The excitation of Li(3s) by electron impact

A comparative study of the excitation of Li(3s) by electron impact in several distorted-wave approximations is made. These include the distorted-wave first and second Born approximations with both first- and second-order distortion in initial and final states. Second-order effects representing both target and projectile distortion are found to give a large correction to the first-order DWBA results. The predictions of the authors' full second-order model are compared with absolute measurements of the differential cross section in the energy range 10-60 eV. Calculations have also been made in the plane-wave second Born approximation, and the results show the failure of a perturbative approach based on the plane-wave Born series at intermediate energies.