WEAKLY-BOUND NEGATIVE-IONS STUDIED BY LASER EXCITATION AND AMS

Electron affinity and absolute photodetachment cross sections of weakl y-bound negative ions are measured by combination of laser excitation and AMS techniques, The photodetachment cross section is determined by measuring the depletion of negative ions through laser light, The ele ctron affinity is obtained by varying: the photon energy and studying the excitation function near a threshold corresponding to a state in t he neutral atom. We measured absolute photodetachment cross sections o f Sr- ions interacting with photons at energies near the threshold cor responding to the first excited state of the neutral Sr atom and deter mined an electron affinity of 48 +/- 6 meV for Sr-. Preliminary result s for Ce- are presented. An alternative technique designed to make mea surements close to threshold more sensitive, is based on the detection of the neutral reaction products rather than the depleted negative io n beam. We performed a test experiment with S- ions that show the capa bility of this last technique.