A measurement based on the fast-atomic-beam separated-oscillatory-field method of sub-natural-linewidth spectroscopy gives, for the Lamb shift in hydrogen, S(w =2) = 1057.845(9) MHz. The result is not in good agreement with theory.

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This paper reports a precision measurement of the 2 *SI/* -2*P1,2 Lamb-shift interval in hydrogen. The fast-beam, separated-oscillatory-field technique was used to obtain resonances that were significantly narrower than the natural linewidth of the transition. The resonances were observed in zero magnetic field by varying the frequency of the microwave field. The final result, 9 (H, n = 2) = 1057.845(9) MHz, is in good agreement with other measurements of this interval. There is sufficient uncertainty in the theoretical value that one cannot make a definitive comparison with theory at the level of precision of this experiment.

Fine-structure intervals in n = 17 and 20 barium Rydberg levels with 6 Յ L Յ 11 have been measured precisely with the resonant excitation Stark ionization spectroscopy microwave technique. The measured intervals were analyzed using the long-range polarization model, including allowance for the large nonadiabatic effects of the Ba + 5d level and the contributions of higher-order terms. The analysis determined improved values in atomic units of both dipole and quadrupole polarizabilities of Ba + , ␣ 1 = 123.88͑5͒ and ␣ 2 = 4420͑250͒.