Photodetachment of Negative Ions. Experimental Investigations of Threshold and Continuum Structures

Doctoral thesis, 2004

In this thesis photodetachment studies of atomic negative ions where one or several electrons are detached from a negative ion are presented. Negative ions are unique objects since their formation and destruction mechanisms strongly depend on the correlated motion of the valence electrons. Studies of low energy photodetachment processes as well as processes that involve a simultaneous excitation of two electrons are therefore particularly suitable to study the electron correlation effect. In order to obtain a high sensitivity and a high resolution the experiments have been performed using a mono­energetic negative ion beam that collinearly has interacted with a photon beam. Partial cross sections have been determined using the resonance ionization method (RIS). The accuracy of the electron affinity measurement of K was improved and the quasi­bound doubly excited states of K- have been investigated. Parameterizations, using a semi­classical model, showed a good agreement with the experimentally observed resonances. In this work experimental investigations of photodetachment thresholds in combination with theoretical interpretations have been used to extract low­energy scattering parameters. Phase shifts and scattering lengths show a strong influence of the binding potentials and the polarizabilities of an atomic state. Experimental studies of the absolute photo double­detachment cross section of Cl- have been performed. Theoretical interpretations predict that the major contribution is related to a direct and non resonant process where two valence electrons are simultaneously detached. Further, an obsolute cross section measurement for double electron­impact detachment of the same ion is made. Finally, an application where the inherited Doppler shift that is introduced in collinear photodetachment experiments have been used to separate 32S- and 34S- and 12C- and 13C-, respectively. The laser photodetachment threshold (LPT) method is here demonstrated to be a sensitivity enhancement tool that can be used in accelerator mass spectrometry (AMS).