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We have measured the absolute frequency of the hydrogen 1S-2S two-photon resonance with an accuracy of 3.4 parts in 10 13 by comparing it with the 28th harmonic of a methane-stabilized 3.39 mm He-Ne laser. A frequency mismatch of 2.1 THz at the 7th harmonic is bridged with a phase-locked chain of five optical frequency interval dividers. From the measured frequency f 1S-2S 2 466 061 413 187.34͑84͒ kHz and published data of other authors we derive precise new values of the Rydberg constant, R` 10 973 731.568 639͑91͒ m 21 and of the Lamb shift of the 1S ground state, L 1S 8172.876͑29͒ MHz. These are now the most accurate values available. [S0031-9007(97)04182-3] PACS numbers: 31.30.Jv, 06.20.Jr, 21.10.FtFor almost three decades, the 1S-2S two-photon transition in atomic hydrogen with its natural linewidth of only 1.3 Hz has inspired advances in high resolution laser spectroscopy and optical frequency metrology [1]. This resonance has become a de facto optical frequency standard. More importantly, it is providing a cornerstone for the determination of fundamental physical constants and for stringent tests of quantum electrodynamic theory. In the future, it may unveil conceivable slow changes of fundamental constants or even differences between matter and antimatter.Here, we report on a new precise measurement of the absolute frequency of the 1S-2S interval which exceeds the accuracy of the best previous measurement [2] by almost 2 orders of magnitude. The 1S-2S resonance is observed by longitudinal Doppler-free two-photon spectroscopy of a cold atomic beam. The resonance frequency is compared with the frequency of a cesium atomic clock with the help of a phase-coherent laser frequency chain, using a transportable CH 4 -stabilized He-Ne laser at 3.39 mm as an intermediate reference.In this way, we have determined a 1S-2S interval of f 1S-2S 2 466 061 413 187.34͑84͒ kHz with an uncertainty of 3.4 parts in 10 13 , limited by the reproducibility of the He-Ne reference laser. This represents now the most accurate measurement of any optical frequency in the ultraviolet and visible region. Together with the results of other authors, in particular, the recent precision measurements of the 2S 1͞2 -8D 5͞2 transition frequency in hydrogen by the group of Biraben [3], we derive new and more precise values for both the Rydberg constant and the 1S Lamb shift. This Lamb shift provides now the best test of quantum electrodynamics for an atom.As in our earlier experiment [2] we are taking advantage of the near coincidence between the 1S-2S interval and the 28th harmonic of the frequency of a CH 4 -stabilized 3.39 mm He-Ne laser. However, a frequency mismatch of 2.

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 ⌬/ϭ8ϫ10 Ϫ13 . 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 1.5ϫ10 Ϫ14 .

The hyperfine coupling (hfc) constants, A, and the g-factors of hydrogen isotopes confined in silasesquioxane (R 8 Si 8 O 12 ) cages show significant deviations from the free-particle vacuum value thus revealing the influence of the environment on the wave function of the trapped atom. Accurate measurements of the hfc constants in the temperature range of 5-300 K show a strong isotope effect, different substituents on the cage corners having a clear influence on both static and dynamic cage effects. Deviations of g-factors from the freeelectron value are independent of temperature, but they depend on substituents. To gain information about the factors influencing the trapped atoms' dynamic behavior inside the cage environments, a phenomenological model developed previously for the description of the dynamics of hydrogen isotopes in liquid water and ice (J. Chem. Phys. 1995, 102, 5989), which is based on the spherical harmonic oscillator, is applied to the more rigid silasesquioxane cage systems. The present study aims at a better understanding of matrix effects on the dynamics of particles in a constraining environment, a problem which represents a challenge for both phenomenological modeling and quantum chemical calculations.