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Journal of Physics Conference Series

Observation of a resonance in the photodetachment of positronium negative ions

Koji Michishio, Susumu Kuma, Tsuneto Kanai, Toshiyuki Azuma, Ken Wada, Izumi Mochizuki, Toshio Hyodo, Akira YAGISHITA, Y. Nagashima

Published Sep 7, 2015• 1 citation
Publisher

Abstract

The positronium negative ion, e − e + e − , is one of the simplest three body bound-states. As its constituents have a unique mass ratio, the ion is an attractive candidate for the investigation of the genuine three-body problem. In the present work, laser spectroscopy of this ion has been demonstrated in the UV region. A shape resonance near the positronium ( n =2) formation threshold was clearly observed and the resonance energy was estimated.

DOI
10.1088/1742-6596/635/5/052050

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The positronium negative ion, e − e + e − , is one of the simplest three body bound-states. As its constituents have a unique mass ratio, the ion is an attractive candidate for the investigation of the genuine three-body problem. In the present work, laser spectroscopy of this ion has been demonstrated in the UV region. A shape resonance near the positronium ( n =2) formation threshold was clearly observed and the resonance energy was estimated.
The negative ion of lanthanum, La–\mathrm{L}{\mathrm{a}}^{\text{--}}La–, has one of the richest bound state spectra observed for an atomic negative ion and has been proposed as a promising candidate for laser-cooling applications. In the present experiments, La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– was investigated using tunable infrared photodetachment spectroscopy. The relative signal for neutral atom production was measured with a crossed ion-beam--laser-beam apparatus over the photon energy range 585--920 meV (2120--1350 nm) to probe the continuum region above the La neutral atom ground state. Eleven prominent peaks were observed in the La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– photodetachment cross section due to resonant excitation of quasibound transient negative ion states in the continuum, which subsequently autodetach. In addition, thresholds were observed for photodetachment from several bound states of La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– to both ground and excited states of La. The present results provide information on the excited state structure and dynamics of La–\mathrm{L}{\mathrm{a}}^{\text{--}}La– that depend crucially on multielectron correlation effects.
An experimental study of the previously unobserved and controversial ${\mathrm{Cs}}^{\ensuremath{-}}(6s6p^{3}P_{J}^{o})$ states has been conducted using a combination of infrared laser and storage ring experiments. This first application of photodetachment spectroscopy to the study of low-lying resonances in atomic negative ions has resulted in the observation of a narrow resonance structure, lying only 8.0(3) meV above the first detachment threshold of ${\mathrm{Cs}}^{\ensuremath{-}}$. The storage ring experiments suggest that all fine structure levels of the 6s6p6s6p6s6p configuration are unbound, thereby eliminating possibly the only serious candidate for a stable atomic negative ion with opposite parity bound states.
The negative ion of cerium is investigated experimentally with tunable infrared laser photodetachment spectroscopy and theoretically with relativistic configuration interaction in the continuum formalism. The relative cross section for neutral atom production is measured with a crossed ion-beam--laser-beam apparatus over the photon energy range of 0.54--0.75 eV. A rich resonance spectrum is revealed near the threshold with, at least, 12 peaks observed due to transitions from bound states of ${\mathrm{Ce}}^{\ensuremath{-}}$ to either bound or quasibound excited states of the negative ion. Theoretical calculations of the photodetachment cross sections enable identification of the transitions responsible for the measured peaks. Two of the peaks are due to electric dipole-allowed bound-bound transitions in ${\mathrm{Ce}}^{\ensuremath{-}}$, making cerium only the second atomic negative ion that has been demonstrated to support multiple bound states of opposite parity. In addition, combining the experimental data with the theoretical analysis determines the electron affinity of cerium to be 0.628(10) eV and the fine structure splitting of the ground state of ${\mathrm{Ce}}^{\ensuremath{-}}$ (4H7/2−−4H9/2{}^{4}\phantom{\rule{-0.16em}{0ex}}{H}_{7/2}--{}^{4}\phantom{\rule{-0.16em}{0ex}}{H}_{9/2}4H7/2​−−4H9/2​) to be 0.097 75(4) eV.
A highly accurate calculation is supplemented by an adiabatic approximation to explore the resonance spectrum of positronium ${\mathrm{Ps}}^{\mathrm{\ensuremath{-}}}$. Surprisingly, the spectrum can be understood and classified with H2+{\mathrm{H}}_{2}^{+}H2+​ quantum numbers by treating the interelectronic axis of ${\mathrm{Ps}}^{\mathrm{\ensuremath{-}}}$ as an adiabatic parameter. We report and interpret the existence of 1^{1}1S shape resonances, a phenomenon so far unknown in three-body Coulomb systems. The new results on ${\mathrm{Ps}}^{\mathrm{\ensuremath{-}}}$ combined with previous results for ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ suggest the existence of a resonance spectrum and its similarity for all ABA Coulomb systems with charges \ensuremath{\Vert}ZA{\mathit{Z}}_{\mathit{A}}ZA​/ZB{\mathit{Z}}_{\mathit{B}}ZB​\ensuremath{\Vert}=1 and masses mA{\mathit{m}}_{\mathit{A}}mA​/mB{\mathit{m}}_{\mathit{B}}mB​\ensuremath{\ge}1.
An adiabatic treatment in hyperspherical coordinates is used to predict the existence of resonant structures in ${\mathrm{Ps}}^{\ensuremath{-}}$ (Ps denotes positronium) photodetachment. This highly correlated system displays a weak series of narrow Feshbach resonances lying just below the Ps(n=2)\mathrm{Ps}(n=2)Ps(n=2) threshold, while a single strong shape resonance dominates the photodetachment spectrum above this threshold as in ${\mathrm{H}}^{\ensuremath{-}}$. Use of a "prediagonalized" representation of hyperspherical harmonics dramatically improves the convergence of the adiabatic calculation.
The photoionization cross section for the positronium (Ps) negative ion, Ps−, at the lowest n = 2 Feshbach resonance, estimated by neglecting the influences of the weakly bound outer electron, is σF = 1.4 × 10−12 cm2, about 3 × 103 times the existing lower limit calculated by Igarashi and co-workers (New J. Phys. 2, 17 (2000)). Although the estimated cross section is 200 times smaller than the analogous cross section for photoexcitation of the Ps Lyman-α transition, including the effect of the broad line width of the resonance shows it will be feasible to observe this resonance to obtain precision information about the three body Ps− system.