Photodetachment Studies of Negative Ion Structure

The structure of atomic negative ions has been investigated experimentally by studying photodetachment, which is the process of photon-induced electron emission from a negative ion. The interaction between the valence electrons and the nucleus is weaker in negative ions than in atoms and positive ions. Therefore, atomic negative ions are fragile systems and their binding energy is sensitive to electron-electron correlation. Quasi-bound doubly excited states are particularly sensitive to this correlation.<p /> A collinear laser-ion beam apparatus has been used to obtain a high sensitivity in combination with high energy-resolution. Total cross sections have been measured by detecting the residual atom created in the photodetachment process. A scheme for state-selective detection of the residual atom, based on resonance ionization spectroscopy (RIS), has been developed for measurements of partial cross sections. Further, a newly developed time-of-flight spectrometer has been used for energy-resolved detection of electrons ejected in the photodetachment process.<p /> The accuracies of the electron affinities of Li and Te have been improved and all fine structure splittings of the As<SUP>-</SUP> ground state have been determined for the first time.<p /> Quasi-bound doubly excited states have been investigated by observing resonance structures in photodetachment spectra. Energies and widths of the resonances have been extracted from the measured cross sections. The high resolution and sensitivity of the apparatus, in combination with state-selective detection, have been necessary for these investigations. Resonance structures, associated with doubly excited states in Li<SUP>-</SUP> have been observed below the photodetachment thresholds corresponding to the 3p, 4p, 6p, and 7p states of the Li atom. Doubly excited states have also been investigated in Na<SUP>-</SUP> below the Na(4d) threshold and in He<SUP>-</SUP> below the He(n=3) thresholds. Photodetachment of C<SUP>-</SUP> has been studied in the vicinity of the C(2s2p<SUP>3</SUP> <SUP>5</SUP>S) threshold, in the search for predicted doubly excited states, but no evidence of resonance structure was found. Measured cross sections and resonance parameters have been compared with recent calculations and have stimulated further theoretical investigations