  Anomalous inter-layer atomic transport of deposited impurity atoms on Al(111)
has been found by constant temperature molecular dynamics simulations. The
low-energy deposition of Pt on Al(111) leads to oscillatory adsorbate-substrate
interaction and to a low-temperature ballistic injection of the deposited
particles to below the topmost layer. The ultrafast injection of a Pt atom
coincides with the ejection of a substrate atom to the surface (ballistic
replacement or exchange mechanism). This is in agreement with the experimental
findings in which thin film rich in Al has been found on the surface after
deposition of few MLs of Pt. We attribute the anomalous inter-layer transport
to the size-mismatched impurity/host interaction and we point out the role of
atomic size mismatch in biasing towards intermixing. The deposition induced
low-temperature disordering of surface Al atoms with few transient Al adatoms
has also been observed. The ultrafast injection of the impurity particles to
the substrate is assisted by the transient out-of-plane and lateral circulating
motion of few surface atoms arranged nearly in a hexagonal symmetry. The atomic
injection of Pt can be regarded as a superexchange process driven by the
transient and collective motion of few surface atoms. A chaotic surface state
assisted mechanism could also be a possible explanation of the unexpected
phenomenon.
