"Polarization-Free" Vacuum-Ultraviolet Excitation of Helium by Electrons

Abstract

"Polarization-free" excitation cross sections for the $1^{1}S\ensuremath{-}n^{1}P$ transitions of He have been measured from threshold to 2 keV using a crossed-beam technique. Results have been made absolute using the accurately known oscillator strengths. Maxima in the cross sections for the $n^{1}P$ levels occurred around 100 eV and were found to be 106\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ ($2^{1}P$), 31\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ ($3^{1}P$), and 13\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ ($4^{1}P$).