Experimental determination of the single-photon transition rate between the2S13and1S01states of He i

Abstract

The highly forbidden single-photon transition rate between the $2^{3}S_{1}$ and $1^{1}S_{0}$ states of Hei has been measured in a radio-frequency He discharge. The population of metastables in the $2^{3}S_{1}$ state was determined by Fabry-Perot interferometric profiles of absorption from the $2^{3}S_{1}$ to the $4^{3}P_{0,1,2}$ states. High spectral resolution and precision ultraviolet radiometry were used to determine the brightness of the emission feature observed at 625.54 \ifmmode\pm\else\textpm\fi{} 0.05 \AA{}, compared to the theoretical value of 625.56 \AA{}. Because of the very low transition rate, this feature is weak, but it is shown that it is due to the $2^{3}S_{1}\ensuremath{-}1^{1}S_{0}$ transition. The value for the radiative transition rate obtained in these experiments was 1.10 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$/sec, with a $2\ensuremath{\sigma}$ uncertainty of \ifmmode\pm\else\textpm\fi{} 30%. A comparison to the theoretical value of 1.27 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$/sec is given. The relevance of this work to the beam-foil experiments which have measured the $2^{3}S_{1}\ensuremath{-}1^{1}S_{0}$ transition rate for high-$Z$ He-like ions is also discussed.