First Penning-Trap Mass Measurement of the Exotic Halo Nucleus 11Li

In this Letter, we report a new mass for Li-11 using the trapping experiment TITAN at TRIUMF's ISAC facility. This is by far the shortest-lived nuclide, t(1/2)=8.8 ms, for which a mass measurement has ever been performed with a Penning trap. Combined with our mass measurements of Li-8,Li-9 we derive a new two-neutron separation energy of 369.15(65) keV: a factor of 7 more precise than the best previous value. This new value is a critical ingredient for the determination of the halo charge radius from isotope-shift measurements. We also report results from state-of-the-art atomic-physics calculations using the new mass and extract a new charge radius for Li-11. This result is a remarkable confluence of nuclear and atomic physics.