Measurements of absolute transition probabilities in Ba ii through optical nutation

We have developed a method to measure absolute transition probabilities for ions by recording optical nutation. Ions in an accelerated beam are brought into resonance with a counterpropagating laser beam by a rapid Doppler switch. From the time evolution of the population of the upper level in the resonant transition, we determine the Rabi frequency of the transition. An accurate measurement of the electric field strength then yields the transition probability. We report measurements for five transitions in Ba ii. For three of these transitions our values are more accurate than those of earlier published results.