Differential cross sections for the electron-impact excitation of He-like ions: 2S1and 2P1states

Differential cross sections for the electron-impact excitation of ions are calculated and compared along the He-like isoelectronic sequence. A distorted-wave method is used to obtain the cross sections for the transitions 1 ${}_{}{}^1{}_{}{}^{}2$ ${}_{}{}^1{}_{}{}^{}S$ or 2 ${}_{}{}^1{}_{}{}^{}P$ in ${\mathrm{Li}}^{+}$, ${\mathrm{O}}^{6+}$, and ${\mathrm{Si}}^{12+}$. To complete the systematic study, a comparison is also made to the cross sections for neutral He and in the limit Z→∞ (Z being the nuclear charge). Dependence on the collision energy and comparisons to the cross sections obtained by other methods are discussed in detail. For the 2 ${}_{}{}^1{}_{}{}^{}P$ excitation, an apparent generalized oscillator strength is derived for the ions using the present differential cross section. Their behavior is very different from the corresponding values of He.