Electron-impact excitation along an isoelectronic sequence: B-like ions.

Abstract

A systematic study of collision strengths for the electron-impact excitation of ions along an isoelec- tronic sequence is presented. 105 excitation transitions between 15 fine-structure levels of the eight [ital LS] terms 2[ital s][sup 2]2[ital p]([sup 2][ital P][sup [ital o]]), 2[ital s]2[ital p][sup 2]([sup 4][ital P],[sup 2][ital D],[sup 2][ital S],[sup 2][ital P]), and 2[ital p][sup 3]([sup 4][ital S][sup [ital o]],[sup 2][ital D][sup [ital o]],[sup 2][ital P][sup [ital o]]) in boronlike ions are considered. The calculations include O IV, Ne VI, Mg VIII, Al IX, Si X, S XII, Ar XIV, Ca XVI, and Fe XXII in an eight-term close-coupling expansion, using the [ital R]-matrix method [H. L. Zhang, M. Graziani, and A. K. Pradhan, Astron. Astrophys. 283, 319 (1994)]. An 11-term expansion, including some [ital n]=3 states, is also carried out for Ne VI to study the effect of the coupling to higher-[ital n] complexes. The results were compared with those with a fully relativistic distorted-wave method by Zhang and Sampson [At. Data Nucl. Data Tables 56, 41 (1994)]. The study shows that the coupling effects and the resonance effects are still pronounced for highly charged ions, even up to Fe XXII. The relativistic effects, which are incorporated into themore » [ital R]-matrix calculations through the term-coupling method [W. Eissner, M. Jones, and H. Nussbaumer, Comput. Phys. Commun. 8, 270 (1974)], begin to manifest themselves for [ital Z][ge]16. This study should provide some guidelines for future [ital R]-matrix calculations for other isoelectronic sequences.« less