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Physical Review A

Measurement of the electron affinity of the lanthanum atom

Yuzhu Lu, Rulin Tang, Xiaoxi Fu, Chuangang Ning

Published Jun 17, 2019• 20 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.99.062507

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•Jan 1, 2015•1 citation
Atomic lanthanum anion La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}La\ensuremath− is the best candidate for laser cooling of any atomic anions found so far. The bound-bound electric dipole transitions of La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}La\ensuremath− have been identified and measured. Theoretical studies have shown that La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}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 La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}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)cm\ensuremath−14496.97(20)\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}4496.97(20)cm\ensuremath−1 or 0.557 553(25) eV. In addition, the energy levels of La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}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.
Synopsis Atomic lanthanum anion La− is the best candidate for laser cooling of any atomic anions by far. The previous theoretical studies showed that La− has very a complicated electronic structure, which cannot be well resolved by the traditional photoelectron spectroscopy. In the present work, we used the slow-electron velocity-map imaging method in combination with an ion trap to measure the electronic structure of La−. The electron affinity of La is accurately determined to be 4496.97(20) cm−1 or 0.557553(25) eV.
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 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.
This study reinterprets an earlier experimental photoelectron kinetic energy spectrum of the negative ion of lanthanum [A. M. Covington, D. Calabrese, J. S. Thompson, and T. J. Kvale, J. Phys. B 31, L855 (1998)] by carrying out relativistic configuration interaction (RCI) photodetachment calculations. The results confirm the earlier RCI calculation for the electron affinity of lanthanum (0.545 eV) [S. M. O'Malley and D. R. Beck, Phys. Rev. A 79, 012511 (2009)] and revise it to a slightly larger value of 0.550 eV, thus modifying the experimental interpretation of $0.47\ifmmode\pm\else\textpm\fi{}0.02$ eV. The calculation also yields the binding energies of the other thirteen bound states of ${\mathrm{La}}^{\ensuremath{-}}$. Good agreement has been found when these energies are compared to the results of a recent experimental study on ${\mathrm{La}}^{\ensuremath{-}}$ [C. W. Walter, N. D. Gibson, D. J. Matyas, C. T. Crocker, K. A. Dungan, B. R. Matola, and J. Rohl\'en, Phys. Rev. Lett. 113, 063001 (2014)]. Finally, our analysis confirms the transition energy for the potential laser cooling transition of ${}^{3}{F}_{2}^{e}\ensuremath{\rightarrow}{}^{3}{D}_{1}^{o}$ in ${\mathrm{La}}^{\ensuremath{-}}$.
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.
The bound-bound transition from the 5d²6s² ³F_2 ground state to the 5d6s²6p ³D_1 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 W/m². A rough estimate of the transition cross section confirms that La‾ is a promising candidate for the first laser cooling of negative ions.