Early Onset of Ground State Deformation in Neutron Deficient Polonium Isotopes

In-source resonant ionization laser spectroscopy of the even-A polonium isotopes 192–210;216;218Po has been performed using the 6p37s 5S2 to 6p37p 5P2 (lambda = 843.38 nm) transition in the polonium atom (Po-I) at the CERN ISOLDE facility. The comparison of the measured isotope shifts in 200–210Po with a previous data set allows us to test for the first time recent large-scale atomic calculations that are essential to extract the changes in the mean-square charge radius of the atomic nucleus. When going to lighter masses, a surprisingly large and early departure from sphericity is observed, which is only partly reproduced by beyond mean field calculations.As submitted to PRLstatus: publishe