Sudden change in the nuclear charge distribution of very light gold isotopes

Abstract

On-line resonance-ionization mass spectrometry has been applied to determine the isotope shift and hyperfine structure of $^{185\mathrm{\ensuremath{-}}189}\mathrm{Au}$ and $^{189\mathrm{m}}\mathrm{Au}$ in the 6s ${}^{2}$${S}_{1/2}$\ensuremath{\rightarrow}6p ${}^{2p}$${}_{1/2}$ (\ensuremath{\lambda}=268 nm) transition. The Au atoms were obtained as daughters of mass-separated Hg isotopes produced at the ISOLDE facility at CERN ionized by a three-color, two-step resonant photoionization process, detected and mass-selected by time of flight. A drastic change of the nuclear charge radius was observed between $^{187}\mathrm{Au}$ and $^{186}\mathrm{Au}$, which is interpreted as an onset of strong deformation of \ensuremath{\beta}\ensuremath{\approxeq}0.25 in $^{186}\mathrm{Au}$ and $^{185}\mathrm{Au}$.