Measurement of the charge radii of the radioactive lithium isotopes 8Li and 9Li

The measurement of the isotope shift of atomic transitions is the only model-independent access to determine nuclear charge radii of short-lived isotopes. Up to now, this was limited to isotopes with nuclear charge Z>9. In this work, a novel method was developed to measure charge radii of short-lived lithium isotopes. It is based on an accurate measurement of an atomic transition combined with an accurate recently performed theoretical calculation of the specific mass shift. The experimental method meets the requested efficiency of >10(-4) and the required accuracy of <200 kHz by Doppler-free two-photon spectroscopy of the 2S→3S transitions in a thermal atomic beam followed by resonance ionization and detection by quadrupole mass spectrometry. The isotope shifts of the isotopes 6Li to 9Li were determined at the online mass separator at GSI with an accuracy better than 180 kHz. The extracted nuclear charge radii decrease monotonically. This can be explained by the strong alpha+d clustering of 6Li, which diminishes with increasing number of neutrons. The results are compared to predictions of different nuclear models and improve the understanding of the structure of these nuclei. With this experiment, the foundation is laid to determine the charge radius of 11Li