Onset of Collectivity in Neutron Deficient ${}^{196,198}$Po

We have studied via in-beam $\gamma$-ray spectroscopy ${}^{196}$Po and ${}^{198}$Po, which are the first neutron-deficient Po isotopes to exhibit a collective low-lying structure. The ratios of yrast state energies and the E2 branching ratios of transitions from non-yrast to yrast states are indicative of a low-lying vibrational structure. The onset of collective motion in these isotopes can be attributed to the opening of the neutron i${}_{13\mathrm{/}2}$ orbital at N$\approx$112 and the resulting large overlap between the two valence protons in the h${}_{9\mathrm{/}2}$ orbital and the valence neutrons in the i${}_{13\mathrm{/}2}$ orbital.