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

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 nrn which is obtained with two frequency doubling stages of a titanium sapphire laser at 820 cm. Its frequency is measured with an optical frequency comb. The second-order Doppler effect is evaluated from the observation of the motional Stark effect clue to a transverse magnetic field perpendicular to the atomic beam. The measured value of the 1S(1/2) (F = 1)-35(1/2)(F = 1) frequency splitting is 2 922 742 936.729(13) MHz with a relative uncertainty of 4.5 x 10(-12). After the measurement of the 1S-2S frequency, this result is the most precise of the optical frequencies in hydrogen.