Energy and Charge Transfer of Hyperthermal-Energy Heavy Ions Scattering on Target Surfaces with Low Atomic Mass

Limited information is available on the scattering dynamics of heavy projectiles from a surface composed of light atoms, especially in the hyperthermal energy regime, in spite of increased importance from both theoretical and practical perspectives. The scattering of hyperthermal Br+ ions on highly oriented pyrolytic graphite (HOPG) serves as a desired model system to investigate detailed energy and charge transfer processes during a violent collision. The angle-resolved velocity distributions of the scattered ions are measured under a wide set of incident conditions. The variation of the distributions with initial kinetic energy reveals unexpected and complicated scattering dynamics which can not be properly described by simple models such as parallel momentum conservation or binary collision models. Instead, it is attributed to successive many-body collisions between the projectile and a dynamically corrugated surface induced by the energetic impact. Br+ ions are efficiently converted to neutral atoms or Br– upon collision with HOPG. The anion yield calculated by integrating the probability density of scattered ions over all exit angles and velocities exhibits a strong resonance at ~30-eV incident energy. For incident energies less than 30 eV, the anion yield is positively correlated to the final exit velocity, in agreement with conventional charge transfer theories applied to light projectiles scattering on a static surface. The steep drop of the anion yield at high incident energy is attributed to surface penetration and trapping of the incident projectile.

The scattering of Cl+ on HOPG as well as Ni(111) supported monolayer graphite surface under analogous incident conditions provides a unique opportunity to examine the effect of mass and physical properties of the surface on the scattering dynamics. The results indicate that for the heavy projectile/light surface atom combination the interaction between surface atoms plays a key role in the energy transfer process.