Absolute Hyperfine Energy Levels and Isotope Shift of Rb 5S – 6S Transition

Using a Rb vapor cell and a wavemeter with a Doppler free two-photon transition near 993 nm, we have measured the hyperfine splittings of the 5S – 6S transition, and for the first time the 5S – 6S isotope shift to be 94 MHz, to an accuracy of 12 MHz. In doing so, we have measured the absolute energy levels of those 6S states to an accuracy of 60 MHz, five times more precise than the best previous measurements. We are proceeding to lock a microwave driven EOM sideband of our laser to an ultra-stable cavity, allowing a measurement of the hyperfine splittings and isotope shift of the 6S state to an accuracy of approximately 50 kHz.