Charge Transfer in Low-Energy Collisions of Protons with a Thick C₆₀ Film

Charge-exchange processes in collisions between H⁺ projectiles and a three-monolayer C₆₀ film grown on Cu(111) are experimentally and theoretically analyzed for a wide range of energies (2–8 keV) in the low-energy regime. The negative, positive, and total projectile scattered ion fractions are experimentally determined by using the low-energy ion scattering technique. Two different collisional setups are studied for a fixed backscattering angle of 135°: 45/90° and 67.5/67.5° incoming/exit angles, determined with respect to the target surface plane. Total ion fractions between 10 and 20% are experimentally found in the whole energy range analyzed, with a slight but sustained increase with the projectile incoming energy. Positive ions were measured to be the majority of the total scattered charged projectiles in almost the whole energy range analyzed. Only for very low incoming energies (2–3 keV), the positive and negative ion fractions become comparable. A first-principle-based theoretical model is used to describe the dynamic charge-exchange processes taking place in the experimental collisional situation. A good description of the experimental results is obtained in the studied energy range. The theoretical and experimental results are also analyzed in comparative terms with previous measurements in the H⁺/highly oriented pyrolytic graphite (HOPG) system. The differences in the measured ion fractions can be directly related to differences in the electronic structures of C₆₀ and HOPG.