A highly efficient method for suppressing isobar contaminants in negative radioactive ion beams by photodetachment is demonstrated. A laser beam having the appropriate photon energy is used to selectively neutralize the contaminants. The efficiency of photodetachment can be substantially improved when the laser-ion interaction takes place inside a radio frequency quadrupole ion cooler. In off-line experiments with ion beams of stable isotopes, more than 99.9% suppression of Co-, S-, and O- ions has been demonstrated while under the identical conditions only 22% reduction in Ni- and no reduction in Cl- and F- ions were observed. This technique is being developed for on-line purification of a number of interesting radioactive beams, such as 56Ni, 17,18F, and 33,36Cl.
