Absolute frequency measurement of the hydrogen 1S-2S transition and a new value of the Rydberg constant.
We have compared the frequency of the hydrogen 1S-2S transition (resonance width \ensuremath{\Delta}\ensuremath{\nu}/\ensuremath{\nu}=2.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$) with a reference frequency synthesized from an 88-THz infrared ${\mathrm{CH}}_{4}$-stabilized HeNe laser. In this first measurement of an optical frequency that overcomes the uncertainty limits of the visible ${\mathrm{I}}_{2}$-stabilized He-Ne laser standard, we find a 1S-2S frequency of 2 466 061 413.182(45) MHz with an 18-fold improved precision of 1.8 parts in ${10}^{11}$. A new value of the Rydberg constant, ${\mathit{R}}_{\mathrm{\ensuremath{\infty}}}$=109 737.315 684 1(42) ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, is deduced.