Measurement of the Charge Radii of Li using Laser Spectroscopy

The radioactive isotopes of lithium have been the subject of numerous investigations, particularly after the discovery of the exotic Halo nucleus Li in 1985 [1]. Matter radii, excited states and electromagnetic moments have been determined. But the charge distribution inside the nuclei and how it is affected by the increasing number of neutrons is still an open question. Charge radii from cross section measurements of nuclear reactions depend strongly on the nuclear model used for the analysis. So far, the only method to obtain a nuclear-model independent value for the root-meansquare (rms) charge radius of short-lived isotopes is an isotope shift measurement in an electronic transition. The socalled field shift, which is proportional to the change in the rms charge radius between the isotopes, must be isolated from the mass shift effect that has its origin in the different nuclear masses and the momentum correlation between the electrons. This can be realized using theoretical calculations of the mass shift. For light elements like lithium the isotope shift is dominated by the mass shift, while the field shift contribution is only of the order of 10. Thus, high accuracy is required for the mass shift calculation, which can, so far, only be reached for twoand three-electron systems. Yan and Drake have recently calculated the mass shift in the 2S → 3S and 2S → 2P transitions of lithium to the required precision [2, 3].