Doppler-Free Two-Photon Spectroscopy of Hydrogen1S−2S

We have observed the $1S-2S$ transition in atomic hydrogen and deuterium by Doppler-free two-photon spectroscopy, using a frequency-doubled pulsed dye laser at 2430 Å. Simultaneous recording of the absorption spectrum of the Balmer-$\beta$ line at 4860 Å, using the fundamental dye-laser output, allowed us to precisely compare the energy intervals $1S-2S$ and $2S$, $P-4S$, $P$, $D$ and to determine the Lamb shift of the $1S$ ground state to be 8.3 ± 0.3 GHz (D) and 8.6 ± 0.8 GHz (H).