Collisions of fullerenes with surfaces at the 5–100 eV impact energy range

Abstract

In this paper, we will present recent studies on the scattering dynamics of neutral and charged fullerenes from surfaces at the hyperthermal impact energy range. The energy range for the neutral colliders is 5–50 eV (aerodynamic acceleration in seeded beams) while with negatively charged ions we extend it up to 100 eV. The smooth overlap of the two energy regions using neutral and charged C 60 is demonstrated. The translational energy of the scattered particles (both C 60 0 and C 60 − ) was found to scale linearly with impact energy for both near normal and near grazing scattering angles. Kinetic energy losses varied with scattering angle from ∼15% (near normal incidence) to ∼40% (near grazing incidence). Analysis of the results in terms of simple kinematical models demonstrates nearly complete decoupling between normal and tangential energy losses. Furthermore, the accuracy of our energy measurements (±0.1 eV) enables us to extract, for the first time, molecule–surface binding energies from kinetic energy losses measured in molecular beam–surface scattering experiments. We compare this dynamically determined value with the equilibrium one as measured by temperature-programmed desorption experiments. The yield of C 60 − negative ions following scattering of hyperthermal neutral C 60 molecules from a graphite monolayer on nickel surface (Ni/C) was measured as a function of impact energy. An increase of two orders of magnitude was observed going from 4 to 42 eV. Energy and angle distributions of both scattered ions (C 60 − ) and neutrals (C 60 0 ) were measured and compared for this impact energy range. The C 60 − ion yield was found to obey an exponential dependence on the inverse of the outgoing normal velocity component similar to that observed before for atom–surface collisions at much higher energies. The shifts observed between the angular and energy distributions maxima of the neutral and negative ion could be analysed and explained in terms of image charge effects on the outgoing trajectory (deflection) and exit energy (retardation) of the C 60 − . A large critical distance for ion formation was extracted from the results. The charge transfer dynamics is consistent with final charge state exclusively determined along the exit trajectory.