Energy and angular distribution of electrons ejected from autoionization states in helium by electron impact

Abstract

Energy spectra of electrons ejected from autoionization states in helium by electron impact have been extensively measured for primary energies from 65 to 1000 eV and for ejection angles from 13\ifmmode^\circ\else\textdegree\fi{} to 142\ifmmode^\circ\else\textdegree\fi{}. The line shape in energy spectra undergoes a noticeable change with ejection angle as well as impact energy, reflecting the characteristics of the corresponding autoionization state. The single differential cross sections (differential in angle) for emissions of autoionized electrons have been obtained by integrating the energy spectra in the vicinity of each isolated resonance with respect to the energy after subtraction of the continuum background, for the $(2{s}^{2})^{1}S$, $(2s2p)^{3}P$, $(2{p}^{2})^{1}D$, and $(2s2p)^{1}P$ states as a function of primary energy at 33\ifmmode^\circ\else\textdegree\fi{}, 127\ifmmode^\circ\else\textdegree\fi{}, and 142\ifmmode^\circ\else\textdegree\fi{}. Profiles of the spectra due to the $(2{s}^{2})^{1}S$ state have been analyzed using Fano's formula, and the shape parameter $q$ is given as a function of ejection angle for 250-eV impact. A number of the autoionization states lying above \ensuremath{\sim}62 eV have also been measured and identified as members of the $(2sns)^{1}S$, $(2pnp)^{1}S$, $(\mathrm{sp}2n\ensuremath{-})^{3}P$, $(2pnp)^{1}D$, and $(\mathrm{sp}2n+)^{1}P$ ($n=2\ensuremath{-}5$) series, and our results are compared with previous results of theory and experiment.