Ionization Efficiency Curves for Ca++, Sr++ and Ba++ near Threshold by Electron Impact

Abstract

Ionization efficiency curves for Ca++, Sr++ and Ba++ near threshold have been studied with the crossed beam method. The difficulties arising from the thermal energy spread of the ionizing electrons are eliminated with the E. D. D. method which was proposed by Winters et al. The I. E. curve obtained for Ca++ has some structures above 24eV, and it increases steeply from 30eV. The shape of the curve agree well with the previous one by Kaneko and Kanomata except for gradually increasing portion above the threshold. In this portion, the present curve shows a linear rather than a second power dependence on the excess energy. The I. E. curve obtained for Sr++ has structures above 20eV, and increases steeply from 28eV. The present curve agrees with the previous one by Fayard et al. in the shape of curve but the absolute energy scale is a little shifted. The shape of the I. E. curve for Sr++ is similar to that of Ca++, and it is suggested that there are three different kinds of processes in the formation of Ca++ and Sr++ that is, direct ionization, autoionization and Auger ejection caused by ionization of 3p- or 4p- electron. The I. E. curve for Ba++ has no structures which can be related to the autoionization processes, although there exists a steep increasing from 20eV, which is attributed to Auger ejection caused by ionization of 5p- electron.