DETERMINATION OF THE RYDBERG CONSTANT BY DIRECT FREQUENCY-MEASUREMENT

We have built a frequency chain to connect the frequencies of the 2S-8 S/D two-photon transitions in hydrogen to two optical standards, the m ethane-stabilized He-Ne laser and the iodine-stabilized He-Ne laser. I n this way, an optical frequency of atomic hydrogen is directly linked for the first time to the cesium clock without any interferometry. We deduce a new value for the Rydberg constant with an uncertainty of 2. 2 parts in 10(11). This value is in very good agreement with published values. The same frequency scheme has been used to measure the freque ncies of the 5S-5D two-photon transition in rubidium and 2(3)S(1-)3(3) P(o) in helium.