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Physical Review A

Structural properties of the negative strontium ion: Binding energy and fine-structure splitting

H. H. Andersen, V. V. Petrunin, P. Kristensen, T. Andersen

Published Apr 1, 1997• 26 citations
Publisher

Abstract

The binding energy and fine-structure splitting of the ground state of the ${\mathrm{Sr}}^{\mathrm{\ensuremath{-}}}$n(${5\mathrm{s}}^{2}$5p${\mathrm{}}^{2}$P) ion have been measured by means of the laser photodetachment threshold technique combined with state-selective detection of neutral atoms using resonant ionization spectroscopy. We find binding energies of 32.17\ifmmode\pm\else\textpm\fi{}0.03 meV (J=3/2) and 52.06\ifmmode\pm\else\textpm\fi{}0.06 meV (J=1/2), yielding a fine-structure splitting of 19.89\ifmmode\pm\else\textpm\fi{}0.07 meV. This represents an improvement of two orders of magnitude compared to the recent data based on a combination of laser photodetachment and accelerator mass spectroscopy [D. Berkovits et al., Phys. Rev. Lett. 75, 414 (1995)].

DOI
10.1103/physreva.55.3247

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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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The binding energy and fine-structure splitting of the ground state of the Sr\ensuremath−{\mathrm{Sr}}^{\mathrm{\ensuremath{-}}}Sr\ensuremath−n(5s2{5\mathrm{s}}^{2}5s25p2{\mathrm{}}^{2}2P) ion have been measured by means of the laser photodetachment threshold technique combined with state-selective detection of neutral atoms using resonant ionization spectroscopy. We find binding energies of 32.17\ifmmode\pm\else\textpm\fi{}0.03 meV (J=3/2) and 52.06\ifmmode\pm\else\textpm\fi{}0.06 meV (J=1/2), yielding a fine-structure splitting of 19.89\ifmmode\pm\else\textpm\fi{}0.07 meV. This represents an improvement of two orders of magnitude compared to the recent data based on a combination of laser photodetachment and accelerator mass spectroscopy [D. Berkovits et al., Phys. Rev. Lett. 75, 414 (1995)].
The binding energies of the 4s24p2P{4s}^{2}4p^{2}P4s24p2P ground state in ${\mathrm{Ca}}^{\ensuremath{-}}$ have been determined by means of state-selective photoionization of Ca atoms produced in s-wave photodetachment to be $24.55\ifmmode\pm\else\textpm\fi{}0.10$ meV for the J=1/2J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2J=1/2 and $19.73\ifmmode\pm\else\textpm\fi{}0.10$ meV for the J=3/2J\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3/2J=3/2 component. The sensitivity of the detection technique applied has also made it possible to disprove previous claims for the observation of a metastable ${4s4p}^{2}^{4}P$ state in ${\mathrm{Ca}}^{\ensuremath{-}}$.
This paper presents a combined experimental and theoretical investigation of the binding energy of the metastable ${\mathrm{Ar}}^{\ensuremath{-}}{(3p}^{5}4s4p{}^{4}{S}_{3/2}^{e})$ ion. Utilizing the laser-photodetachment threshold technique combined with state-selective detection of neutral atoms by resonant ionization spectroscopy, we find a binding energy of $32.5\ifmmode\pm\else\textpm\fi{}1.0$ meV with respect to the 3p54s3P2o{3p}^{5}4s{}^{3}{P}_{2}^{o}3p54s3P2o​ state of neutral argon. Our calculations, based on many-body perturbation theory, yield a binding energy of 24.9 meV. As a test of the present computational method, we also include the results of a comparative study of ${\mathrm{Ca}}^{\ensuremath{-}}$.
We have observed a threshold in the electron photodetachment cross section of ${^{88}S\mathrm{r}}^{\ensuremath{-}}$ ions at a photon energy $h\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.820\mathrm{eV}$ and assign it to a ppp-wave transition from the 5s25p2P{5s}^{2}5p^{2}P5s25p2P ground state in ${\mathrm{Sr}}^{\ensuremath{-}}$ to the 5s5p3P5s5p^{3}P5s5p3P state in neutral Sr. The measurement was made with a new technique combining the tunable laser photodetachment threshold method with accelerator mass spectrometry. We determine for the first time the electron affinity of Sr to be $48\ifmmode\pm\else\textpm\fi{}6\mathrm{meV}$, much smaller than predicted in recent calculations. The 2P1/2^{2}P_{1/2}2P1/2​- 2P3/2^{2}P_{3/2}2P3/2​ fine splitting of the ${\mathrm{Sr}}^{\ensuremath{-}}$ ground state is estimated to be $26\ifmmode\pm\else\textpm\fi{}8\mathrm{meV}$.
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.
Atomic many-body theory methods are used to calculate the fine structure of negative ions formed by binding a p electron into an open shell, ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{Sr}}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{Ba}}^{\mathrm{\ensuremath{-}}}$, and ${\mathrm{Ra}}^{\mathrm{\ensuremath{-}}}$. This binding is due to a strong correlation potential acting between the electron and the neutral atom. Comparison with experimental data shows that the second order many-body perturbation theory calculation overestimates the correlation potential by 10% to 15%. Scaling factors are introduced in the correlation potential to reproduce experimental binding energies of the lower p1/2{\mathrm{p}}_{1\mathrm{/}2}p1/2​ components. This procedure yields fine-structure intervals in excellent agreement with experiment for ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{Sr}}^{\mathrm{\ensuremath{-}}}$, and ${\mathrm{Ba}}^{\mathrm{\ensuremath{-}}}$, and allows us to predict that in ${\mathrm{Ra}}^{\mathrm{\ensuremath{-}}}$ the p1/2{\mathrm{p}}_{1\mathrm{/}2}p1/2​ state is bound by 100 meV, and p3/2{\mathrm{p}}_{3\mathrm{/}2}p3/2​ is a resonance at 16 meV in the continuum.
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.