HIGH-PRECISION MEASUREMENT OF THE HYDROGEN 1S-2S ENERGY

The 1S-2S two-photon transition in atomic hydrogen is resolved to DELT Anu/nu = 1.1 x 10(-11).The transition frequency is compared to a refer ence frequency derived from an infrared CH4-stabilized helium-neon las er at 88 THz with a phase-locked laser frequency chain. The helium-neo n laser is compared to the microwave cesium standard with the help of another phase-locked frequency chain. With our optical frequency measu rement, not limited by the I2-stabilized helium-neon laser standard at 633 nm, we found a 1S-2S transition frequency of 2466061413.182(45) M Hz with an 18-fold improved accuracy. A value of the Rydberg constant, R(infinity) = 109 737.315 684 6(41) cm-1, is deduced.