Excitation of the $4{}_{}{}^3{}_{}{}^{}S$ and $3{}_{}{}^3{}_{}{}^{}P$ Levels of Helium by Electron Impact

The exchange excitation of the $4{}_{}{}^3{}_{}{}^{}S$ and $3{}_{}{}^3{}_{}{}^{}P$ levels of helium by electron impact has been studied by means of time-resolved spectroscopy of the $4{}_{}{}^3{}_{}{}^{}S\to 2{}_{}{}^3{}_{}{}^{}P$ ($\lambda =4713\mathrm{}$-Å) and $3{}_{}{}^3{}_{}{}^{}P\to 2{}_{}{}^3{}_{}{}^{}S$ ($\lambda =3889\mathrm{}$-Å) transitions. Measurements were carried out for incident electron energies in the range 50-400 eV at helium-gas pressures of 8 and 4 mtorr. Radiative cascade transitions from the $n{}_{}{}^3{}_{}{}^{}P$ levels and from the $n{}_{}{}^3{}_{}{}^{}S$ and $n{}_{}{}^3{}_{}{}^{}D$ levels contributed 12 and 10% to the total $4{}_{}{}^3{}_{}{}^{}S$ and $3{}_{}{}^3{}_{}{}^{}P$ populations, respectively, at 50 eV. These fractions increased to about 30 and 40%, respectively, at 400-eV electron-impact energy. The $4{}_{}{}^3{}_{}{}^{}S$ and $3{}_{}{}^3{}_{}{}^{}P$ direct-excitation cross sections were observed to decrease with increasing electron impact according to the relation ${\mathrm{}\left(\mathrm{energy}\right)\mathrm{}}^{-3\mathrm{}}$. This result is in agreement with the theoretical predictions of Ochkur and Bratsev.