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arXiv:physics/9703013 · Physical Review A

Electron affinity of Li: A state-selective measurement

G. Haeffler, D. Hanstorp, Igor Yu. Kiyan, Andreas E. Klinkmüller, Ulric B. Ljungblad, D. J. Pegg

Published Jun 1, 1996• 85 citations
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Abstract

We have investigated the threshold of photodetachment of ${\mathrm{Li}}^{\mathrm{\ensuremath{-}}}$ leading to the formation of the residual Li atom in the 2${\mathit{p}}^{2}$P state. The excited residual atom was selectively photoionized via an intermediate Rydberg state and the resulting ${\mathrm{Li}}^{+}$ 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.

DOI
10.1103/physreva.53.4127

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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
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.
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 formation of Li− by dissociative attachment in e+Li2 collisions is characterized by a large cross section for electron attachment to highly vibrationally excited Li2 molecules. However, the electronic structure of the Li−2 system dictates that low energy electrons (≲0.2 eV) are needed to minimize loss processes. An alternate production route for Li− is possible via direct ion‐pair formation through collisions of highly excited (Rydberg) Li atoms. The mechanisms for ion‐pair formation will be discussed in terms of the high‐lying molecular Rydberg states of Li2.
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.