Resolution dependence and asymmetry of electron capture to the continuum spectra

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Abstract
Velocity distributions of electrons ejected into the forward direction have been measured with proton beams of energies comprised of between 40 and 240 keV interacting with a helium-gas target. An electron spectrometer was used, which permitted us to study the shape of the resulting peaks as a function of the instrumental angular acceptance. These data are analyzed by introducing a general parametric expression for the scattering amplitude of the ionization process. Strong skewness of the electron cusps towards lower electron velocities are observed, which appear to be independent of the large variation of peak widths as determined by different angular acceptances. It is found that this skewness is important up to the closest neighborhood of the peak top and that it leads to the inclusion of an asymmetric singular behavior of the cross section. Partial peak widths at half maximum, measured towards each side of the peak top, and also the total peak width depend linearly on the transverse experimental acceptance, as given by the product of the ion velocity times the half-angle of the electron acceptance cone. The cusp skewness, defined by the ratio of the partial widths, tends to decrease at the higher ion velocities.