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 feature
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 benaeth 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 elsewise 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 left figure. It shows
Na6 impinging on an Ar surface (for the modeling [328]). 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, missing in that figure, is
turned into the large shape changes.
Clusters in the strong fields of extremely intense lasers show a much
different dynamics. The core electrons can be released and contribute
strongly to the process. The detailed description at the fully quantum
mechanical level of TDLDA becomes untractable. However, the
excitations involved validate classical approaches.
The figure to the
right shows the result of a molecular dynamics simulation of
electronic and ionic dynamics of Na41+ under the
influence of
strong laser fields [332].
Ionization is
drawn as function of laser intensity. One sees a sharp kink at a
critical intensity of I=1016 W/cm2. The
critical value is
diistinguished by the fact that the Coulomb force from the laser field
just equals the binding forces of the core electrons. The increase is
due to the core electrons which now start to participate in the
process. This view is checked by sepparating the contributions from
valence (green) and core electrons (red line). There is indeed zero
emission from core electrons up to I=1016 W/cm2
and the
strong increase above that critical intensity is exclusively due to
the core contribution.
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