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Physical Review Letters

Electron capture from circular Rydberg atoms

Søren B. Hansen, Thomas Ehrenreich, Erik Horsdal-Pedersen, Keith B. MacAdam, Louis J. Dubé

Published Sep 6, 1993• 68 citations
Publisher

Abstract

Capture cross sections for circular Rydberg states have been measured as a function of the angle, \ensuremath{\varphi}, between ion velocity and angular momentum of the circular orbital. The system studied is 2.5 keV $^{23}\mathrm{Na}^{+}$ on Li(1${\mathit{s}}^{2}$,nlm) with n=25, l=n-1, and m=+l, where m is defined relative to a weak, external magnetic field. A strong dependence on \ensuremath{\varphi} is found. It is expected that studies such as the present will lead to an improved understanding of the three-body problem in the region of sufficiently large quantum numbers and impact velocities for classical physics to be accurate.

DOI
10.1103/physrevlett.71.1522

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Capture cross sections for circular Rydberg states have been measured as a function of the angle, \ensuremath{\varphi}, between ion velocity and angular momentum of the circular orbital. The system studied is 2.5 keV 23Na+^{23}\mathrm{Na}^{+}23Na+ on Li(1s2{\mathit{s}}^{2}s2,nlm) with n=25, l=n-1, and m=+l, where m is defined relative to a weak, external magnetic field. A strong dependence on \ensuremath{\varphi} is found. It is expected that studies such as the present will lead to an improved understanding of the three-body problem in the region of sufficiently large quantum numbers and impact velocities for classical physics to be accurate.
Electron capture cross sections for coherent elliptic Rydberg states have been measured as a function of eccentricity. The ion impact velocity was parallel to the minor axis of the associated, classical elliptic orbit and both clockwise and anticlockwise orbital motion of the electron were studied. The incoming ions were 2.5 keV 23 Na + and the Rydberg target, with principal quantum number n=25, was formed by laser excitation of Li atoms with subsequent adiabatic switching of crossed, external electric and magnetic fields. A strong dependence on eccentricity and sense of orbital motion was found.
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.
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.
Projectile angular scattering is investigated following electron capture by protons from circular Rydberg atoms oriented along a quantization axis perpendicular to the incident projectile direction. We find strong left-right scattering asymmetry as a function of the orientation angle at collision speeds comparable to the electron orbital speed. The capture cross section is dramatically enhanced due to velocity matching when the angular momenta of the proton and the electron are aligned. The scattering asymmetry exhibits characteristic impact parameter dependencies suitable for studying three-body dynamics influenced by both long and short range forces of comparable strength.
The resonant mechanism of interaction of alkaline-earth atoms having a low electron affinity to Rydberg atoms in circular (l = |m| = n–1) and near-circular states has been studied. To describe the dynamics of resonant processes accompanied by nonadiabatic transitions between ionic and Rydberg covalent terms of a quasimolecule, an approach based on the integration of coupled equations for the probability amplitudes has been developed taking into account the possibility of the decay of an anion in the Coulomb field of the positive ionic core of a highly excited atom. The approach involves the specific features of the problem associated with the structure of the wavefunction of a Rydberg electron in states with high orbital angular momenta l ~ n–1. This approach provides a much more accurate description of the dynamics of electronic transitions at collisions between atoms than that within the modified semiclassical Landau–Zener model. In addition, this approach makes it possible to effectively take into account many channels of the problem. The cross sections for resonant quenching of Rydberg states of the Li(nlm) atom with given principal n, orbital l = n–1, and magnetic m quantum numbers at thermal collisions with the Ca(4s 2) and Sr(5s 2) atoms have been calculated. The dependences of the results on n, m, and angle α between the relative velocity of the atoms and the normal to the plane of the orbit of the Rydberg electron have been obtained. The influence of orientational effects on the efficiency of the collisional destruction of circular and near-circular states has been studied. The results indicate a higher stability of such states to their perturbations by neutral particles as compared to usually studied nl states with low values of l (l ≪ n).
Circular atoms, i.e., Rydberg atoms with maximum angular momentum, are semiclassical atoms of great interest for Rydberg constant metrology and QED tests. Following a suggestion by Delande and Gay, we have prepared circular atoms by a new method, involving the adiabatic switching of crossed electric and magnetic fields. The method, which is demonstrated for Rydberg states of lithium (principal quantum number n=20to25n=20 \mathrm{to} 25n=20to25, is applicable over a wide variety of experiment conditions.