Measurement of the He Ground State Lamb Shift via the Two-Photon 1 1 S -2 1 S Transition

We have extended two-photon Doppler-free spectroscopy to the vacuum ultraviolet spectral region, to accurately measure the He $^{1}1S\ensuremath{-}^{1}2S$ transition at 120 nm. Our result is 4 984 872 315(48) MHz. This yields a ground state Lamb shift of 41 104(48) MHz, in fair agreement with theory and other experiments. This approach has the potential for significantly better accuracy once improvements in the laser and the wavelength metrology are implemented.