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Bulletin of the American Physical Society

Slow collisions of ions with Rydberg atoms

Keith B. MacAdam

Published Dec 1, 1992• 0 citations
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Abstract

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.

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Angular-momentum distributions following impact of slow ions on Na(nd) Rydberg atoms

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Physical Review A•Dec 1, 1998•11 citations
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.
The degree of dipole-forbidden l change in Na(26d) and Na(28d) Rydberg atoms caused by slow Na+{\mathrm{Na}}^{+}Na+ ion impact at reduced velocity v\ifmmode \tilde{}\else \~{}\fi{}=0.1--0.9 has been measured. Na+{\mathrm{Na}}^{+}Na+ beam energies 7.3--684 eV were used. Previous measurements were greatly improved by reducing stray electric fields in the Rydberg collision region to about 10 mV/cm. The distributions of final-angular-momentum states were analyzed using directly measured diabatic selective-field-ionization profiles for l=3, 4, and 5. l-change results in Na(26d) and Na(28d) targets are very similar and clearly show an increasing amount of dipole-forbidden behavior for decreasing reduced velocities v\ifmmode \tilde{}\else \~{}\fi{} and partial agreement with the coupled-channel calculation of Beigman and Syrkin (Zh. Eksp. Teor. Fiz. 89, 400 (1985) [Sov. Phys. JETP 62, 226 (1985)]). The dipole-forbidden process is the main channel for angular-momentum change in ion--Rydberg-atom collisions at v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.7. At very low energy (v\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\le}0.3) the measurements demonstrate that the final-state distributions are dominated by very high angular-momentum states.
An experimental study of the depopulation and final-state distribution of Na 36s and 30p Rydberg states under Na + bombardment in the presence of electric fields up to 50 V cm -1 has been carried out. Incident energies E from 171 to 967 eV span the range from 0.6 to. 1.4 times the mean orbital speed of the excited electron in the initial state. Absolute depopulation cross sections of order 10 -8 cm 2 , believed accurate within 25%, are reported as functions of field and energy. sigma 36s is proportional to E -1 , but sigma 30p is nearly independent of energy. The distribution of final states was measured by selective field ionization. At weak fields for which the initial state is well separated from the nearest Stark manifold, final states are broadly distributed across the nearest manifolds. But for stronger fields that shift the initial state close to a Stark manifold the final states are strongly concentrated among the energetically closest manifold sublevels.
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.
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.
A classical model of singly charged ion collisions with Rydberg atoms in linear Stark states directed toward the oncoming ion beam predicts a bimodal distribution of final states resulting from electron capture. At a reduced velocity $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}{=v}_{\mathrm{ion}}{/v}_{\mathrm{Bohr}}=0.45,$ the calculated energy distribution of captured electrons peaks near the target binding energy Ei,{E}_{i},Ei​, but has a second peak at energies roughly Ei/4.{E}_{i}/4.Ei​/4. Analysis of the impact-parameter dependence of the capture probabilities indicates that the second peak arises from three-swap capture events. The decrease in binding energy of the captured electrons is attributed to excitation via united-atom rotational coupling.
Electron capture by Cs+,{\mathrm{Cs}}^{+},Cs+, K+,{\mathrm{K}}^{+},K+, or Li+{\mathrm{Li}}^{+}Li+ from a Na\mathrm{Na}Na Rydberg target in an extreme Stark state of n=24,m=0n=24, m=0n=24,m=0 has been measured and compared with both a classical and a quantal calculation as a function of the reduced velocity $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=0.15\ensuremath{-}1.6.$ Peaks and shoulders are found at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=0.25,$ 0.5, and 1.0 in the ratio of the measured capture cross sections for electronic charge polarization antiparallel and parallel to the ion-beam direction. A close-coupling calculation reveals similar features in the total capture cross sections near those same scaled velocities. A classical trajectory calculation attributes this structure to swapping of the electron between the two positive charge centers as they pass one another during the collision. The total classical capture cross section of a Rydberg atom having an upstream-directed charge distribution is shown to be the sum of five-and-greater-swap, three-swap, and one-swap contributions, which result in the structure near $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=0.25, 0.5,$ and 1.0, respectively.