Hyperfine Splittings and Isotope Shift of the Rb 5S - 6S Two-Photon Transition*

Using a Rb vapor cell and locking a microwave-driven EOM sideband of our 993 nm laser to an ultrastable very high finesse optical cavity, thus tuning the laser with microwave frequency accuracy, we have conducted Doppler free two-photon spectroscopy of the Rb 5S - 6S transition, in both isotopes.  We have measured the hyperfine splittings of the 6S state, and the isotope shift of the transition, to an accuracy of about 10 kHz.