ANGULAR-DISTRIBUTIONS FOR DOUBLE-ELECTRON CAPTURE IN C4-HE COLLISIONS()

We have studied the angular distributions of double-electron capture i n C4+ + 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 matri x of Danared and Barany [J. Phys. B 19, 3109 (1986)] shows a reasonabl e agreement at 800 eV, while at lower energies the calculations show a shift of the peak positions towards larger angles. An alternative mod el by Barat et al. [J. Phys. B 23, 2811 (1990)], developed for collisi ons in the keV range, was also employed and was found to agree less we ll with our data, succinctly illustrating the increasing sensitivity t o the ''exact'' potential shape with decreasing collision energy.