Nuclear charge radius of 11Li

We have determined the nuclear charge radius of ¹¹Li by high-precision laser spectroscopy. The experiment was performed at the TRIUMF-ISAC facility where the ⁷Li-¹¹Li isotope shift (IS) was measured in the 2s→3s electronic transition using Doppler-free two-photon spectroscopy with a relative accuracy better than 10⁻⁵. The accuracy for the IS of the other lithium isotopes was also improved. IS’s are mainly caused by differences in nuclear mass, but changes in proton distribution also give small contributions. Comparing experimentally measured IS with advanced atomic calculation of purely mass-based shifts, including QED and relativistic effects, allows derivation of the nuclear charge radii. The radii are found to decrease monotonically from ⁶Li to ⁹Li, and then increase with ¹¹Li about 11% larger than ⁹Li. These results are a benchmark for the open question as to whether nuclear core excitation by halo neutrons is necessary to explain the large nuclear matter radius of ¹¹Li; thus, the results are compared with a number of nuclear structure models.