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Journal of Physics Conference Series

Spectroscopy of Os-and a route to laser-cooled negative ions

Ulrich Warring, C. Canali, Arne Fischer, A. Kellerbauer

Published Nov 1, 2009• 2 citations
Publisher

Abstract

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.

DOI
10.1088/1742-6596/194/15/152022

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High-resolution optical spectroscopy of Os−with a view to laser cooling of atomic anions

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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.

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Measurement of the Zeeman effect in an atomic anion: Prospects for laser cooling of Os−

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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.

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Atomic lanthanum anion ${\mathrm{La}}^{\ensuremath{-}}$ is the best candidate for laser cooling of any atomic anions found so far. The bound-bound electric dipole transitions of ${\mathrm{La}}^{\ensuremath{-}}$ have been identified and measured. Theoretical studies have shown that ${\mathrm{La}}^{\ensuremath{-}}$ has a very complicated electronic structure, which cannot be well resolved by the traditional photoelectron spectroscopy. In the present work, we report high-resolution photoelectron spectroscopy of ${\mathrm{La}}^{\ensuremath{-}}$ via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of La was determined to be $4496.97(20)\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ or 0.557 553(25) eV. In addition, the energy levels of ${\mathrm{La}}^{\ensuremath{-}}$, $^{3}F_{3}^{e}$, $^{3}F_{4}^{e}$, $^{1}D_{2}^{e}$, $^{1}D_{2}^{o}$, $^{3}P_{0}^{e}$, $^{3}P_{1}^{e}$, and $^{3}P_{2}^{e}$, were also determined.

Physical Review A•Jun 17, 2019•20 citations
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.
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 osmium ion ${\text{Os}}^{\ensuremath{-}}$ is one of very few atomic anions potentially suitable for laser cooling. We have made a measurement of the Zeeman splitting of a bound-bound transition in 192{}^{192}192${\text{Os}}^{\ensuremath{-}}$ by studying the laser excitation from the 5${d}^{7}666{s}^{2}$ 4{}^{4}4FFF9/2e{}_{9/2}^{e}9/2e​ ground to the 5${d}^{6}666{s}^{2}666p$ 6{}^{6}6DDD9/2o{}_{9/2}^{o}9/2o​ excited state in a homogeneous external magnetic field. The experimental Land\'e factors gJ=1.31(7){g}_{J}=1.31(7)gJ​=1.31(7) and gJ=1.50(8){g}_{J}=1.50(8)gJ​=1.50(8), respectively, agree well with calculated values. Both levels are found to split into 10 Zeeman sublevels, resulting in 28 allowed transitions of different relative intensities, in agreement with calculations based on pure and composite LSLSLS states. In view of the experimental results, the prospects for laser cooling of ${\text{Os}}^{\ensuremath{-}}$ are discussed.
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.
We have investigated the hyperfine structure of the transition between the 5d{7}6s{2} {4}F{9/2}{e} ground state and the 5d{6}6s{2}6p {6}D{J}{o} excited state in the negative osmium ion by high-resolution collinear laser spectroscopy. This transition is unique because it is the only known electric-dipole transition in atomic anions and might be amenable to laser cooling. From the observed hyperfine structure in 187Os- and 189Os- the yet unknown total angular momentum of the bound excited state was found to be J=9/2. The hyperfine structure constants of the {4}F{9/2}{e} ground state and the {6}D{9/2}{o} excited state were determined experimentally and compared to multiconfiguration Dirac-Fock calculations. Using the knowledge of the ground and excited state angular momenta, the full energy level diagram of 192Os- in an external magnetic field was calculated, revealing possible laser cooling transitions.
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.
Atomic lanthanum anion ${\mathrm{La}}^{\ensuremath{-}}$ is the best candidate for laser cooling of any atomic anions found so far. The bound-bound electric dipole transitions of ${\mathrm{La}}^{\ensuremath{-}}$ have been identified and measured. Theoretical studies have shown that ${\mathrm{La}}^{\ensuremath{-}}$ has a very complicated electronic structure, which cannot be well resolved by the traditional photoelectron spectroscopy. In the present work, we report high-resolution photoelectron spectroscopy of ${\mathrm{La}}^{\ensuremath{-}}$ via the slow-electron velocity-map imaging method in combination with an ion trap. The electron affinity of La was determined to be $4496.97(20)\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ or 0.557 553(25) eV. In addition, the energy levels of ${\mathrm{La}}^{\ensuremath{-}}$, 3F3e^{3}F_{3}^{e}3F3e​, 3F4e^{3}F_{4}^{e}3F4e​, 1D2e^{1}D_{2}^{e}1D2e​, 1D2o^{1}D_{2}^{o}1D2o​, 3P0e^{3}P_{0}^{e}3P0e​, 3P1e^{3}P_{1}^{e}3P1e​, and 3P2e^{3}P_{2}^{e}3P2e​, were also determined.