ABSOLUTE FREQUENCY-MEASUREMENT OF 1S-2S T RANSITION IN ATOMIC-HYDROGEN AND A NEW VALUE FOR THE RYDBERG CONSTANT

We have resolved the 1S-2S-transition of atomic hydrogen with a relati ve accuracy of DELTAv/v = 2.8 . 10(-11) and compared this transition f requency to a reference that was synthesized by a phasecoherent freque ncy-chain starting from an infrared CH4-stabilized helium-neon-laser a t 88 THz. In this optical frequency measurement, which is not limited by the less precise I2-stabilized helium-neon laser-standard at 633 nm , we could determine the 1S-2S-transition frequency to be 2 466 061 41 3.182(45) MHz with an 18-fold increased precision of 1.8 . 10(-11). On e can derive a new value for the Rydberg constant R(infinity) = 109 73 7.315 684 1(42) cm-1 where the uncertainty is reduced by a factor of 4 .3 compared to previous measurements.