Excited States inH176,178gand Shape Coexistence in Very Neutron-Deficient Hg Isotopes

Excited states have been observed for the first time in the neutron-deficient ${}^{176,178}\mathrm{Hg}$ nuclei using the recoil-decay tagging (RDT) technique in which prompt $\gamma$ rays are associated with a particular isotope through a correlation with the characteristic ground state $\alpha$ decay. Below $N=102\mathrm{}$, the excitation energy of a rotational band built on a prolate shape $\left({\beta }_2\sim 0.25\right)$ increases with decreasing mass to the point where there is no longer any evidence for its presence at low spin in ${}^{176}\mathrm{Hg}$. The data are in qualitative agreement with recent mean field calculations.