Deformation and mixing of coexisting shapes in neutron-deficient polonium isotopes

Coulomb-excitation experiments are performed with postaccelerated beams of neutron-deficient $^{196,198,200,202}$Poisotopes at the REX-ISOLDE facility. A set of matrix elements, coupling the low-lying states in these isotopes,is extracted. In the two heaviest isotopes, $^{200,202}$Po, the transitional and diagonal matrix elements of the $2^+_1$ state are determined. In $^{196,198}$Po multistep Coulomb excitation is observed, populating the $4^+_1$ , $0^+_2$ , and $2^+_2$ states.The experimental results are compared to the results from the measurement of mean-square charge radii inpolonium isotopes, confirming the onset of deformation from $^{196}$Po onwards. Three model descriptions are usedto compare to the data. Calculations with the beyond-mean-field model, the interacting boson model, and thegeneral Bohr Hamiltonian model show partial agreement with the experimental data. Finally, calculations witha phenomenological two-level mixing model hint at the mixing of a spherical structure with a weakly deformedrotational structure