Helium Symmetric (e,2e) Data
- Excess Energy = 1eV -
page prepared by
Murray (25th July, 1996)
Figure 1. The (e,2e) Differential cross section for an incident electron of energy 25.6eV (1eV excess energy) above the helium ionisation threshold.
For the above results the electrons were detected with equal energy and at equal scattering angles by the Manchester and the Paris experimental groups.
The fit through the data is from parameterisation of the theoretical model in the Wannier picture by Fournier-Legarde et al, J. Phys. B 17:L591 (1984).
This parameterisation, detailed in Hawley-Jones et al, J. Phys.B 25:2393 (1992) has been re-parameterised in terms of the irreducible basis set that is used to fit all of the data taken at Manchester. This allows the results to be presented using a common 'language'. The fits at 30°, 60° and 80° in the above graph are taken from this parameterisation.
The results have been normalised to unity at the common normalisation point at 90 degrees. The perpendicular plane results have an approximately Gaussian angular distribution, whereas the coplanar results have a slight asymmetry in the backward scattering direction.
The parameterisation allows a 3-D picture to be established of the differential cross section surface, where the surface is traced out by the magnitude of the differential cross section at a given detection angle (theta, phi) around the scattering geometry.
Figure 2 shows the result of this 3-D fitting to the data presented above.
Figure 2. A 3-D model of the differential cross section surface at an excess energy of 1eV.
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