We have measured the 1S-2S transition frequency in atomic hydrogen with a precision of 7 parts in ${10}^{10}$ by continuous-wave Doppler-free two-photon spectroscopy. We employ cavity-enhanced multimilliwatt radiation near 243 nm produced by sum-frequency generation, and we observe the 1S-2S transition in a low-pressure hydrogen-helium cell with a resolution of 3 parts in ${10}^9$. For a frequency comparison we detect an optical heterodyne signal at the difference frequency between the 243-nm light used to excite the 1S-2S transition and the second harmonic of a reference laser locked to an interferometrically calibrated ${}_{}{}^{130}{}_{}{}^{}\mathrm{Te}_2^{}{}_{}{}^{}$ absorption line near 486 nm. After determining systematic corrections due to the pressure shift of the F=1 hyperfine component in a 0.7 vol. % hydrogen–99.3 vol. % helium gaseous mixture, we obtain the energy-level separation f(1S-2S) =2 466 061 413.2(18) MHz. Choosing a value of the Rydberg constant measured independently by high-resolution spectroscopy of the hydrogen Balmer-β transition, we find the hydrogen ground-state Lamb shift to be $f_{\mathrm{LS}\left(1\mathrm{}\mathrm{S}\right)=8173.9(19\mathrm{}}$) MHz, in good agreement with the theoretical value of 8172.89(9) MHz.

• Received 14 November 1988

DOI:https://doi.org/10.1103/PhysRevA.39.4591