Cusp-shape studies with He sup + ions at 1. 41- and 2. 41-a. u. impact velocities

The electron loss to the continuum (ELC) process was investigated in He{sup +-}(He,Ar) collisions at 1.41- and 2.41-a.u. impact velocities. The cusp-shaped energy distributions of electrons ejected into the forward direction were measured in coincidence with the scattered He{sup 2+} projectile ions. The shape of these cusps was studied by fitting the multipole series expansion (Meckbach, Nemirovsky, and Garibotti, Phys. Rev. A 24, 1793 (1981)) of the measured spectra using {ital B}{sub {ital n}{ital j}}, the {ital n}th-order expansion coefficients of the {ital j}th-order Legendre polynomials, as fitting parameters. The observed skewness of the spectra (nonzero {beta}{sub 1}={ital B}{sub 01}/{ital B}{sub 00} values) indicates the importance of the higher-order theories in the description of the distribution of ELC electrons. A dip at the top of the peak ( cusp inversion''), reflected by a positive {beta}{sub 2}={ital B}{sub 02}/{ital B}{sub 00} value, was observed in the 1.41-a.u. He{sup +-}He collision.