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Journal of Physics B Atomic Molecular and Optical Physics

The effect of a finite interaction region on the measurement of coherence parameters in electron-photon coincidence experiments

Peter J. M. van der Burgt, J J Corr, J. William McConkey

Published Mar 14, 1991• 13 citations
Publisher

Abstract

Electron-photon coincidence experiments are usually carried out by crossing an electron beam with a beam of atoms emerging from a capillary tube or array. Recently it has been found that the finite dimensions of the interaction region formed by the crossing beams can affect the coherence parameters that are measured in these experiments. The authors have developed models with which these effects and those related to the finite acceptance angles of the detectors can be simulated numerically. This article presents a description of the models and presents results which illustrate the possible magnitude of these effects in electron-rare gas scattering experiments. Other depolarizing effects such as internal atomic interactions are also discussed.

DOI
10.1088/0953-4075/24/5/026

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Electron-photon coincidence experiments are usually carried out by crossing an electron beam with a beam of atoms emerging from a capillary tube or array. Recently it has been found that the finite dimensions of the interaction region formed by the crossing beams can affect the coherence parameters that are measured in these experiments. The authors have developed models with which these effects and those related to the finite acceptance angles of the detectors can be simulated numerically. This article presents a description of the models and presents results which illustrate the possible magnitude of these effects in electron-rare gas scattering experiments. Other depolarizing effects such as internal atomic interactions are also discussed.
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Measurements of superelastic scattering of electrons by laser-excited 138Ba^{138}\mathrm{Ba}138Ba(. . .6s6p 1P1^{1}P_{1}1P1​) atoms were carried out. An asymmetry observed by us and previously by Register et al. [Phys. Rev. A 28, 151 (1983)] has been explained using a model of scattering from a target with finite dimensions. This model employed coherence parameters which we calculated in the distorted-wave approximation. The results indicated that the interpretation of coherence experiments in terms of scattering from a pointlike target can lead to serious errors in the deduction of coherence parameters at low scattering angles.
The analysis of a theoretical equation for the double differential cross-section of electron-atom collisions, in which "bremsstrahlung" (BS) photons are produced, and in particular of its dependence on both photon energy and electron scattering angle, shows that there is an optical sector of electron-atom resonances, that is, the appearance of resonance dips in BS cross-section. When considering, for instance, the resonance that the rare gases exhibit at nearly 0.5 eV below excitation threshold, this optical sector, or in other words the BS photon energy distribution, presents one or two resonancelike structures whose positions depend on both electron energy and scattering angle. By suitably choosing these parameters, one of these optical-resonance images can be made to occur at easily detectable photon energies. A successful experiment could be performed by utilizing an electron-photon coincidence method such as those used for studying coherence and correlation in electron-atom collisions. Particular conditions for such an experiment are considered, and its possible relevance for better insight into electron-atom resonances is outlined.
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Experimental studies concerning elastic and inelastic electron scattering by coherently ensembles of Ba (...6s6p (sub 1)P(sub 1)) atoms with various degrees of alignment will be described. An in-plane, linearly-polarized laser beam was utilized to prepare these target ensembles and the electron scattering signal as a function of polarization angle was measured for several laser geometries at fixed impact energies and scattering angles. From these measurements, we derived cross sections and electron-impact coherence parameters associated with the electron scattering process which is time reverse of the actual experimentally studied process. This interpretation of the experiment is based on the theory of Macek and Herte. The experimental results were also interpreted in terms of cross sections and collision parameters associated with the actual experimental processes. Results obtained so far will be presented and plans for further studies will be discussed.
A brief review is given on development of electron-photon coincidence studies from electron impact of atoms. The theory of electron-photon correlations is outlined in detail in terms of cross sections of excited atoms and excited state charge clouds. The state of a target atom is determined by the various orientation and alignment parameters and the multipole moments of the excited atom. General considerations, such as coherence effects in impact excitation, the influence of fine and hyperfine structures in correlations are taken into account. The relationship between scattering amplitudes (including their phases) and target parameters is worked out for singlet and doublet transitions.