**2.1 Ionic interaction with superficial atoms**

The specific methods by which atoms are ejected from a surface under ionic impact are unknown, but details of the associated interactions can be inferred. Because an ion is about the same size as an atom, when it collides with a surface, it first collides with a surface atom. The sputtering process depends on the value of the energy interchange between an incident ion and a superficial atom; it also depends on the difference in size of the incoming ion and the superficial atom. The detailed description is given below [5].

The energy exchange between a surface atom and incoming ion is substantially greater than the binding energies of the lattice atoms, hence ions impact the bombarded surface in the normal parallel direction of the surface.

As a result, the primary collision is strictly binary, with the incident particle delivering a considerable portion of its core energy to the damaged atom and keeping the rest. If the incident ion's mass is less than the mass of the surface atom it collides with, and the collision occurs in front or close, the incident ion must bounce off the surface, as seen in **Figure 4** (Event-I).

If the mass of incident ion is greater than the affected atom, both the ion and the atom will leave the collision point following inward paths from the surface, regardless of whether the collision is frontal or lateral as seen in **Figure 4** (Event-II). So we have at least

**Figure 3.** *Series of collisions under the ionic impact between atoms of substrate.*

**Figure 4.** *Ionic interaction with superficial atoms at normal incidence.*

one and usually two particles traveling to the surface with energies lower than the impacting ion's fundamental energy but still much greater than the energies of the lattice.
