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Physical Review A

Energy transfer in ion–Rydberg-atom charge exchange

D. S. Fisher, S. R. Lundeen, C. W. Fehrenbach, Brett D DePaola

Published Apr 17, 2001• 12 citations
Publisher

Abstract

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 $q=1--11$ and velocity $v=0.031--0.138\mathrm{a}.\mathrm{u}.$ The measurements are in very good agreement with predictions of the classical trajectory Monte Carlo method, but not with predictions of other classical models.

DOI
10.1103/physreva.63.052712

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•Feb 27, 2015•0 citations
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.
The dynamics of formation of heavy-Rydberg ion-pair states through electron transfer in K(np)-SF6, CCl4 collisions is examined by measuring the velocity, angular, and binding energy distributions of the product ion pairs. The results are analyzed with the aid of a Monte Carlo collision code that models both the initial electron capture and the subsequent evolution of the ion pairs. The model simulations are in good agreement with the experimental data and highlight the factors such as Rydberg atom size, the kinetic energy of relative motion of the Rydberg atom and target particle, and (in the case of attaching targets that dissociate) the energetics of dissociation that can be used to control the properties of the product ion-pair states.
New experimental results have been obtained in collisions of singly charged ions with Na Rydberg atoms. (a) The distribution of final n states populated by electron capture shows non-monotonic variations of width with incident energy over the range 60-2100 eV. (b) The total capture cross section for alkali ions on Rydberg targets shows peaks and dips over the same energy range, correlated with particular reduced velocities v and the polarization of the exciting laser. (c) {ell}-Changing collision of Na{sup +} on Na(nd) targets, n=26 and 28, at low v preferentially transfer multiple units {h_bar} of orbital angular momentum under single-collision conditions, despite very large impact parameters. (d) Inelastic impacts of Na{sup f+} on Na 36s and 30p states, perturbed by electric fields, populate a strongly field-dependent mixture of nearby Stark levels. A synopsis of recent progress will be given.
Charge transfer from high‐n atomic targets populates comparably excited states of ionic projectiles. This paper reviews a decade of accelerating experimental and theoretical progress in the field, which offers an unusual variety of challenges in the preparation of initial states and in the analysis of final states. Interesting dynamical questions at the interface of classical and quantum mechanics are posed by new experimental results. Rydberg atom phenomena are increasingly found to be important for other areas of atomic collision physics in all energy regimes.
Heavy-Rydberg ion-pair states, molecular quantum systems that are bound by their long-range pure Coulomb attraction, are formed through Rydberg atom collisions with molecules that attach low energy electrons. Collisions of potassium Rydberg atoms K(np) with the attaching species CCl4 and SF6 lead, respectively, to dissociative and non-dissociative electron attachment with subsequent formation of bound ion-pair states K+..Cl- and K+..SF6-.\nThe experimental apparatus used to produce and detect heavy-Rydberg ion-pair states consists of a reaction region and a separate analysis region. It separates a fraction of the product ion pairs from the parent Rydberg atoms, eliminating spurious background signals associated with blackbody-induced and electric-field ionization of Rydberg atoms. A position sensitive detector in the analysis region is employed to measure their velocity, angular, and binding energy distribution and lifetimes. Measurements of the spatial distribution of ion pairs provide information on their velocity and angular distributions, and their binding energy distribution is measured using electric field-induced dissociation. The lifetime of the ion-pair states is influenced by multiple processes including internal-to-translational energy transfer, autodetachment of the electron, and neutralization through charge transfer.\nThe experimental results are compared with the results of a Monte Carlo collision code that models both the initial Rydberg electron attachment and the subsequent evolution and molecular dynamics of the ion pairs. This model highlights the factors such as the kinematics of the Rydberg atom and the attaching particle and energy released in dissociation (in the case of dissociative attachment) that are important in governing ion pair formation. The model calculations are in good agreement with the experimental data.