4 lines
6.1 KiB
HTML
4 lines
6.1 KiB
HTML
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<title>Theory of Cluster Dynamics</title>
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<li style="margin-top:1px;border-top:1px solid #B0C4DE; "><a href="../index.html">Home</a></li>
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<p><font size="6" color="white"><b>Theory of Cluster Dynamics</b></font><font
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size="5"><br />
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</font><font size="6"> </font><font size="5">The Toulouse -
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Erlangen Collaboration</font></p>
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</div>
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<a name="oben"> </a>
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<div id="content"><a name="oben"> </a>
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name="oben">
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<div style="width:180px;float:left;text-align:center;font-size:10px"><a
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name="oben">
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</a><a href="formal.html">1. Theoretical developments </a> </div>
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<div style="width:200px;float:left;text-align:center;font-size:10px;">
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<a href="../analysis/detail1.html"> 2. Analysis of cluster
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dynamics </a> </div>
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<div style="width:200px;float:left;text-align:center;font-weight:900;font-size:10px;">
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<a href="../analysis/detail2.html"> 3. Clusters in strong external
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fields </a> </div>
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<div style="width:180px;float:left;text-align:center;font-weight:900;font-size:12px;">
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<a href="detailQMMM.html"> 4. Embedded clusters </a> </div>
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<p> Clusters in strong external perturbations</p>
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<!-- START CONTENT HERE --> <br />
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<p><img src="figs/na8_nacl_SHG.gif" width="300" align="right" />
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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
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cluster and theoretical simulations become more involved.
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However, some features can only be explored in connection with
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a substrate. E.g., the symmetry breaking through a surface
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gives access to second-harmonic generation (SHG). <br />
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<br />
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The figure beneath shows the results from a TDLDA simulation
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of SHG for Na<sub>8</sub> attached to a NaCl surface [<a href="../literatur.html#own1224">248</a>].
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The spectra resulting from irradiation with a 1.4 eV pulse
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shows nicely the peaks at multiple frequencies. The SHG signal
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can be enhanced by increasing the laser intensity. This,
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however, breaks down at some point where the signals are
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substantially broadened. This is caused by a large ionization
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which spoils the otherwise clean dipole response of metal
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clusters. </p>
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<br />
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<br />
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<p> <img src="figs/na6_ar384d_deposit.gif" width="300" align="left" />TDLDA
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coupled
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with molecular dynamics (MD) for ionic motion is a very
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powerfull tool to describe cluster dynamics. One application
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is cluster deposition which is illustrated in the figure on
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the left. It shows Na<sub>6</sub> impinging on an Ar surface
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(see [<a href="../literatur.html#own1303">328</a>] for further
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details). The substrate consists of six layers of Ar taken
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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
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symmetry axix. The Na<sub>6</sub> approaches the surface with
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the symmetry axis in <i>z</i> direction (=perpendicular) and
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the top ion facing away from the surface. <br />
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<br />
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The upper panel shows the evolution of the <i>z</i>
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coordinates, Na ions in red and Ar atoms in green. The cluster
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is immediately stopped by the surface. A large fraction of
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impact momentum is transferred at once to the substrate and
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propagates with velocity of light through the layers. The
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large dissipation through energy transfer and intrinsic
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cluster excitation leads to catching of the cluster by the
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subtrate. The kinetic energies in the lower panel confirm the
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dramatic and very fast energy exchange at the moment of first
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impact. Another fraction of energy, not shown in the figure,
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is turned into the large shape changes. </p>
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