Metastable states of Si- observed in a cryogenic storage ring

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 Si- 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 D-2 term. Furthermore, we find an average lifetime of tau = (22.2 +/- 2.5) s for the weakly bound P-2 states, employing coincidence counting with a pulsed nanosecond laser at 2.45 mu m. Using a laser depletion technique, we produce a pure ground term S-4(3/2) Si- 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 Si-. We find these calculations to be in excellent agreement with our measurements and to improve previous efforts considerably.