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Journal of Physics B Atomic Molecular and Optical Physics

The resonance structures of electron interaction with Sr and Ba atoms: low-energy electron scattering and photodetachment of the negative ions

Jianmin Yuan

Published May 6, 2003• 5 citations
Publisher

Abstract

The resonance and near-threshold structures of low-energy electron interaction with Sr and Ba atoms have been investigated by carrying out respectively 11-state and 21-state close-coupling calculations within the R-matrix scheme. Part of the core–valence correlation is included by exciting one core electron to the valence orbitals. Results for the low-energy electron scattering and the photodetachment of the negative ions are presented. For Sr, the results show that there is a 2D shape resonance around 0.8 eV above the elastic scattering threshold and a remarkable structure just above the first 3Po excitation threshold. For Ba, two 2D shape resonances are predicted, just above the elastic scattering threshold and the 3D excitation threshold, respectively.

DOI
10.1088/0953-4075/36/10/314

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The resonance and near-threshold structures of low-energy electron interaction with Sr and Ba atoms have been investigated by carrying out respectively 11-state and 21-state close-coupling calculations within the R-matrix scheme. Part of the core–valence correlation is included by exciting one core electron to the valence orbitals. Results for the low-energy electron scattering and the photodetachment of the negative ions are presented. For Sr, the results show that there is a 2D shape resonance around 0.8 eV above the elastic scattering threshold and a remarkable structure just above the first 3Po excitation threshold. For Ba, two 2D shape resonances are predicted, just above the elastic scattering threshold and the 3D excitation threshold, respectively.
Three mutually related problems concerning the interaction of an electron with an alkaline-earth atom are examined. (i) The scattering problem: the phaseshifts for p-electron scattering by Ca, Sr and Ba are calculated. (ii) The negative ion problem: the wavefunction and the binding energy are obtained for the outer electron in Ca - 4s 2 4p, Sr - 5s 2 5p and Ba - 6s 2 6p ions. (iii) The photodetachment problem: the photodetachment cross sections are calculated for the outer electron in each of these ions. The most intriguing feature of these problems is the strong correlational potential, attracting the electron to the atom due to the large dipole polarizabilities of Ca, Sr and Ba. The calculations are performed by means of the Dyson equation of the many-body theory.
RRR-matrix calculations have been carried out to investigate the effect of core-valence electron correlation on the scattering of electrons with Ca atoms at low energies. While the major feature of the ddd-wave shape resonance with a peak near 1.3 eV in the total cross section can be reproduced by including only the correlation among the valence electrons, we show that the existence of a low-energy peak near 0.15 eV, which has also been observed experimentally, can be obtained only if the correlations between the valence and core electrons are considered. This low-energy peak is shown not caused by a shape resonance, but rather due to the rapid decrease of the sss- and ppp-wave phase shifts with collision energies. The present result also indirectly supports the previous conclusion that core-valence electron correlation is responsible for bringing the electron affinity of ${\mathrm{Ca}}^{\ensuremath{-}}$ from 72 to 15 meV, i.e., to a value which is much closer to the experiment.
We analyze negative-ion resonances in elastic and inelastic total scattering cross sections for slow electron collisions with the heavy-alkali-metal atoms Rb, Cs, and Fr. Our calculations are based on the Dirac RRR-matrix method [Thumm and Norcross, Phys. Rev. A 45, 6349 (1992)]. For incident electrons of up to 2.8 eV kinetic energy, we compare a 3Po{}^{3}{P}^{o}3Po shape resonance and 3Pe,{}^{3}{P}^{e},3Pe, 1P1o,{}^{1}{P}_{1}^{o},1P1o​, and 1D2o{}^{1}{D}_{2}^{o}1D2o​ Feshbach resonances, located below the first excitation threshold of the atomic target in the low-lying spectra of the ${\mathrm{Rb}}^{\ensuremath{-}},$ ${\mathrm{Cs}}^{\ensuremath{-}},$ and ${\mathrm{Fr}}^{\ensuremath{-}}$ negative ions, with available experimental data and other calculations. We provide the resonance parameters, partial and converged total elastic and inelastic scattering cross sections, and discuss the relative importance of relativistic effects for the three heavy-alkali-metal targets.
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.
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.
The results of a joint experimental and theoretical study concerning elastic electron scattering by laser-excited 138 Ba( ... 6s6p 1 P 1 ) atoms are described. These studies demonstrate several important aspects of elastic electron collisions with coherently excited atoms, and are the first such studies. From the measurements, collision and coherence parameters, as well as cross sections associated with an atomic ensemble prepared with an arbitrary in-plane laser geometry and linear polarization (with respect to the collision frame), or equivalently with any magnetic sublevel superposition, have been obtained at 20 eV impact energy and at 10°, 15° and 20° scattering angles. The convergent close-coupling (CCC) method was used within the non-relativistic LS -coupling framework to calculate the magnetic sublevel scattering amplitudes. From these amplitudes all the parameters and cross sections at 20 eV impact energy were extracted in the full angular range in 1° steps. The experimental and theoretical results were found to be in good agreement, indicating that the CCC method can be reliably applied to elastic scattering by 138 Ba( ... 6s6p 1 P 1 ) atoms, and possibly to other heavy elements when spin-orbit coupling effects are negligible. Small but significant asymmetry was observed in the cross sections for scattering to the left and to the right. It was also found that elastic electron scattering by the initially isotropic atomic ensemble resulted in the creation of significant alignment. As a byproduct of the present studies, elastic scattering cross sections for metastable 138 Ba atoms were also obtained.