Nuclear shape staggering in very neutron deficient Hg isotopes detected by laser spectroscopy

The isotope shift in the lambda =2537 AA line of the even isotopes /sup 206/Hg, /sup 190/Hg, /sup 188/Hg, /sup 186/Hg, /sup 184/Hg as well as of the I=13/2 isomers of /sup 191/Hg, /sup 189/Hg, /sup 187 /Hg, /sup 185/Hg has been measured using a tunable dye laser at the on-line mass separator ISOLDE at CERN. The resulting delta (r/sup 2/) values follow the line, extrapolated from the chain /sup 205/Hg-/sup 187/Hg which is known to have spherical nuclear shape at the heavy end changing smoothly to slight oblate deformation for the lighter isotopes. Previous measurements of the I=1/2 groundstates of /sup 181 /Hg. /sup 183/Hg and /sup 185/Hg revealed a sharp shape transition to strong deformation. Combined with the new results the following effects can be proved from the model-independent quantity delta (r/sup 2/): 1) the existence of odd even-shape staggering, 2) the coexistence of very different shapes in one nucleus, 3) the absence of mixing of the different shapes. (26 refs).