Highly‐excited and oriented Rydberg atoms of varying angular momenta are being used to study atomic interactions. The ‘‘exotic’’ semiclassical properties of these atoms present an opportunity for fine control of dynamic pathways. We report initial application of oriented Rydberg atoms to collision studies of electron capture by protons and positrons at intermediate collision speeds. Several novel features have been revealed, such as the double‐peak structures predicted in the angular scattering of projectiles under certain orientations. These structures are traced to the Thomas two‐step capture mechanism.
Highly‐excited and oriented Rydberg atoms of varying angular momenta are being used to study atomic interactions. The ‘‘exotic’’ semiclassical properties of these atoms present an opportunity for fine control of dynamic pathways. We report initial application of oriented Rydberg atoms to collision studies of electron capture by protons and positrons at intermediate collision speeds. Several novel features have been revealed, such as the double‐peak structures predicted in the angular scattering of projectiles under certain orientations. These structures are traced to the Thomas two‐step capture mechanism.
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.
We present a brief review of recent progress in the transfer of an electron from a circular Rydberg atom to an impinging ion. This field has attracted considerable interest since the "exotic" semiclassical properties of these atoms afford an opportunity to study mass transfer in a regime where dynamic pathways can be well controlled. We report developments in the past two years in electron transfer between oriented circular Rydberg atoms and protons at collision speeds comparable to the electron orbital speed. Newly discovered results have solved a number of outstanding puzzles in the classical mass transfer theory, such as the Thomas two-step mechanism. Also discussed is the application of circular Rydberg atoms as semiclassical objects in a number of very different areas ranging from atomic physics, beam-foil interactions, to astrophysics.
The aim of this work is to investigate the dynamics of collisions between ions and Rydberg atoms by using the Classical Trajectory Monte Carlo (CTMC) method. The study is started by collisions with oriented elliptical Rydberg atom, is then continued to collisions involving hydrogen Rydberg target in an external magnetic field in the l-mixing regime, and later arrives at antihydrogen formation in an external magnetic field where Rydberg positronium is taken as target.\nThe CTMC method has been applied in the study. For different targets, the CTMC method had to be adjusted and reconstructed to properly create the initial target state for the quasi-separable one-body initial system (hydrogen target) and the two-body initial system (positronium target). Charge exchange and ionization cross sections are calculated under special consideration of different initial target states.\nThe velocity matching phenomenon and Thomas capture which appears in the upstream-downstream asymmetry are found in ion-oriented elliptical Rydberg atom collisions, reflecting the important roles of the initial electron spatial and momentum distributions. By increasing the projectile charge, the distortion of the initial states caused by the strong perturbation of approaching projectile induces pronounced changes in the momentum distribution.\nIn the collisions involving hydrogen Rydberg atoms in a magnetic field, a cross section reduction in case of an increasing magnetic field is found for multiply charged projectiles. The structure effect due to the influence of the magnetic field on the initial state distribution results in different capture cross sections for two characterized, kmax and kmin, states. The velocity matching phenomenon as well as the effect of multiply charged projectile ion are observed in the collisions.\nFurthermore, antihydrogen formation in a magnetic field has been studied. Rydberg positroniums are assumed to be the target atoms, colliding with antiprotons. The initial target positronium state is carefully constructed by means of a newly introduced conserved operator, the pseudomomentum K. Similar properties as in the previous hydrogen target atom case were investigated. It is found that the existence of the magnetic field changes the spatial distribution of positronium to irregular motion. By increasing the magnetic field, it induces a decrease of the resulting capture cross sections (i.e. the antihydrogen formation). Thomas capture which happens in the collision plane in the field-free case is foiled. A smaller positronium binding energy and a larger geometrical extension of the target positronium atom yield a larger capture cross section.
Scattering of K(np) Rydberg atoms by electron-attaching molecules can result in the formation of positive-negative ion pairs through electron transfer reactions. These reactions can involve the dissociation of a transient intermediate negative ion into a smaller negative ion and a neutral fragment, or they can result in the parent molecule forming a metastable negative ion, with the excess energy being distributed among its internal modes. For Rydberg atoms of intermediate n, it is possible for these ion pairs to orbit each other at large separation, weakly bound by their mutual electrostatic attraction, forming what are known as heavy-Rydberg states. Measurements of the binding energy and velocity distributions of the ion pairs are used to probe the dynamics of electron transfer reactions. The data are analyzed using a semiclassical Monte Carlo collision code to explore the lifetime of the intermediate, the nature of its dissociation, the distribution of the excess energy of reaction, and the branching ratio of the various reaction channels. Results obtained using a variety of attaching targets including CF3I, CCl4, SF6, CH2Br2, 1,1,1-C2Cl3F3, CCl3Br, Fe(CO)5, and CH3NO2 have demonstrated both the wide variety of different behaviors that can accompany electron capture as well as the different behaviors of the product ion-pair states themselves. Remarkably, despite previous work showing that it was possible for CH3NO2 to form long-lived valence-bound negative ions through Rydberg electron transfer, the present work showed that such collisions do not result in the formation of long-lived bound ion-pair states. Rather, the data show that collisions lead to very strong Rydberg atom scattering through formation of transient ion-pair states. These results are consistent with theoretical work that predicts significant collisional quenching of Rydberg atoms by highly polar molecules and highlight a new mechanism for Rydberg atom scattering.
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.
The intercombination transitions between l-resolved Rydberg levels due to collisions with fast electrons are studied within the Ochkur approximation for principal quantum numbers n\ensuremath{\le}20, where l is the angular momentum. The use of the Heisenberg correspondence principle for radial integrals enables one to obtain analytic expressions for any term in the multipole expansion of the cross section for collisions with an arbitrary n and l change. For transitions involving angular momenta that are small compared to the principal quantum numbers, the semiclassical and the exact results are found to be fairly close, within the accuracy consistent with the Born approximation. As a result, the semiclassical approach furnishes a ready estimate for cross sections, and its validity range proves broad enough to cover the region of large momenta transferred to the atom. The numerical results also confirm that dipole transitions have no dominance over the collisions with other \ensuremath{\Delta}l values, in accord with the general model of intercombination scattering.