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Chinese Physics Letters

Energy Levels and Transition Rates for Laser Cooling Os− and a General Approach to Produce Cold Atoms and Molecules

Yuzhu Lu, Rui Zhang, C. X. Song, Chongyang Chen, R. Si, Chuangang Ning

Published Aug 16, 2023• 8 citations
Publisher

Abstract

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.

DOI
10.1088/0256-307x/40/9/093101

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Journal of Physics Conference Series•Nov 1, 2009•2 citations
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.
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.
Laser cooling is a well-established technique for the creation of ensembles of ultracold neutral atoms or positive ions. This ability has opened many exciting new research fields over the past 40 years. However, no negatively charged ions have been directly laser cooled because a cycling transition is very rare in atomic anions. Efforts of more than a decade currently have La^{-} as the most promising candidate. We report on experimental and theoretical studies supporting Th^{-} as a new promising candidate for laser cooling. The measured and calculated electron affinities of Th are, respectively, 4901.35(48) cm^{-1} and 4832 cm^{-1}, or 0.607 690(60) and 0.599 eV, almost a factor of 2 larger than the previous theoretical value of 0.368 eV. The ground state of Th^{-} is determined to be 6d^{3}7s^{2} ^{4}F_{3/2}^{e} rather than 6d^{2}7s^{2}7p ^{4}G_{5/2}^{o}. The consequence of this is that there are several strong electric dipole transitions between the bound levels arising from configurations 6d^{3}7s^{2} and 6d^{2}7s^{2}7p in Th^{-}. The potential laser-cooling transition is ^{2}S_{1/2}^{o}↔^{4}F_{3/2}^{e} with a wavelength of 2.6 μm. The zero nuclear spin and hence lack of hyperfine structure in Th^{-} reduces the potential complications in laser cooling as encountered in La^{-}, making Th^{-} a new and exciting candidate for laser cooling.
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.
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 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 ${\mathrm{La}}^{\ensuremath{-}}$ (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 $\ensuremath{\sim}4$ times shorter than calculated, highlighting the challenges for this complex ion. The results provide valuable insights into the prospects for laser cooling ${\mathrm{La}}^{\ensuremath{-}}$.
The characteristics of atomic anions differ heavily from cations and neutral atoms, preventing the optical manipulation of their motion, with the remarkable exception of Os - . Since the negative osmium ion exhibits a bound–bound electric-dipole transition, we are currently exploring this unique feature with the aim of laser cooling. In the course of our experiments, we performed high-resolution laser spectroscopy on the relevant transition determining its frequency, cross-section and sub-level splitting in magnetic fields.