Doppler-free resonance ionization spectroscopy of the He 1s{sup 2} {sup 1}S-1s2s {sup 1}S transition at 120.3 nm

An accurate determination of the Lamb shift in the ground state of He is a sensitive check of QED calculations in the 2-electron, 3-body regime. We have begun an experiment that should determine the Lamb shift in the ground state of He with an accuracy better than the uncertainty in the best QED calculations. The measurement will employ a 2-photon Doppler-free resonant excitation of the He 1s2 1S→1s2s 1S transition at 120.3 nm, followed by ionization of the 1s2s 1S state.