Abstract
We have used the Cryogenic Storage Ring (CSR) at the Max Planck Institute for Nuclear Physics to study long-lived metastable states of the silicon anion. A ${\mathrm{Si}}^{\ensuremath{-}}$ beam of 58 keV kinetic energy was stored in the ultrahigh cryogenic vacuum of the CSR, employing only electrostatic deflection elements. We used laser systems at various wavelengths to infer information on the decay of the metastable anionic states by selective photodetachment. Our results give evidence of an excited anionic state for which we determine the extremely long lower lifetime limit of 5.7 h at 90% confidence level, consistent with theoretical predictions for the $^{2}D$ term. Furthermore, we find an average lifetime of $\ensuremath{\tau}=(22.2\ifmmode\pm\else\textpm\fi{}2.5)$ s for the weakly bound $^{2}P$ states, employing coincidence counting with a pulsed nanosecond laser at 2.45 $\ensuremath{\mu}\mathrm{m}$. Using a laser depletion technique, we produce a pure ground term $^{4}S_{3/2}\phantom{\rule{4pt}{0ex}}{\mathrm{Si}}^{\ensuremath{-}}$ beam, and we quantify the fraction of ions in metastable states in our initial ion sample. We combine our experimental efforts with state-of-the-art multiconfiguration Dirac-Hartree-Fock calculations for the radiative lifetimes of all metastable levels of ${\mathrm{Si}}^{\ensuremath{-}}$. We find these calculations to be in excellent agreement with our measurements and to improve previous efforts considerably.
Highlights
Negative ions are excellent benchmark systems to study the electronic structure in atomic physics
Taking into account that the calculated cross sections are somewhat higher for photodetachment of the 4S ground state with the Vis laser, when compared to photodetachment of the 2D states with the IR laser, the ratio of the count rates suggests that the population of metastable states lies in the percent range
By careful stepwise interpretation of the time dependencies in the photodetachment signals at different wavelengths, we were able to extract specific lifetime data on the metastable anionic states that were inaccessible to previous experiments
Summary
Negative ions are excellent benchmark systems to study the electronic structure in atomic physics. Previous storage ring experiments on negative ions [16] included measurements on Si− and studied the temporal behavior of the photodetachment signal selectively for states bound by less than 1.17 eV, excluding the 4S ground state. We begin the experimental section with an overview of the decay of the metastable state signal compared with the overall beam decay in the storage ring, followed by laser depletion measurements and coincidence measurements with a pulsed laser This stepwise analysis allows us to unambiguously identify an average natural lifetime for the weakly bound and sparsely populated 2P states, and a lower lifetime limit for the long-lived 2D states, both of which go considerably beyond the findings of previous experimental studies.
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