Measurement of the 2S-8D Transition in Atomic Hydrogen

High-precision laser spectroscopy of hydrogen provides important input data for determinations of the proton radius and Rydberg constant. At Colorado State University, we are currently measuring the hydrogen 2S-8D_5/2 transition, which was previously measured in 1997 [Beauvoir et al., Phys. Rev. Lett. 78, 440 (1997)].  In our measurement, a cryogenic beam of metastable hydrogen is formed with a cold nozzle and optically excited to the 2S metastable state. The 2S-8D_5/2 two-photon transition is driven in a power enhancement cavity and the remaining metastable atoms are detected to provide a spectroscopic signal. We will carefully describe that status of our measurement, and the characterization of our two main systematic effects – the light shift, and the DC Stark shift.