Low-energy total and differential cross sections for the electron-impact excitation ofSi2+andAr6+

The total and angular differential excitation cross sections for the transitions 3${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$S→3s3p ${}_{}{}^3{}_{}{}^{}$P, 3${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$S→3s3p ${}_{}{}^1{}_{}{}^{}$P, and 3s3p ${}_{}{}^3{}_{}{}^{}$P→3s3p ${}_{}{}^1{}_{}{}^{}$P in the Mg-like ions ${\mathrm{Si}}^{2+}$ and ${\mathrm{Ar}}^{6+}$ are determined in the threshold energy region using the close-coupling R-matrix method. The total cross sections for all three transitions in both ions exhibit strong resonance structures near threshold. We focus our analysis on the angular distribution of the scattered electrons. The differential cross sections are found to undergo rapid and strong variations when the incident energy is tuned through the autoionizing resonance transitions. In ${\mathrm{Ar}}^{6+}$, we also examine the angular distribution of the scattered electrons associated with the nonresonant background cross sections for the two transitions from the ground state. For the dipole-allowed transition, the electrons are preferentially backscattered near the excitation threshold; however, the fraction of electrons scattered in the forward direction increases dramatically with energy. The corresponding behavior for the spin-forbidden transition is quite different. We discuss the implications of these results to possible excitation measurements in which the scattered electrons are collected.