Direct and resonance contributions to electron-impact excitation of n=2 to n=3 transitions in neonlike ions.

Abstract

Rate coefficients for electron-impact excitation have been calculated for transitions from the ground state to n=3 excited states of six neonlike ions with atomic numbers Z=15, 17, 26, 34, 42, and 54. The direct-excitation rate coefficients were calculated using relativistic multiconfiguration target wave functions and a relativistic distorted-wave approximation. The contributions from autoionizing resonances were evaluated using a multiconfiguration Dirac-Fock model. These resonances enhance the rate coefficients for transitions to the 2${p}^{5}$3s states by as much as a factor of 3. The peak rate coefficient and the relative importance of resonance excitation diminish with increasing atomic number. However, we find that resonance enhancements are generally quite large for electron temperatures less than the temperature where peak excitation occurs. Our total rate coefficients for ${\mathrm{Se}}^{24+}$ are in good agreement with close-coupling results obtained with R-matrix calculations. The resonance contributions from the present work differ from the results by Smith et al. [Astrophys. J. 298, 898 (1985)] for the ${\mathrm{Fe}}^{16+}$ ion by as much as a factor of 3--5.