Excitation of Optically Forbidden States in the Ionization Continuum by Electron Impact

Abstract

The inelastic-electron-scattering resonances in the ionization continuum of He, Ne, and Ar were recorded as a function of primary energy. The energy resolution was ∼0.1 eV, the angular resolution $\Delta \theta$ was ∼${10}^{-2\mathrm{}}$ rad about the forward direction, and the primary energy was between 60 and 400 eV. The strength of resonances due to optically forbidden transitions from the ground state compared with those due to optically allowed transitions increases as the energy is reduced, as was expected. A marked difference between ${}_{}{}^1{}_{}{}^{}D$ and ${}_{}{}^1{}_{}{}^{}S$ behavior exists, and certain resonances, notably that due to the $\left(3s3\mathrm{}{p}^{6}4p\right){}_{}{}^1{}_{}{}^{}P$ state in Ar, change shape as the energy is reduced. At the highest energies used the resonances due to optically forbidden transitions were still apparent.