An interferometric measurement of the 1s-2s transition frequency in atomic hydrogen

Two photon Doppler free laser spectroscopy has been used to measure the 1S½--2S½ transition frequency in atomic hydrogen by interferometric comparison with a reference line in 130Te2. The measured 1S½--2S½ transition frequency of 2 466 061 397 (25) MHz is in good agreement with the theoretical value 2 466 061 405.5 (2.9) MHz. This measurement can be used to deduce a value for the ground state Lamb shift giving 8182(25) MHz, which may be compared with a theoretical value 8173.248(81) MHz. Alternatively a value of the Rydberg constant can be deduced as 10 973 731.50 (11) m-1 which is consistent with other measurements but with a slightly larger uncertainty. This is the first time that the Rydberg constant has been measured on a transition in atomic hydrogen other than Balmer (alpha) using techniques of laser spectroscopy