LASER-COOLING EFFECTS IN FEW-ION CLOUDS OF YB+

We report some laser-cooling effects in a few Yb-172+ ions held in a P aul trap. Pronounced cloud-to-crystal phase transitions have been obse rved as discontinuities in the Yb+ fluorescence spectrum of the 369 nm cooling transition. The first reported two-dimensional images of Ybclouds with evidence of crystal structure have been recorded using a p hoton-counting position-sensitive detector. An ion temperature of 100 mK has been estimated from the size of a single ion image. Stepwise co oling of a re-heated, few-ion Yb+ cloud was also observed.