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Physical Review A

Lifetimes of bound excited states of the lanthanum negative ion and implications for laser cooling: Experiment and theory

C. W. Walter, F. E. Vassallo, Julia Karls, D. Leimbach, D. Hanstorp, José E. Navarro Navarrete, O. Hemdal Lundgren, Moa K. Kristiansson, M. Björkhage, Richard Thomas, Henning Zettergren, Henning T. Schmidt, Charles Cheung, Marianna S. Safronova, N. D. Gibson

Published Oct 3, 2025• 0 citations
Publisher

Abstract

The negative ion of lanthanum has one of the richest bound-state spectra observed for an atomic negative ion and it has been proposed as a promising candidate for laser cooling, which has not yet been achieved for any negatively charged system. In the present study, radiative decays of low-lying excited states of ${\mathrm{La}}^{\ensuremath{-}}$ were experimentally investigated at the DESIREE cryogenic electrostatic ion-beam storage ring facility using state-selective photodetachment over storage times of up to 800 s. Independently, the transition rates for the relevant states were calculated using a high-precision hybrid theoretical approach. The measured lifetime of the lowest excited state of ${\mathrm{La}}^{\ensuremath{-}}$ $({}^{3}{F}_{3}^{e})$ of 132(20) s agrees very well with the theoretical calculations and previous predictions. However, a puzzling discrepancy exists for the next-higher state ${}^{3}{F}_{4}^{e}$, with the experiments yielding a lifetime $\ensuremath{\sim}4$ times shorter than calculated, highlighting the challenges for this complex ion. The results provide valuable insights into the prospects for laser cooling ${\mathrm{La}}^{\ensuremath{-}}$.

