Laser excitation of the 4He and 3He 1 1S-2 1P resonance lines at 58.4 nm

In contrast to the extensively studied hydrogen atom, the quantum electrodynamic Lamb shift in helium also contains a two electron contribution. Accurate calculations show that the two electron shift contributes -0.1404 cm^-4 to a total Lamb shift of +1.3745 cm^{-1 1}in the ground state, which is one order of magnitude larger than in excited states of helium. The ground state Lamb shift was first measured by Herzberg in 1958² with an accuracy of 0.15 cm^-1. The main experimental difficulty to improve this accuracy lies in the generation of tunable and narrowband extreme ultraviolet (XUV) at 58.4 nm required to excite the 1 ¹S-2 ¹P transition. Until recently the shortest wavelength produced with nanosecond pulsed lasers in combination with harmonic conversion techniques was 66 nm. We succeeded in the generation of narrow band (0.3 cm^-1) XUV radiation at 58.4 nm and reported the first laser spectroscopic measurement of the ⁴He 1 ¹S-2 ¹P resonance line,³ resulting in accurate values for the binding energy and the Lamb-shift in the ground state.