Classical trajectory Monte Carlo calculations of electron capture and ionization in collisions of multiply charged ions with elliptical Rydberg atoms

Abstract

We have performed classical trajectory Monte Carlo studies of electron capture and ionization in multiply charged (Qр8) ion-Rydberg-atom collisions at intermediate impact velocities. Impact parallel to the minor and to the major axis, respectively, of the initial Kepler electron ellipse has been investigated. The important role of the initial electron momentum distribution found for singly charged ion impact is strongly diminished for higher projectile charge, while the initial spatial distribution remains important for all values of Q studied. PACS number͑s͒: 34.60.ϩz, 34.70.ϩe The use of coherent elliptical Rydberg states in ion-atom collision studies ͑for recent papers on the subject cf. ͓1-3͔ and references therein͒ has not only aided the intuitive understanding of the interaction dynamics, it also illuminates the roles of the momentum and the spatial distributions of the target electron states. In classical terms, the momentum distribution can be widely varied simply by changing the eccentricity of the Rydberg ellipse without affecting the energy of the state. In particular, for impact perpendicular to the major axis of the ellipse, the capture cross section displays a maximum if v p ͑the perihelion electron velocity͒ is parallel and equal to the projectile velocity v; this is believed to be due to the matching electron momenta in the initial target and the final projectile state. In contrast, the role of the spatial distribution becomes most clearly visible if the impact velocity vector is adjusted perpendicular to the minor axis of the Rydberg ellipse; in this case, the electrons can be located either between the approaching ion and the target nucleus ͑''upstream geometry''͒ or behind the target nucleus, as seen from the projectile ͑''downstream geometry''͒ without otherwise changing the momentum distribution of the Rydberg state ͑i.e., its angular momentum l and the principal quantum number n). The capture cross section in both cases turns out to be quite different: it is much larger in the upstream case as compared to the downstream case; apparently, in the corresponding region of parameter space the spatial characteristics of the initial state determine the outcome of the collision. These investigations have so far been restricted to collisions with singly charged ions. Recently, however, it has become possible to employ such targets in studies involving multiply charged ions ͓1,4͔. In another context ͑electron capture by multiply charged ions in the presence of an external magnetic field͒ we have found indications ͓5͔ that for increasing projectile charge Q the distortion of the initial state increasingly dominates the influence of different target electron distributions. We have therefore performed an exploratory study of such systems in which a spatially oriented Rydberg atom collides with a multiply charged ion. This is the topic of this paper.We employ the classical trajectory Monte Carlo ͑CTMC͒ method, which is quite useful in particular for the description of quantum-mechanically complex syste...