1ST PURE FREQUENCY-MEASUREMENT OF AN OPTICAL-TRANSITION IN HELIUM - LAMB SHIFT OF THE 2(3)S(1) METASTABLE LEVEL

The 2(3)S1-3(3)P0, He-4 transition at lambda0 = 389 nm is measured with respect to a previously frequency calibrated Rb-87 two-photon transition at 2lambda0 = 778 nm. The He-4 absolute frequency is 770732839058 (190) kHz, with an accuracy of 2.4 parts in 10(10). A Lamb shift value of +4057.61 (79) MHz. with 600 kHz uncertainty arising from the theoretical position of the 3(3)P0 energy level, is extracted for the 2(3)S1 level. This value is more than 2 orders of magnitude more accurate than the current best theoretical predictions.