Exact and approximate second order calculations for electron excitation of the 2s state of hydrogen

In previous works, exact second-order amplitudes for electron excitation of the 2p state of hydrogen have been reported. These exact results were then used to evaluate the errors associated with various approximations for the second-order amplitude and they were also used to evaluate the accuracy of various pseudostate basis sets for calculating second-order amplitudes. The present work extends the previous results to the excitation of the 2s state by electron impact. The 2s results are given, comparisons are made with the corresponding 2p results and potential generalisations are discussed.