QED TEST AND NUCLEAR-RADIUS DETERMINATION OF HELIUM ATOM BY LASER SPECTROSCOPY

The 2(3)S(1)-3(3)P(0) He-4 transition at lambda = 389 nm is measured w ith respect to a previously frequency calibrated Rb-87 two-photon tran sition at 2 lambda = 778 nm. The He-4 absolute frequency is 770 732 83 9 058 (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)P(0) energy level, is extracted for th e 2(3)S(1). This value is more than two orders of magnitude more accur ate than the current best theoretical predictions. By means of a novel heterodyne spectrometer using only one laser tuned at the atomic freq uency, we have measured for the first time the He-3-He-4 isotope separ ation for the 2(3)S(1)-3(3)P(0) transition at 389 nm. Our experimental result of 45 394 425 (140) kHz, for the F = 1/2-1/2 He-3 component, i s compared with the theoretical prediction which has similar accuracy and depends also on the rms nuclear radius of He-3. For this we determ ine a preliminary value of 1.923 (37) fm which shows a slight discrepa ncy with previous values obtained by extrapolation to zero momentum tr ansfer from high energy scattering measurements.