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Physical Review A

Zeeman transition strengths in photodetachment from O− and S−

A. K. Langworthy, D. M. Pendergrast, John N. Yukich

Published Feb 26, 2004• 7 citations
Publisher

Abstract

We have probed the relative weight of the first Zeeman transition in photodetachment from ${\mathrm{O}}^{\ensuremath{-}}$ and ${\mathrm{S}}^{\ensuremath{-}}$ at the ${}^{2}{\stackrel{\ensuremath{\rightarrow}}{{P}_{3/2}}}^{3}{P}_{2}$ detachment threshold, using laser light polarized perpendicular to a 1T field. For both ion species, we find a nonzero transition strength at the first threshold, a clear discrepancy with previously published theory based on LS coupling in the ion and the atom.

DOI
10.1103/physreva.69.025401

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We have probed the relative weight of the first Zeeman transition in photodetachment from O\ensuremath−{\mathrm{O}}^{\ensuremath{-}}O\ensuremath− and S\ensuremath−{\mathrm{S}}^{\ensuremath{-}}S\ensuremath− at the 2P3/2\ensuremath→3P2{}^{2}{\stackrel{\ensuremath{\rightarrow}}{{P}_{3/2}}}^{3}{P}_{2}2P3/2​\ensuremath→​3P2​ detachment threshold, using laser light polarized perpendicular to a 1T field. For both ion species, we find a nonzero transition strength at the first threshold, a clear discrepancy with previously published theory based on LS coupling in the ion and the atom.
Photodetachment from negative ions in a static electric field has been studied using ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$. These experiments were done with greater precision than previous studies and allow a quantitative comparison of data with theory. A 10-keV ion beam was sent through a region with fields of up to 1.5 kV/cm applied parallel to the beam velocity. A pulsed dye-laser beam, perpendicular to the ion beam, photoneutralized the ions. The relative cross sections were measured by detecting the resulting fast neutral atoms. Detachment below threshold and oscillations on the cross section above threshold were observed near the ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ threshold at 16 269.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and near the ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ threshold at 29 138.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The phase of the oscillations in the data is in good agreement with predictions for s-wave photodetachment in a static electric field, but the amplitude of the oscillations is found to be slightly reduced.
The behavior of the photodetachment cross section of the ${S}^{\ensuremath{-}}$ ion near thresholds in a magnetic field is derived. This cross section is shown to have an oscillatory dependence on photon energy. The calculation is based on first-order time-dependent perturbation theory, and final-state interactions are ignored. The weights of individual transitions between the various Zeeman sublevels for different light polarizations are given in terms of the appropriate angular momentum coupling coefficients. Broadening of the features of the cross section due to the motion of the ions is included in the calculation. The Wigner-law behavior known to be valid for zero magnetic field is shown to result from the magnetic field dependent cross section in the limit of low magnetic field. The theory is shown to be in good agreement with experimental results. The extension of the theory to other atomic negative ions is discussed.
The effect of final-state interactions on the photodetachment cross section of negative ions in a magnetic field is discussed. Experiments on the location of the 2P32^{2}P_{\frac{3}{2}}2P23​​ threshold in ${\mathbf{S}}^{\ensuremath{-}}$ as a function of magnetic field show clearly the effect of the zero-point energy of an electron in a magnetic field. The experiments also give a precise value for the electron affinity and provide information about the interaction between the atom and the detached electron.
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 $26\ifmmode\pm\else\textpm\fi{}8\mathrm{meV}$.
A multichannel study of the photodetachment of a negative ion such as ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ in the presence of a constant electric field is presented. The analysis is carried out as a series of frame transformations for the outgoing photoelectron. An earlier analysis in the absence of the external field is now augmented by the further transformations from spherical to cylindrical symmetry that are required. The interplay between the various thresholds due to fine structure in the negative ion and residual atom and the effects of the external field are analytically treated. The net effect is to eliminate the thresholds, to induce modulations in the detachment cross section, and to introduce field-assisted-tunneling contributions to the photodetachment. Numerical illustrations are provided. Our treatment closely parallels similar treatments of photoionization of neutral atoms in an electric field and of photodetachment in a magnetic field.
The authors study threshold shifts theoretically in negative-ion photodetachment, using the approach of Rzezewski, Lewenstein and Eberly (1982), by considering two model couplings for the discrete-state continuum interaction specific to outgoing p and s electrons. They apply their results to the photodetachment of Pt - and predict a threshold shift which is directly proportional to laser intensity and is approximately 2.3*10 -10 I cm -1 (W cm -2 ) -1 . This shift appears too small to be measured by present day techniques.