Continuous-Wave Two-Photon Spectroscopy of the1S2STransition in Hydrogen

Abstract
We have observed the 1S2S transition in atomic hydrogen gas with a resolution of 5 parts in 109 by Doppler-free two-photon spectroscopy, using multimilliwatt cw radiation near 243 nm generated by sum-frequency mixing. The experiment opens the way to substantial increases in the accuracy of the Rydberg constant, the electron/proton mass ratio, and the Lamb shift of the hydrogen 1S ground state.