281 lines
11 KiB
HTML
281 lines
11 KiB
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<title>Theory of Cluster Dynamics</title>
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<div id="navMenu">
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<ul>
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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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<li><a href="intro.html">Introductory Overview</a></li>
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<li><a href="research.html">Scientific Information</a></li>
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<li><a href="staff.html">Staff</a></li>
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<li><a href="publications.html">Publications/Talks</a></li>
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<li><a href="contact.html">Contact</a></li>
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</div>
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<div id="image">
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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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</div>
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<div id="content">
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<div style="margin:15px;width:770px;border:1px solid gray;float:left;font-size:10px;">
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<div style="width:220px;float:left;text-align:center;">
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<a href="intro.html">1. What are clusters? </a>
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</div>
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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="dynamics.html"> 2. Why study cluster dynamics?</a>
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</div>
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<div style="width:220px;float:left;text-align:center;">
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<a href="ourdynamics/our_dynamics.html"> 3. How we deal with cluster dynamics </a>
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</div>
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</div>
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<div id="WideContent">
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<div id="contentBoxWide">
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<div id="contentBoxHeader">
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Why study cluster dynamics ?
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</div>
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<div id="contentBoxContent">
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<br>
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Cluster dynamics represents a fast developing area of cluster
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physics. The field covers various phenomena with impact both
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on fundamental cluster research and on potential applications, for
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example in cluster engineering. We show in the following a few
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emblematic examples of the field. <br>
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<br>
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Clusters are made of electrons and ions. Both are charged particles
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which can then be excited by an electromagnetic field. A favorite and
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fashionable tool of investigation of cluster dynamics is thus provided
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by lasers. The latter deliver to the system electromagnetic
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pulses whose characteristics can be tailored
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almost at will both in terms of deposited energy and time profile. Some
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experiments are also performed by means of collisions between clusters
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and highly charged projectiles, as delivered by heavy-ion sources
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and facilities. <br> <br>
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In both cases (lasers, ions), electrons and ions
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strongly couple to the delivered
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electromagnetic field, but at different time scales. Indeed, electrons
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are light particles which thus react and evolve at short time scales,
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typically the fs (10<sup>-15</sup>s). In turn, the much heavier ions
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(several thousand times heavier than electrons) evolve on a much longer time scale of
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order 100-1000 fs. Of course these time
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scales are not fully independent of each other, through the natural
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coupling between electrons and ions, and the actual relation
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between these two time scales somewhat depends on the deposited energy.<br>
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Let us illustrate the two coupled electron and ion dynamics on a few
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examples. <br>
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<br>
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Metal clusters couple especially well to an electromagnetic
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perturbation because their electrons are only moderately bound to the
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ionic cores. They thus react strongly, for instance to a
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laser excitation. The response, called "optical response" (because the emitted light
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is to a large extent visible), is the
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fingerprint of this coupling. <br><br>
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The optical response
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is caused by the collective oscillations of the cluster electrons
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following an excitation by the electromagnetic pulse. The electron
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cloud, elastically bound to the ionic cores, oscillates around
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them, once displaced from its original position, and radiates
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visible light. This collective response provides a signature of the
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underlying structure of the irradiated cluster. The "color" of
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the irradiated cluster, for example, significantly depends on the size
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of the cluster. We thus have here an example where electron dynamics
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provides a direct means of investigation of
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structure properties. The case is illustrated on Figure 1 where the
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frequency (the color) of the optical response of mixed gold and silver
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clusters (embedded in an inert glass) is plotted as a function of
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cluster size. One can see that the cluster color significantly depends
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on size. It means that such golden inclusions in a glass (of course of
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various sizes) would deliver a variety of colors, as it was already
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well-known by ancient artcrafters (see in the cluster <a
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href="intro.html">introduction</a> page).<br>
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<br>
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<table style="width: 100%; text-align: left;" border="0" cellpadding="2"
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cellspacing="2">
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<tbody>
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<tr>
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<td style="vertical-align: top;"><img
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alt="silver optical response" src="images/opt2.jpg"
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style="width: 620px;"></td>
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<td style="vertical-align: middle;">Fig.1: Optical response of mixed
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gold and silver clusters, embedded in inert glass, as a function of
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size (<a href="javascript:lade(1)">details</a>).
