First Pure Frequency Measurement of an Optical Transition in Atomic Hydrogen: Better Determination of the Rydberg Constant

We have performed a pure frequency measurement of the 2 S -8 S/D two-photon transition in atomic hydrogen. The hydrogen frequencies are compared with the difference of two optical standards, the methane-stabilized He-Ne laser and the iodine-stabilized He-Ne laser. In this way, an optical frequency of atomic hydrogen is directly linked for the first time to the caesium clock without any interferometry. We deduce a new value for the Rydberg constant, R ∞ = 109 737.315 683 4(24) cm -1 with an uncertainty of 2.2 parts in 10 11 . This value is currently the most precise available.