First-order exchange approximation: the excitation of the 2³S and 2³P states of helium by electron impact

Cross sections for the excitation of the 2 3 S and 2 3 P states of helium from the ground state by electron impact are calculated using the first-order exchange approximation. In the case of the excitation of the 2 3 S state it is found that at low electron energies the cross section given by this approximation is about a factor of 10 smaller than that obtained by using the Born-Oppenheimer approximation, although it is still somewhat larger than the experimental data. For the case of the excitation of the 2 3 P 0 state, the reduction in the cross section brought about by employing the first-order exchange approximation rather than the Born-Oppenheimer approximation is less marked, while the two approximations are identical for the excitation of the 2 3 P ±1 states.