Weakly-bound negative ions studied by laser excitation and AMS

Electron affinity and absolute photodetachment cross sections of weakly-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 electron affinity is obtained by varying the photon energy and studying the excitation function near a threshold corresponding to a state in the neutral atom. We measured absolute photodetachment cross sections of Sr^- ions interacting with photons at energies near the threshold corresponding to the first excited state of the neutral Sr and determined an electron affinity of 48 ± 6 meV for Sr^-. Preliminary results for Ce^- are presented. An alternative technique designed to make measurements close to threshold more sensitive, is based on the detection of the neutral reaction products rather than the depleted negative ion beam. We performed a test experiment with S^- ions that show the capability of this last technique.