Relativistic distorted-wave cross sections for electron-impact excitations of berylliumlike ions

Abstract

Relativistic distorted-wave Born (RDWB) cross sections of Be-like ions for excitations from the ground state 2$s^2$ ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ to the 2s2p ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ states by electron impact are reported for ${\mathrm{Ne}}^{6+}$ through ${\mathrm{U}}^{88+}$. Multiconfiguration Dirac-Fock wave functions were used to describe the target-ion states, and the relativistic continuum wave functions were calculated in the field of (frozen) target-ion charge distribution with the configuration-average exchange potential. We found, however, that cross sections hardly changed even if the exchange terms were omitted. In contrast, adding the 2$p^2$ configuration to the ground-state wave function reduced cross sections by 10–60 %. RDWB cross sections were calculated from thresholds to T=10 keV, where T is the incident electron energy. For Z≃25 and above, where Z is the nuclear charge, intermediate coupling mixes the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ states such that the transition to the former becomes practically spin- and electric-dipole allowed, changing the high-T behavior of the excitation cross section. Cross sections are presented in compact fitting formulas that allow interpolations to determine cross sections with high precision for arbitrary Be-like ions (10≤Z≤92) and arbitrary incident energies (T≤10 keV).