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Physical Review A

Anomalous final-state distributions of electrons captured from directed Rydberg states

Dean M. Homan, Keith B. MacAdam, M. J. Cavagnero

Published Jan 1, 1998• 12 citations
Publisher

Abstract

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 ${E}_{i},$ but has a second peak at energies roughly ${E}_{i}/4.$ 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.

DOI
10.1103/physreva.57.r13

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Energy Transfer in Charge Exchange Collisions between Slow Ions and Rydberg Atoms

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Charge transfer collisions between slow ions and Rydberg atoms have been studied for ion charges $q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1--4$, 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.

Physical Review Letters•Aug 31, 1998•11 citations
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 v\ifmmode~\else˜\fi=vion/vBohr=0.45,\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}{=v}_{\mathrm{ion}}{/v}_{\mathrm{Bohr}}=0.45,v\ifmmode~\else˜\fi=vion​/vBohr​=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.
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 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.
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.
Thermal collisions between potassium Rydberg atoms with intermediate values of quantum number n and target molecules that capture free low-energy electrons are investigated. The velocity, binding energy, and angular distributions of heavy-Rydberg ion-pair states formed through such collisions are measured. The experimental data are analyzed with the aid of semiclassical Monte Carlo simulations to investigate the dynamics of electron attachment to a variety of target species. Multiple reaction channels involving short- and long-lived intermediates are seen for electron capture by 1,1,1-C2Cl3F3 and CBrCl3 (where dissociation of CBrCl3 intermediates involves production of Br- and Cl- ions). Electron attachment to BrCN produces long-lived BrCN-* intermediates whose dissociation results in the creation of rotationally-hot CN- anions. Electron capture by Fe(CO)5 leads to formation of Fe(CO)5-* intermediates which can fragment producing Fe(CO)4- anions, either directly or after partial redistribution of the excess energy within the intermediate has occurred. Electron transfer in Rydberg atom collisions with C2Cl4 provide no evidence of formation of long-lived C2Cl4 ions, only Cl- ions are seen. Moreover, strong Rydberg atom scattering is observed following collisions of K(12p) Rydberg states with CH3NO2. This finding is consistent with a theory which predicts that collisions between Rydberg atoms with intermediate values of n and polar molecules can lead to strong resonant quenching of Rydberg states, i.e., n and l changing, through formation of short-lived K+...CH3NO2- ion-pair states. Finally, the improvements in sensitivity to electron capture reaction dynamics that might be achieved using different Rydberg species and a crossed molecular beam are examined. Simulations are performed to demonstrate that the use of a hyperthermal helium Rydberg atomic beam and a crossed beam of target molecules such as CCl4, BrCN and CF3I can provide new insights into dissociative electron attachment reactions. The calculations show that not only can much detailed information on translational energy release distributions and intermediate lifetimes be obtained, but also on the electron radial probability density distributions in the Rydberg atoms themselves.
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.