164 lines
5.8 KiB
HTML
164 lines
5.8 KiB
HTML
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<title>Theory of Cluster Dynamics</title>
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<p><font color="white" size="6"><b>Theory of Cluster Dynamics</b></font><font size="5"><br>
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</font><font size="6">
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</font><font size="5">The Toulouse - Erlangen Collaboration</font></p>
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<a href="../analysis/detail1.html">1. Analysis of cluster dynamics</a>
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<div style="width:220px;float:left;text-align:center;font-weight:900;font-size:12px;">
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<a href="../analysis/detail2.html"> 2. Clusters in external fields</a>
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<div style="width:220px;float:left;text-align:center;">
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<a href="../tddft-md/formal.html"> 3. Theoretical developments </a>
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<p> Clusters in strong external perturbations</p>
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<!-- START CONTENT HERE -->
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<br>
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<p><img src="figs/na8_nacl_SHG.gif" align="right" width="300">
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Many experiments are done for clusters in contact with a
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substrate. The strong interface interaction modifies the cluster and
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theoretical simulations become more involved. However, some features
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can only be explored in connection with a substrate. E.g., the
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symmetry breaking through a surface gives access to second-harmonic
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generation (SHG). </br></br>
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The figure beneath shows the results from a TDLDA
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simulation of SHG for Na<sub>8</sub> attached to a NaCl surface
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[<a href="../literatur.html#own1224">248</a>]. The spectra resulting
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from
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irradiation with a 1.4 eV pulse shows nicely the peaks at multiple
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frequencies. The SHG signal can be enhanced by increasing the laser
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intensity. This, however, breaks down at some point where the signals
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are substantially broadened. This is caused by a large ionization
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which spoils the otherwise clean dipole response of metal clusters.
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</p>
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<br>
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<br>
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<p>
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<img src="figs/na6_ar384d_deposit.gif" align="left" width="300">TDLDA
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coupled with molecular dynamics (MD) for ionic motion is a very
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powerfull tool to describe cluster dynamics. One application is
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cluster deposition which is illustrated in the figure on the left. It shows
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Na<sub>6</sub> impinging on an Ar surface (see [<a
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href="../literatur.html#own1303">328</a>] for further details). The substrate consists of
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six layers of Ar
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taken from an appropriate cut of the Ar fcc structure. The Na<sub>6</sub>
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cluster consist in a ring of 5 ions topped by one ion on the symmetry
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axix. The Na<sub>6</sub> approaches the surface with the symmetry axis
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in <i>z</i>
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direction (=perpendicular) and the top ion facing away from the
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surface. </br></br>
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The upper panel shows the evolution of the <i>z</i>
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coordinates,
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Na ions in red and Ar atoms in green. The cluster is immediately
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stopped by the surface. A large fraction of impact momentum is
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transferred at once to the substrate and propagates with velocity of
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light through the layers. The large dissipation through energy
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transfer and intrinsic cluster excitation leads to catching of the
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cluster by the subtrate. The kinetic energies in the lower panel
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confirm the dramatic and very fast energy exchange at the moment of
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first impact. Another fraction of energy, not shown in the figure, is
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turned into the large shape changes.
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</p>
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<br><br>
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<p>
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Clusters in the strong fields of extremely intense lasers show a much
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different dynamics. Core electrons can be released and contribute
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strongly to the process. The detailed description at the fully quantum
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mechanical level of TDLDA becomes untractable. However, the
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excitations involved validate classical approaches. </br></br>
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<img src="figs/MD_fig5.gif" align="right" width="300">
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The figure to the
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right shows the result of a molecular dynamics simulation of
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electronic and ionic dynamics of Na<sub>41</sub><sup>+</sup> under the
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influence of
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strong laser fields [<a href="../literatur.html#own1308">332</a>].
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Ionization is
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drawn as function of laser intensity. One sees a sharp kink at a
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critical intensity of I = 10<sup>16</sup> W/cm<sup>2</sup>. This
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threshold value is
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explained by the fact that the Coulomb force from the laser field
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at the threshold just equals the binding forces of the core electrons. The increase is
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due to the core electrons which now start to participate in the
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dynamics. This view is illustrated by separating the contributions from
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valence (green line) and core electrons (red line). There is indeed zero
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emission from core electrons up to I = 10<sup>16</sup> W/cm<sup>2</sup>
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and the
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strong increase above that critical intensity is exclusively due to
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the contribution from core electrons.
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