The NPL Rydberg Constant Experiment

We describe the characterization of the apparatus that has been constructed at the National Physical Laboratory for absolute optical frequency measurements of a number of two-photon 2S-nS,D transitions in the hydrogen atom. The use of a femtosecond optical frequency comb will enable many transitions to be studied under near-identical conditions, whereas the use of optical excitation for efficient production of the metastable 2S state will reduce ac Stark shifts. These improvements are expected to lead to significantly reduced uncertainty in the measured transition frequencies. Together with results from other experiments worldwide, this paper will contribute to an improved determination of the Rydberg constant