Resonance contributions to the electron-impact excitation of ions in the distorted-wave approximation

In analogy with recent work on dielectronic recombination, an average-configuration distorted-wave method is developed to calculate resonance structures in the electron excitation of ions. Confirming previous results, large resonance contributions are found for the 2s→3s and 2p→3s excitations of ${\mathrm{O}}^{5+}$. Large resonance contributions are also found for the 2$p^6$→2$p^5$3s excitation of ${\mathrm{Ti}}^{12+}$. Radiative-coupling and electric-field-mixing effects are discussed.