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Physical Review A

Low-lying negative-ion states of calcium

A. R. Johnston, G. A. Gallup, P. D. Burrow

Published Oct 1, 1989• 23 citations
Publisher

Abstract

Studies of electron transmission through Ca vapor reveal two prominent resonances in the range 0--4 eV. The energies and widths of these temporary anion states suggest that they may be assigned to the strongly mixed (4${s}^{2}$3d${)}^{2}$D and (4s4${p}^{2}$${)}^{2}$D states of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$. This interpretation is supported by ab initio configuration-interaction calculations and is consistent with previously unassigned structure observed in the photodetachment cross section.

DOI
10.1103/physreva.40.4770

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Physical Review A•Aug 1, 1996•5 citations
Studies of electron transmission through Ca vapor reveal two prominent resonances in the range 0--4 eV. The energies and widths of these temporary anion states suggest that they may be assigned to the strongly mixed (4s2{s}^{2}s23d)2{)}^{2})2D and (4s4{p}^{2}$${)}^{2}D states of Ca\ensuremath−{\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}Ca\ensuremath−. This interpretation is supported by ab initio configuration-interaction calculations and is consistent with previously unassigned structure observed in the photodetachment cross section.
We have performed close-coupling calculations on electron scattering by Ca atoms and on photodetachment of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ ions by using a six-state (1{(}^{1}(1S- $^{3}\mathrm{\ensuremath{-}}$ $^{3}\mathrm{\ensuremath{-}}$ $^{1}\mathrm{\ensuremath{-}}$ $^{1}\mathrm{\ensuremath{-}}$ 1P^{1}\mathrm{P}1P) R-matrix method. We predict a strong interaction between (4s21{4\mathrm{s}}^{21}4s21S+\ensuremath{\epsilon}d)2{)}^{2})2D and (4s4p3{4\mathrm{s}4\mathrm{p}}^{3}4s4p3P+\ensuremath{\epsilon}p)2{)}^{2})2D channels that gives rise to two relatively broad resonance peaks in the d-wave cross section. In addition, one obtains a strong and a weak peak in the total scattering cross section. Our calculations on the photodetachment cross section of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ ions yield results that are in good qualitative agreement with the experimental data by allowing for the opening of a detachment channel connected to the excited state 4s4p3{4\mathrm{s}4\mathrm{p}}^{3}4s4p3P.
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Our Dirac R-matrix calculations suggest that none of the heavy alkali-metal negative ions, Rb, Cs, and Fr, has an excited bound state. Their lowest excited state appears to be a multiplet of 3PJo{}^{3}{P}_{J}^{o}3PJo​-shape resonances, the J=1J=1J=1 component of which was recently observed in photodetachment experiments on ${\mathrm{Cs}}^{\ensuremath{-}}.$ We analyze these 3PJo{}^{3}{P}_{J}^{o}3PJo​ and the 3Se{}^{3}{S}^{e}3Se excited negative ion states in partial and converged total scattering cross sections for slow electrons with incident kinetic energies below 120 meV. Our results are in excellent agreement with available experimental data. We also propose a new value for the electron affinity of Fr, provide the scattering length for electronic collisions with Rb, Cs, and Fr, and discuss the nuclear charge dependence of relativistic effects in the resonance profiles.
Electron-transmission spectroscopy is used to observe structure in the electron-scattering cross section associated with the formation of temporary negative ions and channel-opening effects. A strong ``family'' resemblance is apparent among the resonances of the alkali metals. From published calculations and angular scattering studies, the lower-lying features are known with some confidence. Based on these, plausible assignments for some of the higher-lying resonances are put forth. We propose that the absence of resonances near the Cs 5 2^{2}2D and possibly the Na 4 2^{2}2P and 5 2^{2}2P thresholds is associated with the negative polarizabilities of these excited states.
Relative yields of Ca+{\mathrm{Ca}}^{+}Ca+ and ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ ions from thermal-energy collisions between highly excited Ca∗{\mathrm{Ca}}^{\mathrm{*}}Ca∗ 4snd 1^{1}1D Rydberg atoms and Ca ground-state atoms are measured as a function of n. The collisions take place within an effusive Ca atomic beam. The cross section for a collision resulting in a bound ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ and a ground-state Ca+{\mathrm{Ca}}^{+}Ca+ is expected to be large when the binding energy of the excited Rydberg state nearly matches the electron affinity of Ca. We observe a large increase in positive and negative ions for n=25 only. This result strongly suggests that the binding energy of ${\mathrm{Ca}}^{\mathrm{\ensuremath{-}}}$ is 24\ifmmode\pm\else\textpm\fi{}1.4 meV.
The formation of Ca+{\mathrm{Ca}}^{+}Ca+ ions is measured in an effusive Ca beam by exciting through either the 4s4p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ or the 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ state to highly excited Ca*(4snd1{\mathit{snd}}^{1}snd1D2{\mathit{D}}_{2}D2​) Rydberg states. In agreement with previous work by McLaughlin and Duquette [Phys. Rev. Lett. 72, 1176 (1994)], when the Rydberg states are accessed through the 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ state, we detect a large increase in Ca+{\mathrm{Ca}}^{+}Ca+ ions for n=25, compared to other high-n Rydberg states. It had been reported (Fabrikant [Phys. Rev. A 48, R3411 (1993)]; McLaughlin and Duquette) that ``ground-state--Rydberg-state'' collisions cause this large increase in ion production and that the result provides a precise value of the electron affinity of Ca through the binding energy of the Rydberg electron. However, a variation of the original experiment in which the same Rydberg states are accessed through the 4s4p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ level instead of the 4s5p1{\mathit{p}}^{1}p1P1{\mathit{P}}_{1}P1​ level shows no enhancement of ions for any particular Rydberg level. With a three-laser experiment we show that the enhanced ionization effect for the 25d state is due to the 3${\mathit{d}}_{3/2}999{\mathit{f}}_{5/2}$, ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${2}^{\mathrm{\ensuremath{-}}}$ autoionizing resonance, which is coincidentally accessed from the Ca(4s25d1{\mathit{d}}^{1}d1D2{\mathit{D}}_{2}D2​) state by the 808-nm photon involved in the original experiment by McLaughlin and Duquette. The existence of this resonance close to the experimentally observed energy is also established by R-matrix calculations. \textcopyright{} 1996 The American Physical Society.