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Journal of Physics B Atomic and Molecular Physics

Some consequences of atomic orientation in electron-atom scattering by consideration of attractive and repulsive forces

H. J. Beyer, Hans Kleinpoppen, Ian A. McGregor, L.C. McIntyre

Published Aug 14, 1982• 11 citations

Abstract

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.

DOI: 10.1088/0022-3700/15/15/008

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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.

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 density matrix parametrisation of collisionally excited atomic ensembles is generalised to account for projectile and target spin polarisations. The density matrix elements, containing spin-resolved alignment and orientation parameters, can be determined by measurements of the 'generalised Stokes parameters' introduced by Andersen and Bartschat (1994) to describe scattered-projectile-polarised-photon coincidence experiments. Focusing on electron impact excitation of light alkali-type targets and mercury, the present experimental status of such experiments is reviewed, in particular with regard to the 'perfect scattering experiment' whereby all independent scattering amplitudes are determined.

It is shown that new effects in light scattering by oriented atoms and in orientation of an unpolarized atomic target due to the light scattering arise when channels of dissipation for the light energy are open. Thus, in scattering of linearly-polarized or unpolarized light, the degree of linear polarization and the angular distribution of scattered light depend on the initial atomic orientation. On the other hand, the orientation of unpolarized atoms appears in the scattering of unpolarized photons without registration of the scattered photon polarization. The orientation vector is perpendicular to the scattering plane, and the effect can be observed in a coincidence experiment.