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</td>
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</tr>
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</tbody>
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</table>
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<br>
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<br>
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The optical response is a rather simple process, involving mostly electrons
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(although ions may also interfere, for example when temperatures
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are involved). Another interesting case is provided by cluster fission
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where ionic motion then plays a key role. When sufficiently charged
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(for example after a laser irradiation and escape of several electrons)
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a metal cluster may become unstable with respect to fission, exactly as
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massive atomic nuclei. It then becomes preferable for the
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system to break into two smaller clusters, the fission fragments. In
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such processes electrons play a relatively passive role (once the
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system is properly charged) and tend to follow the ions during the fission
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process. Fission is furthermore characterized by a potential barrier
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over which the system has to pass in order to evolve from one
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piece to two. This is illustrated in Figure 2 where the
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fission barrier of a small metal cluster is shown, together with the different shapes
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taken by the system at different deformations (from the smallest: 1
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piece, to the largest: 2 pieces). Figure 3 presents an example of
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fission dynamics for another cluster.<br>
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<br>
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<table style="width: 100%; text-align: left;" border="0" cellpadding="2"
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cellspacing="2">
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<tbody>
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<tr>
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<td style="vertical-align: middle;"><img alt="K12++ fission"
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src="images/fission2.jpg" style="width: 420px;"><br>
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</td>
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<td style="vertical-align: middle;">Fig.2: Potential energy of K<sub>12</sub><sup>++</sup>
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as a function of the extension of the cluster (<a
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href="javascript:lade(2)">details</a>).<br>
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<br>
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Fig.3: Movie of the fission of Na<sub>14</sub>, induced by a laser
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irradiation (<a href="javascript:lade(3)">details</a>).<br>
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<object data="images/film_fission.mpg" type="video/mpeg" width="300" height="300">
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<param name="src" value="film_fission.mpg">
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<object data="images/film_fission.mpg" type="video/mpeg" width="300" height="300">
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<param name="src" value="film_fission.mpg">
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<br><i>
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Your browser is unable to open the video. You need a suitable MPEG plugin to watch
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it inside this window.
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</i>
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</object>
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</td>
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</tr>
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</tbody>
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</table>
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<br>
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<br>
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A most interesting situation is attained when both electron and
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ion dynamics explicitely couple to produce elaborate dynamical
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scenarios. This is illustrated on the third example we want to present
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here. We consider the case of embedded silver clusters, the shape of
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which can be tailored, as one can see on Figure 2. Metal clusters
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possess a specific frequency at which they couple to light (the optical
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response frequency seen above). If one shines a cluster with a laser
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precisely tuned at that frequency, one will so much excite the
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cluster that it will emit several electrons. This is a typical resonant
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behavior as is well known in any oscillating system. <!--Think for example to
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the case of noise associated to mechanical vibrations in a car or the
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search of a TV or a radio channel by tuning reception to the right
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frequency of the electromagnetic signal.--> <br> <br>
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In this case <!-- the case of irradiated
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clusters-->, if several electrons are stripped during the exposure to the
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laser, the cluster may become highly charged and will consequently
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expand because of the net charge acquired, as in fission. But full
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ionic expansion is hindered here by the fact that the cluster is
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included in a matrix. The final result is a somewhat expanded cluster.
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This expansion can be further analyzed by irradiating again the cluster
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and recording its optical response (pump and probe experiment). As seen above the optical response
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provides a signature of the cluster size. A variation in the optical
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response thus indicates a structure modification. This is exactly what
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one can see on Figure 4. The peak is at the same time broadened and
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shifted to a higher wavelength. Moreover the optical response depends
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on the laser polarization, reflecting a non-spherical shape of the
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cluster. The laser and its preferred coupling to the cluster has thus
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allowed to tailor the system shape. This allows to envision
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potential applications in laser assisted tailoring of materials at the
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nanometer scale. <br>
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<br>
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<table style="width: 100%; text-align: left;" border="0" cellpadding="1"
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cellspacing="0">
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<tbody>
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<tr>
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<td style="vertical-align: middle;" width="100"><img alt="Ag burning"
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src="images/burning2.jpg" width="600" ><br>
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</td>
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<td style="vertical-align: middle;" width="100">Fig.4: Optical response of
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silver clusters embedded in an inert glass (<a href="javascript:lade(4)">details</a>).<br>
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</td>
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</tr>
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</tbody>
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</table>
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<br>
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<center>
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<table width="70%">
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<tr>
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<td align="right">
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<a href="#top">Back to top </a>
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</center>
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