Cold target recoil ion momentum spectroscopy has been used to image the momentum distributions of continuum electrons liberated in the impact of slow He 1 and He 21 ions on He. The distributions were measured for fully determined vector impact parameter. The spectra show that the electron momenta lie mainly in the collision plane and display a structure which strongly suggests that it is p states which are dominantly promoted into the continuum in such collisions, but that interfering s and perhaps d amplitudes are also important. [S0031-9007(98)07407-9]
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 He by C 6ϩ and proton projectiles at low velocities. Projectile velocities of 1.63, 1.38, and 1.16 a.u. for C 6ϩ 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 C 6ϩ 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 C 6ϩ data.
Cold-target recoil-ion-momentum spectroscopy ͑COLTRIMS͒ has been used to study single electron capture from He by Ar 8ϩ ions at projectile velocities between 0.2 and 1.0 a.u. Populations of 3d through 7l states on the final ion are resolved, and angular distributions are presented for separated major final channels. As the projectile velocity is raised, the reaction window is observed to spread. Contrary to expectations based on a Landau-Zener picture of the process, higher n and l become favored with higher v. The results are in excellent agreement with coupled-channel calculations. ͓S1050-2947͑98͒01306-7͔
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