On distorted-wave approximations for excitation

A comparative study of several distorted-wave approximations for s-s excitation in electron-atom collisions is presented. in particular, it is shown that two methods due to Bransden and Winters (J. Phys. B.; 8:1236 (1975)) are extensions of the well known distorted-wave first and second Born approximations (DWB1 and DWB2). The discussion is illustrated with the results of several distorted-wave calculations for the direct excitation of the 2s state of hydrogen by 100 and 300 eV electrons. These include the DWB2 approximation, for which a straightforward method of calculation is proposed, and its extension by Bransden and Winters. The results demonstrate the importance of distortion in both initial and final states. It is also found that higher-order corrections to the distorted waves in the DWB1 approximation have a small effect compared with that arising from the distorted-wave second Born term. (author)