Abstract
The orientation ((Ly)) and alignment angle ( gamma ) of the excited 2p state of hydrogen-like positive ions by high-energy electron and positron impact are calculated in the Coulomb-Born approximation. The angular dependence of (Ly) and gamma is found to be qualitatively similar to the case of e+or- collisional excitation of the helium atom. The authors also analyse the shape and rotation of the excited state in terms of the dipole and velocity vectors which result from the coherence between the 2s and 2p levels. The dependence of (Ly) and gamma on the projectile velocity and target nuclear charge Z is also investigated. The results reveal that the classical grazing model, which interprets positive (negative) (Ly) in terms of an attractive (repulsive) force between the projectile and the target, is not valid in general. The authors also report the orientation and alignment angle for the 3d state of He+ excited by electron and positron impact.
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