Cluster impact at surfaces
A) In most cases, ion-impact induced sputtering of solids follows the well known law of linear-cascade theory. However, if the energy-density near the surface is very high, the sputter yield gets much higher than can be expected from linear-cascade theory.
Using molecular-dynamics simulation, we study the sputtering of a Au (111) surface due to impact of Aun (n = 1, 2, 4, 8, 13 and 43, 87, 134, 201) with energies in the range of 1 keV/atom up to 64 keV/atom. We measure the initial sputter yield Yinitial at the time when the maximum number of atoms are energized to above the cohesive energy of the material, the energy which is deposited near the surface at that time, and the total sputter yield after the crystal is annealed (after 100ps). Than we correlate these early values to the final sputter yield, Yfinal in order to examine the influences of cluster size and impact velocity.
Additionally, for the larger clusters, we determine that part of the yield which is due to reflected projectile atoms, not to the truly sputtered atoms.
(The figure shows the Au4 (64keV/atom) impact at a crystal with 1.26 millions of atoms.)
B) We look for the fragmentation behaviour of slow clusters (up to 20 eV). In this regard, we focus especially on the excitation of the cluster, its energy distribution and the fragmentation pattern. We are interested in the difference between atomic clusters and molecular clusters and the influence of their additional degrees of freedom on the stability of the cluster. (The figure shows the increase in the different energy forms related to the increasing impact energy for a synthetic nitrogen-cluster with a weak intramolecular binding energy.)