Single ionization of He by low-velocity protons and C6+: Ejected electron momentum distributions

A technique for electron spectroscopy which yields full two-dimensional momentum distributions for continuum electrons has been used to study ejected electrons from single ionization of lie by C6+ and proton projectiles at low velocities. Projectile velocities of 1.63, 1.38, and 1.16 a.u, for C6+ and 2.39, 1.71, 1.15, .85, and 0.63 a.u. for protons were used. All spectra show much broader distributions along the beam than transverse to the beam. For the case of proton bombardment, the spectra are strongly influenced by both target and projectile potentials, maximizing near the velocity of the saddle in the potential between the two, receding ion cores for the lowest projectile velocities. For C6+ projectiles, the spectra appear to be dominated by the projectile potential and the center of the distribution is strongly shifted toward the projectile velocity. Theoretical results from the continuum-distorted-wave-eikonal-initial state and classical-trajectory-Monte Carlo methods are in rather good agreement with the proton data but do not agree well with the C6+ data.