High-resolution spectroscopy of the hydrogen 1S-2Stransition in an atomic beam

We have observed the 1S-2S transition of atomic hydrogen to a resolution of 5 parts in ${10}^{11}$ by means of Doppler-free two-photon spectroscopy in an atomic beam. The significantly asymmetric line shape is strongly affected by the relativistic Doppler effect. The new high resolution can be used for improved measurements of the Rydberg constant, the 1S Lamb shift, and the isotope shift of hydrogen and deuterium.