Clusters in strong external perturbations
Many experiments are done for clusters in contact with a
substrate. The strong interface interaction modifies the
cluster and theoretical simulations become more involved.
However, some features can only be explored in connection with
a substrate. E.g., the symmetry breaking through a surface
gives access to second-harmonic generation (SHG).
The figure beneath shows the results from a TDLDA simulation
of SHG for Na8 attached to a NaCl surface [248].
The spectra resulting from irradiation with a 1.4 eV pulse
shows nicely the peaks at multiple frequencies. The SHG signal
can be enhanced by increasing the laser intensity. This,
however, breaks down at some point where the signals are
substantially broadened. This is caused by a large ionization
which spoils the otherwise clean dipole response of metal
clusters.
TDLDA
coupled
with molecular dynamics (MD) for ionic motion is a very
powerfull tool to describe cluster dynamics. One application
is cluster deposition which is illustrated in the figure on
the left. It shows Na6 impinging on an Ar surface
(see [328] for further
details). The substrate consists of six layers of Ar taken
from an appropriate cut of the Ar fcc structure. The Na6
cluster consist in a ring of 5 ions topped by one ion on the
symmetry axix. The Na6 approaches the surface with
the symmetry axis in z direction (=perpendicular) and
the top ion facing away from the surface.
The upper panel shows the evolution of the z
coordinates, Na ions in red and Ar atoms in green. The cluster
is immediately stopped by the surface. A large fraction of
impact momentum is transferred at once to the substrate and
propagates with velocity of light through the layers. The
large dissipation through energy transfer and intrinsic
cluster excitation leads to catching of the cluster by the
subtrate. The kinetic energies in the lower panel confirm the
dramatic and very fast energy exchange at the moment of first
impact. Another fraction of energy, not shown in the figure,
is turned into the large shape changes.
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