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Journal of Physics B Atomic Molecular and Optical Physics

The electron affinity of rubidium: a state selective measurement

Annie Ringvall-Moberg, Miranda Nichols, José E. Navarro Navarrete, Uldis Berzinsh, Viola Caroline D’mello, Julia Karls, D. C. Lu, Yazareth Peña Rodríguez, Rachel Poulose, Andrea Morales Rodríguez, Keerthana Ravi, Meera Ramachandran, Vitali Zhaunerchyk, D. Hanstorp, D. Leimbach

Published Jul 2, 2024• 3 citations
Publisher

Abstract

Abstract Negative ions, which are formed when an electron is attached to a neutral system, are unique quantum systems. The lack of a long-range Coulomb force causes the inter-electronic interactions to become relatively more important. As a consequence, the independent particle model, which adequately describes atomic structure under normal conditions, breaks down. The alkali negative ions, with a closed valence s-shell, are among the simplest anionic systems. Hence, they can favorably be used to benchmark atomic theory. In this work, we have determined the electron affinity of 85Rb by measuring the relative partial photodetachment cross section of the negative ion, leaving the residual atom in the 5p 2 P 3 / 2 excited state. Resonance ionization spectroscopy allows for state selectivity and the ability to measure the Wigner s-wave threshold onset of the photodetachment process. The electron affinity of 85Rb was determined to be 485.887(6) meV.

DOI
10.1088/1361-6455/ad5e25

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Abstract Negative ions, which are formed when an electron is attached to a neutral system, are unique quantum systems. The lack of a long-range Coulomb force causes the inter-electronic interactions to become relatively more important. As a consequence, the independent particle model, which adequately describes atomic structure under normal conditions, breaks down. The alkali negative ions, with a closed valence s-shell, are among the simplest anionic systems. Hence, they can favorably be used to benchmark atomic theory. In this work, we have determined the electron affinity of 85Rb by measuring the relative partial photodetachment cross section of the negative ion, leaving the residual atom in the 5p 2 P 3 / 2 excited state. Resonance ionization spectroscopy allows for state selectivity and the ability to measure the Wigner s-wave threshold onset of the photodetachment process. The electron affinity of 85Rb was determined to be 485.887(6) meV.
Negative ions are unique quantum systems where electron correlation plays a decisive role in determining their properties. The lack of optically allowed transitions prevents traditional optical spectroscopy and the electron affinity is, therefore, for most elements, the only atomic quantity that can be determined with high accuracy. In this work, we present a high-precision experimental determination of the electron affinity of cesium. A collinear laser-ion beam apparatus was used to investigate the partial photodetachment cross section for the cesium anion, leaving the neutral atom in the 6p 2P3/2 excited state. A resonance ionization scheme was used to obtain final-state selectivity, which enabled the investigation of a sharp onset of the cross section associated with a Wigner s -wave threshold behavior. The electron affinity was determined to be 0.4715983(38)eV . Published by the American Physical Society 2024
We have investigated the threshold of photodetachment of ${\mathrm{Li}}^{\mathrm{\ensuremath{-}}}$ leading to the formation of the residual Li atom in the 2p2{\mathit{p}}^{2}p2P state. The excited residual atom was selectively photoionized via an intermediate Rydberg state and the resulting Li+{\mathrm{Li}}^{+}Li+ ion was detected. A collinear laser-ion beam geometry enabled both high resolution and sensitivity to be attained. We have demonstrated the potential of this state-selective photodetachment spectroscopic method by improving the accuracy of Li electron affinity measurements an order of magnitude. From a fit to the Wigner law in the threshold region, we obtained a Li electron affinity of 0.618 049(20) eV. \textcopyright{} 1996 The American Physical Society.
In a recent experiment, Lindahl et al. ( Lindahl A. O. et al. , Phys. Rev. Lett. , 108 (2012) 033004) observed a new threshold behavior in the photodetachment of the potassium negative ion when the residual atom is left in the state. Unfortunately the resonance close to threshold made it more difficult to apply the semi-classical model that was developed to describe the threshold behavior. In this paper we present a study of the threshold behavior of the partial cross-section in the photodetachment of the sodium negative ion. The experiment was conducted using a collinear beams apparatus where the detection was performed using a resonance ionization scheme. We observe the same threshold behavior as in the previous experiment, but without any obfuscating resonance. The model derived in the previous paper is found to fit the observed cross-section up to 35 meV above threshold. The present experiment clearly demonstrates the threshold behaviour for the fundamental process of a free electron moving in the field of an atom with a large negative polarizability.
We calculate near-threshold photodetachment cross sections for ${\mathrm{Rb}}^{\ensuremath{-}},$ ${\mathrm{Cs}}^{\ensuremath{-}},$ and ${\mathrm{Fr}}^{\ensuremath{-}}$ using the Pauli equation method with a model potential describing the effective electron-atom interaction. Parameters of the model potential are fitted to reproduce ab initio scattering phase shifts obtained from Dirac R-matrix calculations. Special care is taken to formulate the boundary conditions near the atomic nucleus for solving the Pauli equation, based on the analytic solution of the Dirac equation for a Coulomb potential. We find a 3P1o{}^{3}{P}_{1}^{o}3P1o​ resonance contribution to the photodetachment cross section of ${\mathrm{Rb}}^{\ensuremath{-}},$ ${\mathrm{Cs}}^{\ensuremath{-}},$ and ${\mathrm{Fr}}^{\ensuremath{-}}$ ions. Our calculated total photodetachment cross sections for Cs agree with experiments after tuning the resonance position by 2.4 meV. For ${\mathrm{Rb}}^{\ensuremath{-}}$ and ${\mathrm{Fr}}^{\ensuremath{-}}$ the resonance contribution is much smaller than for Cs. We therefore also provide angle-differential cross sections and asymmetry parameters which are much more sensitive to the resonant contribution than total cross sections.
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 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.