Transition probability of the Si III 189.2-nm intersystem line

Abstract

A radio-frequency ion trap has been used to store ${\mathrm{Si}}^{2+}$ ions created by electron bombardment on Si${\mathrm{H}}_{4}$ or Si${\mathrm{F}}_{4}$. The lifetime of the metastable $3s3p^{3}P_{1}^{o}$ level, measured by direct observation of the $^{1}S_{0}$-$^{3}P_{1}^{o}$ decay at 189.2 nm, was 59.9\ifmmode\pm\else\textpm\fi{}3.6 \ensuremath{\mu}s; $A=(1.67\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{4}$ ${\mathrm{s}}^{\ensuremath{-}1}$. The uncertainties indicate the 90%-confidence limit. Statistical uncertainty dominated. The measured value agrees with calculated values to within their quoted accuracies. Our result therefore confirms the calculated data which have been used heretofore in determinations of electron densities in astrophysical plasmas such as the solar transition zone.