DOI
10.1103/6641-mdhr

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The negative ion of lanthanum has one of the richest bound-state spectra observed for an atomic negative ion and it has been proposed as a promising candidate for laser cooling, which has not yet been achieved for any negatively charged system. In the present study, radiative decays of low-lying excited states of La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}La\ensuremath− were experimentally investigated at the DESIREE cryogenic electrostatic ion-beam storage ring facility using state-selective photodetachment over storage times of up to 800 s. Independently, the transition rates for the relevant states were calculated using a high-precision hybrid theoretical approach. The measured lifetime of the lowest excited state of La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}La\ensuremath− (3F3e)({}^{3}{F}_{3}^{e})(3F3e​) of 132(20) s agrees very well with the theoretical calculations and previous predictions. However, a puzzling discrepancy exists for the next-higher state 3F4e{}^{3}{F}_{4}^{e}3F4e​, with the experiments yielding a lifetime \ensuremath∼4\ensuremath{\sim}4\ensuremath∼4 times shorter than calculated, highlighting the challenges for this complex ion. The results provide valuable insights into the prospects for laser cooling La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}La\ensuremath−.
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.
Despite the tremendous advances in laser cooling of neutral atoms and positive ions, no negatively charged ion has been directly laser cooled. The negative ion of lanthanum, La(-), has been proposed as the best candidate for laser cooling of any atomic anion [ and , Phys. Rev. A 81, 032503 (2010)]. Tunable infrared laser photodetachment spectroscopy is used to measure the bound-state structure of La(-), revealing a spectrum of unprecedented richness with multiple bound-bound electric dipole transitions. The potential laser-cooling transition ((3)F(2)(e)→(3)D(1)(o)) is identified and its excitation energy is measured. The results confirm that La^{-} is a very promising negative ion for laser-cooling applications.
The energy levels and transition rates of promising laser-cooling candidate thorium anion ($\mathrm{T}{\mathrm{h}}^{\ensuremath{-}}$) are experimentally measured in the current work. Three excited states between the lower and upper states for the bound-bound electric dipole (E1) transition ${\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}^{o}\ensuremath{\leftrightarrow}{\phantom{\rule{4pt}{0ex}}}^{4}{F}_{3/2}^{e}$ of $\mathrm{T}{\mathrm{h}}^{\ensuremath{-}}$ are observed: 4G5/2o,4F9/2e{\phantom{\rule{4pt}{0ex}}}^{4}{G}_{5/2}^{o}, {\phantom{\rule{4pt}{0ex}}}^{4}{F}_{9/2}^{e}4G5/2o​,4F9/2e​, and 4F7/2o{\phantom{\rule{4pt}{0ex}}}^{4}{F}_{7/2}^{o}4F7/2o​, and their energy levels are determined to be 1847(13), 3166.8(59), and 3666(12) $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, respectively. Meanwhile, the lifetime of the upper state 2S1/2o{\phantom{\rule{4pt}{0ex}}}^{2}{S}_{1/2}^{o}2S1/2o​ of the E1 transition is experimentally determined to be 30(2) \textmu{}s, substantially shorter than the previously calculated result, which makes $\mathrm{T}{\mathrm{h}}^{\ensuremath{-}}$ the most promising candidate for laser cooling of negative ions. Furthermore, the lifetimes of two other short-lived odd-parity excited states of $\mathrm{T}{\mathrm{h}}^{\ensuremath{-}}$ are also measured to be 62(5) and 53(3) \textmu{}s, respectively.
The bound-bound transition from the 5d26s2 3Fe2 ground state to the 5d6s26p 3D° 2 excited state in negative lanthanum has been proposed as a candidate for laser cooling, which has not yet been achieved for negative ions. Anion laser cooling holds the potential to allow the production of ultracold ensembles of any negatively charged species. In this work the aforementioned transition was studied in a beam of negative La ions by high-resolution laser spectroscopy. The center-of-gravity frequency of the transition was measured to be 96.592 80(10) THz. Seven of the nine expected hyperfine structure resonances were resolved. The observed peaks were unambiguously assigned to the predicted hyperfine transitions by a fit. From the determined hyperfine structure for this transition it was concluded that only three lasers would be required to cool and re-pump all hyperfine levels. Furthermore, the observed relative transition amplitudes suggest that in resonance the transitions are saturated at a laser power of 45 Wm-2. A rough estimate of the transition cross section confirms that La- is a promising candidate for the first laser cooling of negative ions.
High-resolution photoelectron energy spectra of osmium anions are obtained using the slow-electron velocity-map imaging method. The energy levels of excited states 4 F 7/2, 4 F 5/2 and 4 F 3/2 of Os− are determined to be 148.730(13), 155.69(15), and 176.76(13) THz [or 4961.09(41), 5193.4(49), and 5896.1(42) cm−1], respectively. The lifetime of the opposite-parity excited state 6 D 9 / 2 o is determined to be 201(10) μs using a cold ion trap, about 15 times shorter than the previous result 3(1) ms. Our high-level multi-configuration Dirac–Hartree–Fock calculations yield a theoretical lifetime 527 μs. Our work shows that the laser cooling rate of Os− is as fast as that of Th−. The advantages of Os− are its near-IR range cooling transition and simple electronic structure, which make Os− a promising candidate for laser cooling of negative ions. We propose a general approach to produce cold atoms and molecules based on the sympathetic cooling of negative ions in combination with a threshold photodetachment.
The bound-bound transition from the 5d^{2}6s^{2} ^{3}F_{2}^{e} ground state to the 5d6s^{2}6p ^{3}D_{1}^{o} excited state in negative lanthanum has been proposed as a candidate for laser cooling, which has not yet been achieved for negative ions. Anion laser cooling holds the potential to allow the production of ultracold ensembles of any negatively charged species. We have studied the aforementioned transition in a beam of negative La ions by high-resolution laser spectroscopy. The center-of-gravity frequency was measured to be 96.592 80(10) THz. Seven of the nine expected hyperfine structure transitions were resolved. The observed peaks were unambiguously assigned to the predicted hyperfine transitions by a fit, confirmed by multiconfigurational self-consistent field calculations. From the determined hyperfine structure we conclude that La^{-} is a promising laser cooling candidate. Using this transition, only three laser beams would be required to repump all hyperfine levels of the ground state.