Angular distributions for double-electron capture inC4+-He collisions

We have studied the angular distributions of double-electron capture in ${\mathrm{C}}^{4+}$+He collisions in the 400–800-eV collision energy range. At all measured energies an interference pattern has been observed which is attributed to Stueckelberg oscillations. A comparison with calculated differential cross sections using a two-state diabatic potential matrix of Danared and Bárány [J. Phys. B 19, 3109 (1986)] shows a reasonable agreement at 800 eV, while at lower energies the calculations show a shift of the peak positions towards larger angles. An alternative model by Barat et al. [J. Phys. B 23, 2811 (1990)], developed for collisions in the keV range, was also employed and was found to agree less well with our data, succinctly illustrating the increasing sensitivity to the ‘‘exact’’ potential shape with decreasing collision energy.