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Physical Review A

Extension of classical trajectory Monte Carlo calculations to ion–Rydberg-atom collisionsin a magnetic field

S Bradenbrink, Emil Y. Sidky, Zvjezdana Roller-Lutz, H Reihl, Hans O. Lutz

Published Jun 1, 1997• 14 citations
Publisher

Abstract

Theoretical electron transfer cross sections for 1.3--130 eV/amu singly charged ions colliding with Rydberg atoms (n=28, m=2) in a magnetic field up to 4 T are presented. Both paramagnetic and diamagnetic interactions are taken into account for the initial state and the collision. Cross sections are calculated within the classical trajectory Monte Carlo model. A method to create a classical ensemble of electrons in a nonseparable Hamiltonian allows the initial quantum stationary states to be modeled. The effect of the magnetic field on electron capture is analyzed in terms of the field-induced alteration of the initial state, as well as the direct influence on the collision dynamics.

DOI
10.1103/physreva.55.4290

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Theoretical electron transfer cross sections for 1.3--130 eV/amu singly charged ions colliding with Rydberg atoms (n=28, m=2) in a magnetic field up to 4 T are presented. Both paramagnetic and diamagnetic interactions are taken into account for the initial state and the collision. Cross sections are calculated within the classical trajectory Monte Carlo model. A method to create a classical ensemble of electrons in a nonseparable Hamiltonian allows the initial quantum stationary states to be modeled. The effect of the magnetic field on electron capture is analyzed in terms of the field-induced alteration of the initial state, as well as the direct influence on the collision dynamics.
Within the classical trajectory Monte Carlo (CTMC) model, we calculate electron transfer and ionization cross sections for singly charged ions colliding with Rydberg atoms in the presence of a laboratory-strength magnetic field of 4 tesla. A new method for generating a stationary microcanonical ensemble for a quasi-integrable initial-state Hamiltonian is presented. The calculated cross sections show signatures of electron capture and ionization mechanisms for the field-free case, e.g. multiple swaps and saddle-point electrons; their structure as well as their magnitude, however, are strongly modified by the presence of the magnetic field.
With an extended version of the classical trajectory Monte Carlo method to take into account the quasi-integrable initial state, we calculate the ionization cross section for singly charged ions colliding with Rydberg atoms in the presence of a magnetic field of 4 Tesla. Strong evidence for long-lived quasi-bound states, created and located around three centres (target nucleus, projectile nucleus and potential saddle between the two) is found which are caused by the trapping and multiple scattering of emitted electrons in the external fields.
The aim of this work is to investigate the dynamics of collisions between ions and Rydberg atoms by using the Classical Trajectory Monte Carlo (CTMC) method. The study is started by collisions with oriented elliptical Rydberg atom, is then continued to collisions involving hydrogen Rydberg target in an external magnetic field in the l-mixing regime, and later arrives at antihydrogen formation in an external magnetic field where Rydberg positronium is taken as target.\nThe CTMC method has been applied in the study. For different targets, the CTMC method had to be adjusted and reconstructed to properly create the initial target state for the quasi-separable one-body initial system (hydrogen target) and the two-body initial system (positronium target). Charge exchange and ionization cross sections are calculated under special consideration of different initial target states.\nThe velocity matching phenomenon and Thomas capture which appears in the upstream-downstream asymmetry are found in ion-oriented elliptical Rydberg atom collisions, reflecting the important roles of the initial electron spatial and momentum distributions. By increasing the projectile charge, the distortion of the initial states caused by the strong perturbation of approaching projectile induces pronounced changes in the momentum distribution.\nIn the collisions involving hydrogen Rydberg atoms in a magnetic field, a cross section reduction in case of an increasing magnetic field is found for multiply charged projectiles. The structure effect due to the influence of the magnetic field on the initial state distribution results in different capture cross sections for two characterized, kmax and kmin, states. The velocity matching phenomenon as well as the effect of multiply charged projectile ion are observed in the collisions.\nFurthermore, antihydrogen formation in a magnetic field has been studied. Rydberg positroniums are assumed to be the target atoms, colliding with antiprotons. The initial target positronium state is carefully constructed by means of a newly introduced conserved operator, the pseudomomentum K. Similar properties as in the previous hydrogen target atom case were investigated. It is found that the existence of the magnetic field changes the spatial distribution of positronium to irregular motion. By increasing the magnetic field, it induces a decrease of the resulting capture cross sections (i.e. the antihydrogen formation). Thomas capture which happens in the collision plane in the field-free case is foiled. A smaller positronium binding energy and a larger geometrical extension of the target positronium atom yield a larger capture cross section.
We report a study of charge transfer from laser-aligned Rydberg atoms in collisions with Ar+{\mathrm{Ar}}^{+}Ar+ ions in the velocity-matching regime. The Rydberg atoms in well-defined magnetic substates were prepared by two-photon laser optical pumping in a weak magnetic field. The results are compared with classical-trajectory Monte Carlo calculations and can also be understood in terms of a simple and ``intuitive'' picture.
The classical trajectory Monte Carlo method has been used to simulate fully stripped ion-circular Rydberg atom collisions. We report that the mechanisms that govern the capture process for ions colliding with aligned Rydberg targets, namely direct and indirect velocity matching, alternate as to which is dominant. Previous experimental and theoretical results for singly charged ion-Rydberg atom collisions demonstrate that direct capture processes dominate at intermediate collision speeds. We have found that for highly charged ion-Rydberg atom collisions electron capture via indirect velocity matching is dominant at intermediate collision speeds and contributes up to 90% of the total capture cross-section.
Low-energy collisions of Rydberg atom-ion systems are investigated theoretically. We present the parameter space associated with suitable approaches for the dynamics of Rydberg atom-ion collisions, i.e., quantum, Langevin, and classical exchange regimes, showing that for the lowest reachable temperatures a classical treatment is appropriate. A quasiclassical trajectory method is used to study charge exchange cross sections for ${\mathrm{Li}}^{*}\text{\ensuremath{-}}{\mathrm{Li}}^{+}$ and ${\mathrm{Li}}^{*}\text{\ensuremath{-}}{\mathrm{Cs}}^{+}$ at collision energies down to 1K1\phantom{\rule{0.28em}{0ex}}\mathrm{K}1K. For cold collisions we find the charge exchange cross section deviating from the n4{n}^{4}n4 geometric scaling. Furthermore, for low-energy collisions, we find both an influence of the ionic core repulsion as well as variations for two different models used for describing the electron-core potential.