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G. Betz, W. Husinsky:
"Modelling of cluster emission from metal surfaces under ion impact";
Philosophical Transactions of the Royal Society of London A, 362 (2004), 177 - 194.

Using the molecular-dynamics technique, cluster emission for 5 keV Ar bombardment
of a Cu (111) surface has been investigated using a many-body (tight binding)
potential for the Cu-Cu interaction. The calculations allow us to analyse the basic
processes underlying cluster emission. It is found that two distinct processes can be
distinguished which lead to cluster emission under energetic ion bombardment.
The .rst process causes the emission of small clusters, which are emitted by a
collective motion during the development of the collision cascade within the .rst
picosecond after impact. Thus, emission times of such clusters agree with the emission
times of atoms in sputtering. Such a process can be envisioned if, for example, a few
layers below the surface, an energetic recoil causes the development of a subcascade.
Energy transferred by this event to the surface is strongly directional and can lead to
the simultaneous emission of a group of neighbouring surface atoms, which in some
cases will remain bounded and form a cluster after emission. Typically, clusters
emitted by this mechanism consist of atoms, which are neighbouring in the target
and are almost exclusively surface atoms, similar to all sputtered atoms.
Emission of large clusters (cluster sizes of 10 or more atoms), as observed experimentally,
is a puzzling phenomenon. From our calculations we conclude that the
emission of such large clusters does not occur during the collisional phase of sputtering,
but happens much later (5-10 ps after ion impact). Emission can occur for
spike events, where all the energy of the impinging ion is deposited locally in a small
volume near to the surface, and the sputtering yield is 3-5 times the average yield.
Such events are rare, but we have found a few cases in our calculations where stable
clusters consisting of more than 20 atoms were emitted. Melting of the spike volume
occurs, and the high temperatures and pressures produced can cause emission of
large fragments during the thermal phase. The composition of such large clusters
is quite di.erent from that of small clusters. They consist of atoms from di.erent
layers and the constituents are also generally not next-neighbour atoms. This change
in origin of the cluster atoms re.ects the mixing and di.usion processes occurring
in the melted zone before emission. The calculations indicate that hydrodynamical
phenomena might play a role in the emission of large fragments. Additional calculations,
where the energy was distributed `thermally’ in a three-dimensional volume
under the surface for 500 fs, give very similar results, even in such cases where the
kinetic phase of the collision-cascade development was absent.
Keywords: sputtering; ion bombardment; cluster emission;
computer simulation; molecular dynamics