The 21S and 21P excitations of helium by proton and electron impact

Abstract

The 2 1S and 2 1P excitations of helium by electron and proton impact are considered by use of Born, two-state and four-state versions of the impact-parameter treatment. The cross sections so obtained are modified by a transformation that takes account (in a plane-wave approximation) of the maximum and minimum momentum changes that occur during the collision. Agreement with the accurate Born values of Bell et al. (1968- 9) is found, while comparison with the proton-impact cross sections of Van den Bos (1969) suggests that the 2s-2p dipole coupling is more sensitive to the choice of the 2 1S helium wave function than is the Born 1s-2s short-range coupling. The inclusion of wavelength distortion, back-couplings, 2p-2s couplings and rotational coupling between the magnetic substates affects the proton-impact excitations more than it does the electron-impact excitations.