperfine anomaly difference has a value of ${}_{87}{\delta }_{85}=-0.0036(2)$ due to the finite distribution of nuclear magnetization, the Bohr-Weisskopf effect.