H-ION EMISSION PRODUCED BY PRE-IMPACT SURFACE-IONIZATION( SECONDARY)

A model is described in which the ion emission from solid surfaces is triggered by the primary projectile Coulomb field, before the projecti le-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 rela xation of the surface ionized species via ion desorption. A q(3)-depen dence is predicted for the desorption yield of H+ ions; this result ha s been observed experimentally for different projectile-target pairs a nd for several projectile energies in the keV-MeV range. The model pre dicts a maximum desorption yield occurring at projectile velocities ar ound the Bohr velocity, a linear dependence with nu for very low veloc ities and a 1/nu dependence in the very high nu regime. Analytical exp ressions are also obtained for the surface charge distribution produce d around single impacts.