Shape of the ‘‘cusp’’ inHe2+-He andHe+-He collisions in the 0.6–1.6-MeV impact-energy region

The shape of the distribution of electrons ejected into the forward direction in the ${\mathrm{He}}^{+}$-He and ${\mathrm{He}}^{2+}$-He collisions in the 0.6–1.6-MeV impact-energy region are investigated. In both cases the anisotropies observed in the measured double-differential electron spectrum are analyzed by the help of a series expansion and a fitting procedure for a better characterization of the cusp. The theoretical peak shapes are also analyzed by this method. In the case of ${\mathrm{He}}^{2+}$ the agreement between the measured and the calculated cusp shapes is especially poor.