Distorted-wave-method calculation of electron-impact excitation of atomic ions: He- and Be-like ions

A distorted-wave method is applied to electron-impact excitations of He-like (Li i i and O v i i) and Be-like (C i i i) ions. In the method, the same distortion potential is assumed for both the initial and the final states, and electron exchange is taken into account only between interacting electrons. The collision strengths are calculated for the excitations of 2 ${}^1$P, 2 ${}^1$S, 2 ${}^3$P, and 2 ${}^3$S states of the He-like ions and 2s2p ${}^1$P and 2s2p ${}^3$P states of the Be-like ion, and compared with the results of more elaborate calculations (e.g., the close-coupling-type ones). Good agreement is found in most cases if use is made of the target wave functions of the same or comparable accuracy. Also examined is the Coulomb-wave approximation, in which the distorted wave is replaced by the corresponding Coulomb wave.