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

"Polarization-free" excitation cross sections for the $1{}_{}{}^1{}_{}{}^{}S-n{}_{}{}^1{}_{}{}^{}P$ 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×${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ ($2{}_{}{}^1{}_{}{}^{}P$), 31×${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ ($3{}_{}{}^1{}_{}{}^{}P$), and 13×${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ ($4{}_{}{}^1{}_{}{}^{}P$).