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Physical Review Letters

Experimental Studies of Os− : Observation of a Bound-Bound Electric Dipole Transition in an Atomic Negative Ion

R. C. Bilodeau, Harold K. Haugen

Published Jul 17, 2000• 83 citations
Publisher

Abstract

The first experimental studies of Os(-) reveal the presence of two very strong resonances. One state is a narrow shape resonance lying only 3.52(12) meV above the ground state detachment threshold, while the other is bound by 11.48(12) meV. Convincing evidence that these states have odd parity is presented, making Os(-) the only known atomic negative ion with bound states of opposite parity. Additionally, the binding energies for the Os(-) ground state ((4)F(e)(9/2)) and first excited state ((4)F(e)(7/2)) are measured to be 1.077 80(12) eV and 0.553(3) eV, respectively.

DOI
10.1103/physrevlett.85.534

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•Jan 1, 2015•0 citations
The first experimental studies of Os(-) reveal the presence of two very strong resonances. One state is a narrow shape resonance lying only 3.52(12) meV above the ground state detachment threshold, while the other is bound by 11.48(12) meV. Convincing evidence that these states have odd parity is presented, making Os(-) the only known atomic negative ion with bound states of opposite parity. Additionally, the binding energies for the Os(-) ground state ((4)F(e)(9/2)) and first excited state ((4)F(e)(7/2)) are measured to be 1.077 80(12) eV and 0.553(3) eV, respectively.
Atomic anions are generally not amenable to optical spectroscopy because they are loosely bound systems and rarely have bound excited states. Until recently, there was only one known negative ion with a strong bound–bound electronic transition, the osmium anion Os − . The electric-dipole transition between the 4 F e 9/2 ground and 6 D o J excited state of this ion provides unique insight into the structure of atomic anions. In addition, it may enable the preparation of ultracold ensembles of negative ions. Laser excitation of the electric-dipole transition in Os − ions could be used to laser-cool them to microkelvin temperatures. If demonstrated to be successful, the technique would allow the cooling of any species of negatively charged ions - from subatomic particles to molecular anions - to ultracold temperatures by sympathetic cooling. We have been investigating the bound-bound electric-dipole transition in Os − by high-resolution laser spectroscopy with a view to using it for the first laser cooling of negative ions. The principle of the method, its potential applications, as well as experimental results are presented.
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.
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.
Laser cooling of neutral atoms or positive ions is today routinely employed in numerous experiments. Negative ions, in contrast, have distinct characteristics which hamper the application of lasers for cooling. But in 1999, the discovery of the unique bound–bound electric dipole transition in the negative osmium ion provided the motivation for a first cooling attempt. This thesis presents the first milestones toward the ultimate goal of laser cooling negative osmium, including high-resolution laser spectroscopy of the relevant bound–bound E1 transition. Its frequency – between the ground 4F9/2 and the 6DJe1 (bound) excited states – was determined to be 257.831190(35) THz in 192Os-, in agreement\nwith a previous measurement, but two orders of magnitude more precise. The determination of the resonant cross-section implicitly provided the corresponding Einstein A coefficient, which was found to be A ~ 330 s-1. Furthermore, the isotope shift of the E1 transition and the hyperfine structure constants of the ground and excited state were obtained, for the first time, from the analysis of the spectra of all naturally abundant isotopes. The hyperfine structure revealed the heretofore unknown total angular momentum of the excited state to be Je1 = 9/2. Finally, laser spectroscopy in an external magnetic field confirmed the expected line splitting for Os- due to the Zeeman effect. Based on all these experimental results the prospect of laser cooling negative osmium is reviewed.
By identifying a special class of atomic negative ion states where all electron spins are unpaired and where the nuclear charge is partially unscreened, we predict, via large-scale state-specific calculations, that and have states which belong to the discrete spectrum even though they are very highly excited and embedded in a multitude of ionization thresholds. For example, the state lies 600.6 eV above the O ground state and yet it is bound. For , the two maximum spin states, and , are connected by an electric dipole transition with and with transition rate . For these unique states, the reliable prediction of the relevant energy differences demanded the execution of systematic all-electron calculations, with inter- and intra-shell correlations included.
Among the three candidates for negative ion laser cooling, La- is the most promising due to the strong transitions between its bound states. A direct energy calculation at the valence level [1] had been applied to La-. However, a comparison with the most recent experimental study [2] has shown there exists a range of disagreement of 17 ~ 90 meV in the energy values relative to the ground state. This research is a computational effort to determine the energies using a different method. By reproducing the photo-detachment cross section of La- and then lining it up with the experimental plot, the amount of shifts needed for the excited states and the ground states can be determined. In doing so, the main sources to the above-mentioned discrepancies can be identified, e.g. the differing amount of missing correlation energy in the ground state and the excited states. This presentation will be a summary of the progress that has been made toward the goal. [1] S. M. O'Malley and D. R. Beck, Phys. Rev. A 79, 023622 (2009). [2] C. W. Walter, et al, Phys. Rev. Lett. 113, 063001 (2014).