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Physical Review A

Electron capture in Ar+ -ion collisions with laser-aligned Rydberg atoms

Th. Wörmann, Zvjezdana Roller-Lutz, Hans O. Lutz

Published Mar 1, 1993• 29 citations
Publisher

Abstract

We report a study of charge transfer from laser-aligned Rydberg atoms in collisions with ${\mathrm{Ar}}^{+}$ 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.

DOI
10.1103/physreva.47.r1594

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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.
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.
Charge-transfer collisions between slow multiply charged ions and highly excited Rydberg atoms have been studied to determine the details of the internal energy transfer accompanying charge exchange. The experiment uses lasers to define uniquely the binding energies of both initial and final states of the collision, thereby eliminating the ambiguity inherent in the selective field ionization method used for previous studies. The results clearly characterize the capture energetics over the range of ion charge q=1−−11q=1--11q=1−−11 and velocity v=0.031−−0.138a.u.v=0.031--0.138\mathrm{a}.\mathrm{u}.v=0.031−−0.138a.u. The measurements are in very good agreement with predictions of the classical trajectory Monte Carlo method, but not with predictions of other classical models.
Charge transfer collisions between slow ions and Rydberg atoms have been studied for ion charges q=1−−4q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1--4q=1−−4, using resonant laser excitation to detect specific energy states of the collision products. The data collected show a clear resonance in the capture cross section to a particular energy state as the binding energy of the Rydberg target is varied. The resonance position differs significantly from the predictions of the classical overbarrier model, and its width becomes very small (0.025 eV) at the lowest velocities studied.
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.