Abstract
Second-order effects for the problem of electron excitation of the 2p state of hydrogen are examined. Some different types of second-order amplitudes which result from the distorted-wave series and the plane-wave series are discussed and fundamental amplitudes of each type are evaluated exactly. The effect of using different Green's functions as well as the effect of distortion in both the first- and second-order terms is analysed. The validity of the closure approximation for both the second-order Born approximation and the second-order distorted-wave approximation is examined through comparison with equivalent exact results. The accuracy of the local-potential approximation is discussed.