Nuclear Shape Staggering in Very Neutron-Deficient Hg Isotopes Detected by Laser Spectroscopy

The isotope shifts of ${}_{}{}^{188}{}_{}{}^{}\mathrm{Hg}$, ${}_{}{}^{186}{}_{}{}^{}\mathrm{Hg}$, and ${}_{}{}^{184}{}_{}{}^{}\mathrm{Hg}$ in the 2537-Å line have been measured by use of tunable dye laser at the on-line mass separator ISOLDE at CERN. The results are $\delta \nu ({}_{}{}^{188}{}_{}{}^{}\mathrm{Hg}-{}_{}{}^{204}{}_{}{}^{}\mathrm{Hg})=35.8\mathrm{}\left(2\right)\mathrm{}$ GHz; $\delta \nu ({}_{}{}^{186}{}_{}{}^{}\mathrm{Hg}-{}_{}{}^{204}{}_{}{}^{}\mathrm{Hg})=39.4\mathrm{}\left(2\right)\mathrm{}$ GHz; and $\delta \nu ({}_{}{}^{184}{}_{}{}^{}\mathrm{Hg}-{}_{}{}^{204}{}_{}{}^{}\mathrm{Hg})=43.1\mathrm{}\left(2\right)\mathrm{}$ GHz. These data combined with those obtained by $\beta$-radiation-detected optical pumping ($\beta$-RADOP) on the odd Hg isotopes yield a huge odd-even staggering for ${}_{}{}^{185}{}_{}{}^{}\mathrm{Hg}$ of $\gamma =\frac{\left(2\mathrm{}\delta {\nu }^{\frac{185}{184}}\right)}{\delta {\nu }^{\frac{186}{184}}}=13\mathrm{}\left(1\right)\mathrm{}$ which has to be interpreted as nuclear shape staggering.