A comparison of the 1S-2S and 2S-4P frequencies in atomic hydrogen

An experiment has been carried out to compare directly the frequencies of the 1S1/2-2S1/2 and 2S1/2-4P1/2 transitions in atomic hydrogen and hence obtain a value for the 1S1/2 Lamb shift. It is the first of a new generation of 1S1/2 Lamb shift experiments which are much less dependent on external frequency standards than earlier work [1,2] because no absolute measurement of the 1S1/2-2S1/2 frequency is required. This pilot experiment gave a result of 8167(10) MHz for the 1S1/2 Lamb shift. The 1S1/2-2S1/2 transition was excited by two-photon Doppler-free spectroscopy. The necessary radiation at 243 nm was generated by intra-cavity frequency doubling with a crystal of β-barium borate (BBO) placed inside a ring dye laser operating at 486 nm. For the first time, the 1S1/2-2S1/2 transition was excited in an atomic beam, thereby removing the uncertainties inherent in cell experiments and generating an optically-excited beam of metastable (2S1/2) hydrogen atoms. The single-photon 2S1/2-4P1/2 transition was excited by radiation at 486 nm from a second ring dye laser, incident transversely on the optically excited metastable beam. The small frequency difference (~ 4.7 GHz) between the fundamental output of the doubled laser and that of the second laser when both were on resonance was directly measured using a stabilised optical cavity. From this measured frequency interval, the value quoted above for the 1S1/2 Lamb shift was obtained. The result is in agreement with theory and is considerably more precise than that obtained from earlier direct comparisons of 1S1/2-2S1/2 and Balmer-β [3]. The error on the result was largely due to the limited amount of data available. However, the aim of the experiment was to demonstrate the feasibility of the method. This has been achieved and considerable improvements in precision should be possible in the future.