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Physical Review A

La− binding energies by analysis of its photodetachment spectra

Lin Pan, Donald R. Beck

Published Jun 1, 2016• 10 citations
Publisher

Abstract

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 ${\mathrm{La}}^{\ensuremath{-}}$.

DOI
10.1103/physreva.93.062501

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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 ${\mathrm{Ce}}^{\ensuremath{-}}$ (${}^{4}\phantom{\rule{-0.16em}{0ex}}{H}_{7/2}--{}^{4}\phantom{\rule{-0.16em}{0ex}}{H}_{9/2}$) to be 0.097 75(4) eV.

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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±\else\textpm\fi0.020.47\ifmmode\pm\else\textpm\fi{}0.020.47\ifmmode±\else\textpm\fi0.02 eV. The calculation also yields the binding energies of the other thirteen bound states of La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}La\ensuremath−. Good agreement has been found when these energies are compared to the results of a recent experimental study on La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}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 3F2e\ensuremath→3D1o{}^{3}{F}_{2}^{e}\ensuremath{\rightarrow}{}^{3}{D}_{1}^{o}3F2e​\ensuremath→3D1o​ in La\ensuremath−{\mathrm{La}}^{\ensuremath{-}}La\ensuremath−.
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).
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.
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.
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 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.