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

On-line resonance-ionization mass spectrometry has been applied to determine the isotope shift and hyperfine structure of ${}_{}{}^{185\mathrm{-}189}{}_{}{}^{}\mathrm{Au}$ and ${}_{}{}^{189\mathrm{m}}{}_{}{}^{}\mathrm{Au}$ in the 6s ${}^2$ $S_{1/2}$→6p ${}^{2p}$ ${}_{1/2}$ (λ=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 β≊0.25 in ${}_{}{}^{186}{}_{}{}^{}\mathrm{Au}$ and ${}_{}{}^{185}{}_{}{}^{}\mathrm{Au}$.