Photodetachment Studies of Atomic Negative Ions Through Velocity-Map Imaging Spectroscopy
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The technique of velocity-map imaging (VMI) spectroscopy as been adapted to a keV-level negative ion beamline for studies of photon-negative ion collisions. The design and operation of the VMI spectrometer takes into consideration the use of continuous, fast-moving (5 keV to 10 keV) ion beams, as well as a continuous wave (CW) laser as the source of photons. The VMI spectrometer has been used in photodetachment studies of the Group 14 negative ions Ge−, Sn−, and Pb− at a photon wavelength of 532 nm. Measurements of the photoelectron angular distributions and asymmetry parameters for Ge− and Sn− were benchmarked against those measured previously [W. W. Williams, D. L. Carpenter, A. M. Covington, and J. S. Thompson, Phys. Rev. A 59, 4368 (1999), V. T. Davis, J. Ashokkumar, and J. S. Thompson, Phys. Rev. A 65, 024702 (2002)], while fine-structure-resolved asymmetry parameters for Pb− were measured for the first time. Definitive evidence of a “forbidden” 4S3/2→1D2 transition was observed in both the Ge− and Sn− photoelectron kinetic energy spectra. This transition is explained in terms of the inadequacy of the single-configuration description for the 1D2 excited state in the corresponding neutral.Near-threshold photodetachment studies of S− were carried out in order to measure the spectral dependence of the photoelectron angular distribution. The resulting asymmetry parameters were measured at several photon wavelengths in the range of 575 nm (2.156 eV photon energy) to 615 nm (2.016 eV photon energy). Comparison of the measurements to a qualitative model of p-electron photodetachment [D. Hanstorp,C. Bengtsson, and D. J. Larson, Phys. Rev. A 40, 670 (1989)] were made. Deviations of the measured asymmetry parameters from the Hanstorp model near photodetachment thresholds suggests a reduced degree of suppression of d partial-waves than predicted by models.Measurement of the electron affinity of terbium was performed along with a determination of the structure of Tb−. The energy scale for the Tb− photoelectron kinetic energy spectrum was calibrated to the photoelectron kinetic energy spectrum of Cs−, whose electron affinity is well-known [T. A. Patterson, H. Hotop, A. Kasdan, D. W. Norcross, and W. C. Lineberger, Phys. Rev. Lett. 32, 189 (1974)]. Comparison to a previous experimental measurement of the electron affinity of terbium [S. S. Duvvuri, Ph. D. dissertation, University of Nevada, Reno (2006)] and to theoretical calculations of the electron affinity [S. M. O’Malley and D. R. Beck, Phys. Rev. A 79, 012511 (2009)] were made. In contrast to the [Xe]4f106s2 5I8 ground state configuration proposed in the experimental study and the [Xe]4f85d6s26p 9G7 ground state configuration proposed in the theoretical study, the present study suggests a Tb− ground state of [Xe]4f96s26p 7I3 and an electron affinity of 0.13 ± 0.07 eV for terbium.