H+ SECONDARY-ION EMISSION PRODUCED BY PRE-IMPACT SURFACE-IONIZATION

A model is described in which the ion emission from solid surfaces is triggered by the primary projectile Coulomb field, before the projectile-surface collision. This Pre-Impact Desorption (PID) model considers that projectiles, with charge qe and velocity nu, cause: i) electron emission prior to their collision with the target and ii) partial relaxation of the surface ionized species via ion desorption. A q(3)-dependence is predicted for the desorption yield of H+ ions; this result has been observed experimentally for different projectile-target pairs and for several projectile energies in the keV-MeV range. The model predicts a maximum desorption yield occurring at projectile velocities around the Bohr velocity, a linear dependence with nu for very low velocities and a 1/nu dependence in the very high nu regime. Analytical expressions are also obtained for the surface charge distribution produced around single impacts.