Absolute frequency measurement of the 2S‐8S/D transitions in atomic hydrogen

We have performed an absolute frequency measurement of the 2S‐8S/D two‐photon transition in atomic hydrogen. We have compared the hydrogen frequencies with the difference of two optical standards, namely the methane stabilized and iodine stabilized He‐Ne laser. In this way, we have linked the 2S‐8S/D frequencies to the cesium clock. We have deduced a new value for the Rydberg constant with an uncertainty of 2.2 parts in 1011.