High-resolution spectroscopy of the 1S-2S transition in atomic hydrogen

The sharp hydrogen 1S-2S two-photon transition is a promising candidate for the realization of a frequency standard based on an atomic transition in the optical region. In recent work we have used this transition to precisely determine the Rydberg constant, the IS Lamb shift and the hydrogen-deuterium isotope shift. In this paper we focus on substantially improved spectroscopic methods leading to a much higher spectral resolution of the 1S-2S transition in hydrogen and deuterium. We have successfully applied a time-delayed measurement scheme, which allowed us to reduce the linewidth to 1 kHz at 243 nm corresponding to a spectral resolution of Delta v/v=8x10(-13). A theoretical line-shape model bused on a solution of the Master equation allows us to determine the unperturbed hydrogen 1S-2S two-photon transition frequency from our spectra to a level of 1.5x10(-14). [S1050-2947(99)06202-2].