DOPPLER-FREE RESONANCE IONIZATION SPECTROSCOPY OF THE He 1s2 1S - 1s2s 1S TRANSITION AT 120.3 NM. Scott D. Bergeson Jesse Wen, Thomas B. Lucatorto, Buildiing 221, Room A253, National Institute of Standards and Technology, Gaithersburg, MD, USA. (301-975-3394, email:

An accurate determination of the Lamb shift in the ground state of He is a sensitive check of quantum electrodynamics (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. The laser system for the generation of the 120.3 nm radiation has been completed. The 120.3 nm photons are created in a 4-wave mixing scheme. A 10 mW cw diode laser at 842 nm is pulse amplified to about 30 mJ in 5 ns. The 842 nm pulse is then doubled twice in a sequential BBO crystals to 210 nm. This fourth harmonic is then mixed with the 842 nm fundamental in a phase matched mixture of Kr and Ar in a 2w1 - w2 scheme to generate the 120.3 nm radiation. The technology developed for this measurement can be used for EUV wavelength standards and metrology. In addition, when large collider facilities produce anti-hydrogen, this laser source could be used to trap it for detailed spectroscopic investigations.