Application of the second Born approximation to the 3s-4s excitation of sodium by electron impact

Differential and integrated cross sections for 3s-4s excitation of sodium by electrons are given in an energy range from 25-500 eV, using third order approximations. The 3s, 3p and 4s intermediate states are included in the calculation. Large deviations from the first Born approximation are found, caused mainly by the 3p state (polarization). Above 500 eV first Born and third order results are in fair agreement. Third order results compare well with some experimental data, while first Born results are much too low.