Optical frequency measurement of the 1S–3S two-photon transition in hydrogen

Abstract

This article reports the first optical frequency measurement of the 1S–3S transition in hydrogen. The excitation of this transition occurs at a wavelength of 205 nm which is obtained with two frequency doubling stages of a titanium sapphire laser at 820 nm. Its frequency is measured with an optical frequency comb. The second-order Doppler effect is evaluated from the observation of the motional Stark effect due to a transverse magnetic field perpendicular to the atomic beam. The measured value of the $1\mathrm{S}_{1/2}$ (F = 1)-3S1/2(F = 1) frequency splitting is 2 922 742 936.729(13) MHz with a relative uncertainty of 4.5 × 10-12. After the measurement of the 1S–2S frequency, this result is the most precise of the optical frequencies in hydrogen.