Atomic collision sequences in crystals of copper, silver and gold revealed by sputtering in energetic ion beams

Specimens of copper, silver and gold have been bombarded with 0.3 MeV protons and 10 keV ions of argon and xenon. Atoms were ejected from crystal surfaces with maximum intensity along low-index crystal directions. The effects are interpreted as being due to the arrival at the surface of three types of focused atomic collision sequence. The first travels along $\langle$110$\rangle$ directions, momentum being focused by the geometrical properties of a close-packed line of spheres. No mass transfer is thought to occur in this type of sequence and experiment shows its maximum range in gold to be roughly 350 $^\overset\circ{A}$. The second and third types travel along $\langle$100$\rangle$ and $\langle$111$\rangle$ directions, and are thought to consist of successive collisions in which the projectile atom replaces the target; focusing being due to the rings of atoms which encircle the projectile's path. A theoretical study shows these rings to behave with formal similarity to the converging lens in optics, and an upper energy limit for focusing is derived. It is predicted that the maximum ranges in copper are roughly 35 $^\overset\circ{A}$ for $\langle$100$\rangle$ sequences and 120 $^\overset\circ{A}$ for $\langle$111$\rangle$ sequences, the corresponding values in gold being 50 and 300 $^\overset\circ{A}$, respectively. These predictions are shown to be in agreement with experiment.