Absolute Measurement of Differential Cross Sections for Electron Scattering in Helium

Absolute measurements of cross sections for electron-impact scattering in helium at an angle of 5\ifmmode^\circ\else\textdegree\fi{} have been made for elastic scattering and excitation of the $2^{1}P$ and $2^{1}S$ states and for incident energies of 50-400 eV. Cross-section values of $\ensuremath{\sigma}(2^{1}P,{5}^{\ensuremath{\circ}})$ are found to be lower than theoretical Born values by (9.5 \ifmmode\pm\else\textpm\fi{} 5.4)% at 400 eV, (31.5 \ifmmode\pm\else\textpm\fi{} 4.6)% at 100 eV, and (62.5 \ifmmode\pm\else\textpm\fi{} 3.4)% at 50 eV. Deduced values of total $2^{1}P$ excitation cross sections ($E\ensuremath{\ge}100$ eV) are in agreement with other experimental values. Our measurements for elastic scattering agree well with recent theoretical calculations.