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- 189 Au and 189Aum in the 6s 2S 1/ 2 - 6p 2Pl/2 (A =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 n1ass selected by time of flight. A drastic change of the nuclear charge radius was observed between 187Au and 186Au, which is interpreted as an onset of strong deformation of f3~ 0.25 in 186Au and 185Au.