Phase-coherent measurement of the hydrogen 1S-2S frequency with an optical frequency interval divider chain

We have measured the absolute frequency of the hydrogen 1S-2S two-photon resonance with an uncertainty of 3.4 parts in 10/sup 13/ by comparing it with the 28th harmonic of a methane-stabilized 3.39 /spl mu/m He-Ne-laser. A frequency mismatch of 2 THz at the 7th harmonic was bridged with the help of a phase-locked chain of five optical frequency interval dividers.