On the mechanisms of target modification by ion beams and laser pulses

Abstract

A comparison is made between the importance of the following categories of processes in ion-surface and laser-surface interactions: ballistic, thermal-spike, residual-defect-induced, and electronic. It is shown that ballistic processes are important for ion sputtering, ion mixing, and ion composition change. Also, they exist with laser-pulse sputtering but are not important. The picture with thermal-spike processes is the inverse, such processes being unimportant in most ion-surface interactions, but very important indeed with laser-pulse sputtering. In the latter case one must distinguish between normal vaporization and phase explosion, both of which are fundamental. On the other hand, normal boiling may possibly be unimportant owing to the negligible density of heterogeneous nuclei formed in the bulk, ∼ 10 6 kg −1. However, it is essential to note that, if nuclei can be formed at the surface, the density will be much higher. Sputtering due to subsurface heating probably does not exist. Residual defects are basic to all ion-surface interactions, but most especially to ion mixing. Here most (∼90%) of the observed mixing is due to residual defects and only a lesser amount is due to ballistic events. But the role of thermal spikes in ion mixing appears to have been exaggerated. Finally, electronic processes are somewhat important to ion sputtering but have not yet been quantified for ion mixing and composition change. On the other hand, although they are very important indeed with laser-surface interactions, a basic qualification must be made: the primary laser-surface interactions are dominantly electronic, but most of the deposited energy is rapidly converted from excitation to heat, and it is for this reason that thermal-spike sputtering is so prominent. Finally, we consider the fact that laser sputtering has two aspects. These are the primary processes, which lead to particle expulsion, and the secondary processes. The latter arise due to collisions amongst the emitted particles, with the result that the particles, effectively, “lose memory” of the primary process.