N. D. Gibson

Depletion of the excited state population in negative ions using laser photodetachment in a gas-filled RF quadrupole ion guide

The depopulation of excited states in beams of negatively charged carbon and silicon ions was demonstrated using collisional detachment and laser photodetachment in a radio-frequency quadrupole ion guide filled with helium. The high-lying, loosely bound 2D excited state in C− was completely depleted through collisional detachment alone, which was quantitatively determined within 6%. For Si− the combined signal from the population in the 2P and 2D excited states was only partly depleted through collisions in the cooler. The loosely bound 2P state was likely to be completely depopulated, and the more tightly bound 2D state was partly depopulated through collisions. 98(2)% of the remaining 2D population was removed by photodetachment in the cooler using less than 2 W laser power. The total reduction of the excited population in Si−, including collisional detachment and photodetachment, was estimated to be 99(1)%. Employing this novel technique to produce a pure ground state negative ion beam offers possibilities of enhancing selectivity, as well as accuracy, in high-precision experiments on atomic as well as molecular negative ions.

Observation of an excited C2−4 ion

This paper reports an experimental investigation of the electron impact detachment of C−4. We observe structure in the electron impact cross section for detaching a single electron from a C−4 cluster anion, which we attribute to the formation and decay of the C2−4 dianion. The system is energetically unstable and very rapidly decays via double autodetachment. The energy and width of the resonance were determined to be 8.8(5) eV and 1.4(5) eV, respectively, and the resonance lies 1.5(5) eV above the ground state of the neutral system. The experiment was conducted by merging monoenergetic electron and ion beams in the heavy ion storage ring CRYRING. The detachment channel was monitored by detecting neutral C4 fragments.

K-shell Photodetachment from O?

The K-shell photodetachment spectrum of O- has been investigated using the merged ion-photon beam photo-detachment technique. O- ions were produced in a Cs sputtered ion source on a new Movable Ion Photon Beamline (MIPB) while the photons were produced at the Advanced Light Source Beamline 8.0.1. Positive oxygen ions formed by multiple detachment were detected as a function of photon energy. Photoexcitation of a 1s electron leads to a short-lived Feshbach resonance ~3 eV below the 1s detachment threshold due to the extra stability of the now full 2p6 shell.

Photo-multidetachment and Fragmentation of C60 Anions

Absolute single-photon multi-electron ionization and fragmentation cross sections for C−60 ions are measured for the first time. It is determined that the excess electron in the anion causes a significant increase in cross section and an overall energy scaling, but otherwise has little effect on the observed structure. Appearance energies (apparent thresholds) of the fragmentation products are also observed to be significantly lower for the anion, as compared to the neutral molecule.

Inner-shell photodetachment from Se− negative ions at the ALS

The photodetachment spectrum of Se− from 50 – 62 eV has been investigated using the merged ion-photon beam photodetachment technique. Se+ and Se++ ions formed by double and triple detachment were detected as a function of photon energy. Correlations in short-lived negative ion resonances formed by completely filling the valence 4p shell in Se- by photoexcitation of 3d electrons lead to three resonance structures. The resonances are observed in both the Se+ and Se++ decay channels bound by several eV below the 3d detachment threshold near 54 eV.

Inner-shell photodetachment from Ru{sup -}

Inner-shell photodetachment from Ru{sup -} was studied near and above the 4p excitation region, 29-to-91-eV photon energy range, using a merged ion-photon-beam technique. The absolute photodetachment cross sections of Ru{sup -} ([Kr]4d{sup 7}5s{sup 2}) leading to Ru{sup +}, Ru{sup 2+}, and Ru{sup 3+} ion production were measured. In the near-threshold region, a Wigner s-wave law, including estimated postcollision interaction effects, locates the 4p{sub 3/2} detachment threshold between 40.10 and 40.27 eV. Additionally, the Ru{sup 2+} product spectrum provides evidence for simultaneous two-electron photodetachment (likely to the Ru{sup +} 4p{sup 5}4d{sup 6}5s{sup 2} state) located near 49 eV. Resonance effects are observed due to interference between transitions of the 4p electrons to the quasibound 4p{sup 5}4d{sup 8}5s{sup 2} states and the 4d{yields}{epsilon}f continuum. Despite the large number of possible terms resulting from the Ru{sup -} 4d open shell, the cross section obtained from a 51-state LS-coupled R-matrix calculation agrees qualitatively well with the experimental data.

Promoting a core electron to fill a d-shell in negative ions: Shape versus Feshbach resonances and a novel threshold law

Two new results emerging from inner-shell photodetachment of transition metal negative ions are obtained, following studies in Pt−. First, the d-wave form of the Wigner threshold law is observed for the first time in single-photon measurements. Second, single-vacancy valence shells are filled with a core electron. While this should result in stabilization of the core excited state, producing Feshbach resonances, we find that stabilization does not occur for some core excitations, dramatically demonstrating the importance of core-valence interactions.

Shape resonances in inner-shell photodetachment of transition metal negative ions

Inner-shell photodetachment of transition metal negative ions was studied using photoion spectroscopy. We focused on shape resonances, their behavior, and their decay mechanisms produced by np → (nd+d) (n = 3, 4, 5) photoexcitation for Fe, Ru, Ni, Pd, and Pt transition metal negative ions.

Experimental investigation of electron impact on Si{sub 2}{sup -}

A merged beams technique has been used to investigate collisions between electrons and

Shape resonance in K -shell photodetachment from C-

\text{Si}_{2}{}^{\ensuremath{-}}