Studies of beams of nanoclusters (small-sized aggregates made up of several to several thousand atoms or molecules) utilize atomic and molecular beam concepts and methods to gain insight into the microscopic mechanisms of nano-scale as well as bulk-scale and surface phenomena. This project will utilize laser-induced fragmentation and electric field-induced deviation of metal and water cluster systems possessing strong permanent or induced electric dipole moments. The main goal is to investigate the electric susceptibility of water, metal, and "superatom" clusters as a function of their internal temperature. "Superatoms" are particularly stable and symmetric clusters which have been proposed to possess closed electronic and geometrical shells. The results will reveal such effects as the melting transition of a nanoscale icicle, charge transfer between gold and silver clusters and surface impurities, the formation of "superatomic molecules," and the interplay between electronic and structural motifs as the particle size increases atom-by-atom. The experiments will provide stringent tests and benchmarks for a number of current theoretical models. On the technical side, the broader impact of the project lies in its relevance for understanding nanomaterial building blocks and for optimization of molecular sensors and nanocatalysts. On the human side, the program will offer graduate students thorough training in an inherently interdisciplinary field, promote undergraduate involvement in research, and contribute to interdepartmental recruitment and outreach activities.
Clusters are agglomerates, clumps, of a finite number of atoms or molecules, bigger than a small molecule but smaller than a 'crumb' of bulk-like matter. By studying them flying in beam, one is able to investigate fundamental quantum phenomena at the intersection of atomic, molecular and nanometer scales by taking advantage, on one hand of precise molecular-beam techniques, and on the other hand of a variety of unique and advantageous cluster properties. For example, metal clusters can behave as "artificial atoms;" clusters can react strongly to external electric and magnetic fields and to being illuminated by light; and by varying the cluster size one is able to tweak and tune various characteristics of the particle. The subject matter is interdisciplinary, with relevance to such active fields of physics research as collisions between cold polar molecules, avenues for controlling and understanding scattering and chemical interactions, nanoscale optics, atmospheric and climate-related phenomena, quantum-mechanical design of electronic materials, and other interesting and important topics. The present project focused on two types of clusters: those composed of metal atoms, and of water molecules. The former where investigated by impacting them with laser pulses, and with a low-energy electron beam. The fundamental aim is to understand what happens to the energy deposited into the cluster by the captured photon or electron. It is found that absorption of light gives rise to characteristic oscillations of the cloud of electrons within the cluster, including oscillations of the type that cannot be produced by illuminating a bulk material with a laser beam. This shows that clusters may hold promise as building blocks of materials with novel optical properties, as well as the basis for sensitive optical detectors. As a final step, clusters that absorb a photon or an electron heat up and shrink by evaporation. A detailed analysis of this process enables one to understand the microscopic mechanisms of evaporation. Interestingly, the process has close similarities to how highly-excited nuclei disintegrate. The work on water clusters focused on the following issue. It is well known that in a water solution of an acid, the acid molecules separate into positive and negative constituents (forming a so-called ion pair). One of the active areas of research is the question, how many water molecules need to be attached to an acid molecule for the latter to fall apart? This is important for gaining an essential microscopic understanding of the process of solvation; acid-water agglomerates also play an essential role in the chemistry of the atmosphere, such as ozone depletion and engine exhaust. In the present project, the answer to the aforementioned question is sought be measuring how strongly a beam of water clusters with an embedded acid molecule deflects if acted upon by an electric field. This behavior yields insight into the structure of the acid-water complex.
