Absolute Measurements of the21Pand23PElectron Excitation Cross Sections of Helium Atoms

The apparent cross sections for electron excitation of the $2^{1}P$ and $2^{3}P$ states of helium are examined in the light of new and detailed information concerning the cascading to these levels from higher states, particularly in the low-energy region near onset. Excitation functions of the ${}_{}{}^1{}_{}{}^{}S$, ${}_{}{}^1{}_{}{}^{}D$, ${}_{}{}^3{}_{}{}^{}S$, and ${}_{}{}^3{}_{}{}^{}D$ states from $n=3\mathrm{}$ to $n=7\mathrm{} or 8$ are presented in this energy range. Absolute measurements of the apparent cross sections of 22 of these states are also given. Excitation functions for the $2P$ states are presented for low-energy and extended-energy ranges. After allowance is made for imprisonment in the $2^{1}P$ state, the cascading corrections yield the cross sections for direct electron excitation to the $2^{1}P$ and $2^{3}P$ states over the energy range from onset to 450 eV. The maximum value for the $2^{1}P$ state is 9.2×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$, and for the $2^{3}P$ state is 2.2×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$. The results show good agreement with theoretical calculations for energies greater than 100 eV, but poor agreement in the low-energy range.