Temperature is surprisingly influential on clusters, the tiny building blocks of nanotechnology. For example, thermal energy alone makes some clusters fluctuate in shape. The greater a cluster’s thermal energy, the faster its shape changes and the wider the range of shapes it explores. We have used laser light to selectively destroy clusters in a particular shape, thereby altering the energy distri- bution of clusters in a thermal ensemble and changing that ensemble’s overall temperature. We have then followed the shape fluctuations with ultrafast lasers. Invited Talk, ACS Annual Meeting, Washington, August 30, The cesium-iodide cluster (Cs 4 I 3 – ) changes shape easily between a cube, a ladder, and ring. Laser light that destroys only cube-shaped clusters tends to deplete a thermal ensemble of its lowest-energy clusters, which spend most of their time in the cube shape. The average cluster energy rises and so does the ensemble temperature. Using Laser Light to Control the Temperature of an Ensemble of Clusters Louis A. Bloomfield, University of Virginia, DMR

Clusters composed of cesium-chloride and cesium-iodide are surprisingly soft and many of them change shape easily near room temperature. We have watched such shape-changing for several years with the help of ultrafast laser pulses. In the present work, we use laser light to manipulate and control the thermal ensemble of clusters in our apparatus. Although the ensemble produced by our cluster source has a single overall temperature, the individual clusters have a distribution of energies. As shown in the figure, this thermal spread means that the ensemble contains some clusters with relatively little energy and some with considerably more energy. Each cluster in the ensemble has some extra energy that allows it to vibrate and occasionally to change shape. Those vibrations and shape explorations are represented schematically in the figure as excursions on a potential energy surface. A particular cluster has a specific total energy and therefore explores the potential energy surface at a particular altitude. Explorations at two different altitudes are shown in the figure, one at low altitude for a low-energy cluster and one at high altitude for a high-energy cluster. The low-energy cluster is able to explore only a small region of possible vibrations and shapes, and therefore spends much of its time in the cube shape shown on the left. The high-energy cluster can explore a much broader region of vibrations and shapes and spends far less of its time as a cube. A thermal distribution of clusters near room temperature contains clusters at both energy levels, as well as clusters with many other energy levels. Since the low-energy clusters spend a large fraction of their time in the cube shape, those clusters are especially vulnerable to laser light that destroys cube clusters. When we expose the ensemble of clusters to such light, destroying the cubes, we preferentially destroy the low-energy clusters. The remaining clusters are mostly high- energy clusters that don’t spend much time in the cube form. Subsequent studies using ultrafast lasers pulses confirm that the average energy of the ensemble rises after we deplete the cubes and that the ensemble temperature of the remaining clusters is higher than before the exposure to light.

Education: Five graduate students have contributed to our NSF research program on magnetism and dynamics in clusters. Songbai Ye has been conducting the experimental Cs 4 I 3 – studies using an apparatus in the UVa ultrafast laser laboratory, while Jin Liu has been simulating those experiments computationally. Forrest Payne and Wei Jiang have been measuring magnetism in cobalt and chromium clusters using another experimental facility. The fifth graduate student, Jeff Emmert, is finish up his dissertation on magnetism in cobalt clusters. Societal Impact: Understanding thermal dynamics and magnetic order in small particles is essential to the development of nanotechnology. Our pioneering work on isomerization dynamics provides insights into the long-term thermal stabilities of tiny structures. Our seminal measurements of magnetic order and thermal fluctuations in clusters provides basic information about finite-size effects on spin systems and magnetism, and supports experimental and theoretical work on magnetic nanostructures and magnetic memory. Using Laser Light to Control the Temperature of an Ensemble of Clusters Louis A. Bloomfield, University of Virginia, DMR

Our studies of magnetism in clusters, though not mentioned on the previous slide, are of great interest to theoreticians trying to understand how magnetism is affect by enhanced surface to volume ratios, loss of long-range order, unusual crystalline symmetries, and low-dimensionality in general. They are also critical to experimentalists trying use magnetic nanoparticles to store information. Those magnetic experiments continue apace and detailed measurements will be published shortly.