The Lamb Shift in the Ground State of Helium
The goal of this project is to determine the Lamb shift in the 1s2 1S0 ground state of atomic helium in a laserspectroscopic experiment. An XUV-laser is used to excite the 1s2 1S0-1s2p 1P1 transition at 58.4 nm. Detection takes place via 1 XUV + 1 UV photoionization and mass-selection of He-ions. Initially our "broadband" XUV-laser setup was used to measure this resonance line resulting in an accuracy of 0.03 cm-1 for the transition frequency and for the value of the Lamb shift (Phys. Rev. Lett. 71, 1993, 1690). In subsequent experiments at increasingly higher resolution and accuracy (see: Phys. Rev. Lett. 76, 1996, 1216) we improved the value for the ground state Lamb shift in He. The Figure shows a spectral recording of the most recent experiment with the best accuracy.
The transition frequency is calibrated against iodine reference lines measured with saturated absorption spectroscopy. One of these lines, used as an absolute reference standard for the present experiment in Helium is shown in the figure with an *. The helium resonance was recorded while scanning the PDA-laser near 584 nm and using its 10th harmonic (first frequency doubling in a crystal and generation the 5th harminic of the UV-output) for the spectral recording. In the figure below we give an historic impression of the progress of the determination of the Lamb shift in the He ground state, for experiment and theory.
Now a value for the ground state Lamb shift is obtained: 41224 (45) MHz
which compares well with the theoretical value of Drake: 41233 (35) MHz
A final account of this work is given in: K. Eikema et al. Phys. Rev. A55 (1997) 1866.