Precision spectroscopy of high-L,n=10 Rydberg helium: An improved test of relativistic, radiative, and retardation effects

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Abstract
We report improved measurements of the 10G-H, 10H-I, 10I-K, and 10K-L fine-structure intervals in atomic helium, obtained using fast-beam microwave-optical techniques. The measurement uncertainty is less than 1 kHz in each of sixteen measured intervals. The existence of very precise calculations of the nonrelativistic contributions to these intervals allows the measurements to be used to test relativistic and QED effects directly, and to confirm the predicted contributions from ‘‘retardation’’ forces due to two-photon exchange to better than 10%.