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Journal of Physics B Atomic Molecular and Optical Physics

Orientation induced scattering asymmetry

J Wang, R. E. Olson

Published Aug 28, 1994• 13 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/27/16/017

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Journal of Physics B Atomic Molecular and Optical Physics•Jul 11, 2000•0 citations
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.
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.
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.
Discusses some consequences of the classical grazing model of Kohmoto and Fano (1981) to the problem of atomic orientation by electron collisions. The authors introduce amplitudes for repulsive and attractive scattering interactions and related them to the magnetic sublevel; amplitudes f 0 and f 1 of the coherent 1 S to 1 P excitation process. An important consequence of the grazing model is a left-right asymmetry in the scattering of electrons having excited the atom with a given definite orientation. It follows from parity invariance that the circular polarisation of the 1 P to 1 S radiation emitted perpendicular to the scattering plane is identical to the left-right intensity asymmetry of the scattered electrons measured in coincidence with right- and left-hand circularly polarised light. The magnitudes and the phase difference of the amplitudes for attractive and repulsive interactions of the coherent 1 S to 1 P excitation can be calculated from the knowledge of the lambda and chi parameters.
Circular Rydberg states offer advantages for quantum information and quantum simulation platforms due to their long lifetimes and strong dipole-dipole interactions. Unfortunately, current techniques for the production of these states remain technically challenging. Here we investigate the ability of twisted electron collisions to produce circular Rydberg states. Twisted electrons carry quantized orbital angular momentum that can be transferred to the electronic state of the atom, potentially providing an efficient means to generate circular Rydberg states. Using a fully quantum mechanical framework, we compute total excitation cross sections for circular Rydberg states of hydrogen, rubidium, and cesium targets using Bessel electron beams. Our models account for the full Bessel-beam structure of the incident electron and incorporate macroscopic target effects to model experimentally-relevant conditions. Our results show that twisted electrons with large opening angles produce significant enhancements in the excitation probability relative to plane-wave electrons, particularly for large opening angles and low energies. We trace this enhancement to contributions from projectiles with large values of orbital angular momentum. These findings demonstrate that twisted-electron excitation may provide a feasible and potentially advantageous pathway for generating circular Rydberg states.
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.
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.