Measurement of the Lamb shifts in singlet levels of atomic helium

We report laser spectroscopic results that extend the precision of earlier measurements substantially and provide tests of recent calculations on the singlet S, P, and D levels of atomic helium. We have determined the Lamb shift of the He 2 ¹S₀, 3 ¹P₁ states by unambiguously establishing the 2 ¹S₀ absolute level position, by Doppler-free, two-photon transitions from 2 ¹S₀ to n ¹D₂ (7≤n≤20). The Lamb shift of 0.093 761(5) cm^-1 [2810.88(15) MHz] agrees with a less precise experiment of Sansonetti, Gillaspy, and Cromer [Phys. Rev. Lett. 65, 2539 (1990)], but differs from theory by 98.36(15) MHz. In combination with the above-noted experimental result, the Lamb shift of the 3 ¹P₁ level is 0.000 522(45) cm^-1, in agreement with theory. Our relative spacings of the D levels is in disagreement with published theoretical calculations, but is in agreement with recent unpublished corrections of this work. This D-level agreement also confirms Casimir forces to the 2% level.