Electron excitation of the2sand2pstates of hydrogen from threshold to 50 eV

The $2s$ and $2p$ excitation cross sections for electron scattering on hydrogen in the energy range from threshold to 50 eV are calculated by two similar methods. The distorted-wave method using Hartree-Fock continuum wave functions in the frozen-core approximation is applied. The effect of calculating the distorted waves in the potential of the initial or final state is investigated. The imaginary part of the optical potential, or self-energy of the single-particle Green's function, is also calculated in lowest order by the use of many-body diagrams. Comparison is made with recent experimental results.