First pure frequency measurement of an optical transition in helium: Lamb shift on the 2(3)S1 metastable level.

The 2[sup 3][ital S][sub 1-]3[sup 3][ital P][sub 0][sup 4]He transition at [lambda][sub 0]=389 nm is measured with respect to a previously frequency calibrated [sup 87]Rb two-photon transition at 2[lambda][sub 0]=778 nm. The [sup 4]He absolute frequency is 770 732 839 058 (190) kHz, with an accuracy of 2.4 parts in 10[sup 10]. A Lamb shift value of +4057.61 (79) MHz, with 600 kHz uncertainty arising from the theoretical position of the 3[sup 3][ital P][sub 0] energy level, is extracted for the 2[sup 3][ital S][sub 1] level. This value is more than 2 orders of magnitude more accurate than the current best theoretical predictions.