The impulse approximation for excitation of atomic hydrogen

Cross sections for the process H + +H(1s) -> H + +H(2s) have been calculated in the quantal impulse approximation for proton energies in the range 1-500 keV. It is shown that a peaking approximation previously used in the evaluation of impulse approximation matrix elements is invalid and greatly underestimates the true impulse approximation cross section for this process.