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Physical Review A

Binding energy of the metastable Ar− ion

U. V. Pedersen, H. H. Andersen, T. Andersen, Leif Veseth

Published Jul 1, 1998• 9 citations
Publisher

Abstract

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 ${3p}^{5}4s{}^{3}{P}_{2}^{o}$ 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 ${\mathrm{Ca}}^{\ensuremath{-}}$.

DOI
10.1103/physreva.58.258

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This paper presents a combined experimental and theoretical investigation of the binding energy of the metastable Ar\ensuremath−(3p54s4p4S3/2e){\mathrm{Ar}}^{\ensuremath{-}}{(3p}^{5}4s4p{}^{4}{S}_{3/2}^{e})Ar\ensuremath−(3p54s4p4S3/2e​) 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±\else\textpm\fi1.032.5\ifmmode\pm\else\textpm\fi{}1.032.5\ifmmode±\else\textpm\fi1.0 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 Ca\ensuremath−{\mathrm{Ca}}^{\ensuremath{-}}Ca\ensuremath−.
The binding energy and fine-structure splitting of the ground state of the ${\mathrm{Sr}}^{\mathrm{\ensuremath{-}}}$n(${5\mathrm{s}}^{2}5p5p5p{\mathrm{}}^{2}$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)].
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 ${\mathrm{Ca}}^{\ensuremath{-}}$.
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.
Theoretical binding energies have been obtained for the metastable np34So{\mathrm{np}}^{3}{}^{4}{S}^{o}np34So states in the alkaline-earth sequence using a B-spline configuration-interaction approach. The results agree with available experimental and theoretical data within 5 meV for ${\mathrm{Be}}^{\ensuremath{-}}$ and within 10 meV for ${\mathrm{Mg}}^{\ensuremath{-}}.$ The binding energies of the p34So{p}^{3}{}^{4}{S}^{o}p34So state in ${\mathrm{Mg}}^{\ensuremath{-}},$ ${\mathrm{Ca}}^{\ensuremath{-}},$ and ${\mathrm{Sr}}^{\ensuremath{-}}$ are larger than the one in ${\mathrm{Be}}^{\ensuremath{-}}$ by roughly 250 meV due to the stronger interactions with the d2p{d}^{2}pd2p configurations. For ${\mathrm{Ca}}^{\ensuremath{-}},$ the 4So{}^{4}{S}^{o}4So binding energy is determined to be 555.5 meV, including a contribution of $\ensuremath{-}70$ meV due to dielectronic core polarization. For ${\mathrm{Sr}}^{\ensuremath{-}}$ the 4So{}^{4}{S}^{o}4So binding energy is 557.9 meV, including a decrease of only 6 meV due to dielectronic core polarization. This behavior for the 4So{}^{4}{S}^{o}4So states in ${\mathrm{Ca}}^{\ensuremath{-}}$ and ${\mathrm{Sr}}^{\ensuremath{-}}$ is ascribed to the different neutral-atom states to which the 4So{}^{4}{S}^{o}4So states are bound.
The resonant metastability exchange process in low-energy collinear collisions between metastable argon atoms (Ar* 3 P 2 ) polarized in spin ( M = +2) and ground-state Ar atoms from a nozzle beam is studied by means of a time-of-flight technique. A wide range of metastable atom velocities in the laboratory frame (275 m s −1 down to 50 m s −1 ) is obtained by use of a Zeeman slower, the counter-propagating laser beam of which is locked in frequency onto the 3 P 2 – 3 D 3 closed transition (λ = 811.5 nm). The accessible centre-of-mass energy range (8–27 meV) has not been explored so far, to our knowledge. Calculations based upon existing interatomic potentials of 2 g and 2 u symmetries are in reasonable agreement with experiment.
Abstract The collision between a spin-polarized metastable argon atom in Ar* (3p 5 4s, 3 P 2 , M = +2) state slightly decelerated by the Zeeman slower-laser technique and a co-propagating thermal ground state argon atom Ar (3p 6 , 1 S 0 ), both merged from the same supersonic beam, but coming through adjacent slots of a rotating disk, is investigated at the center of mass energies ranging from 1 to 10 meV. The duration of the laser pulse synchronised with the disk allows the tuning of the relative velocity and thus the collision energy. At these sub-thermal energies, the ‘resonant metastability transfer’ signal is too small to be evidenced. The explored energy range requires using indiscernibility amplitudes for identical isotopes to have a correct interpretation of the experimental results. Nevertheless, excitation transfers are expected to increase significantly at much lower energies as suggested by previous theoretical predictions of potentials 2g( 3 P 2 ) and 2u( 3 P 2 ). Limits at ultra-low collisional energies of the order of 1 mK (0.086 μ eV) or less, where gigantic elastic cross sections are expected, will also be discussed. The experimental method is versatile and could be applied using different isotopes of Argon like 36 Ar combined with 40 Ar, as well as other rare gases among which Krypton should be of great interest thanks to the available numerous isotopes present in a natural gas mixture.