High-resolution spectroscopy of the1S−2Stransition 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 $1S$ 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 $\Delta \nu /\nu =8\mathrm{}\times {10}^{-13}.$ A theoretical line-shape model based 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 $1.5\times {10}^{-14}.$