Absolute Measurements of the21Pand23PElectron Excitation Cross Sections of Helium Atoms

Abstract

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 $n=3$ to $n=7 or 8$ 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\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$, and for the ${2}^{3}P$ state is 2.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$. The results show good agreement with theoretical calculations for energies greater than 100 eV, but poor agreement in the low-energy range.