In this award, funded by the Experimental Physical Chemistry Program of the Division of Chemistry, Professor Andrey Vilesov of the University of Southern California, together with his graduate student researchers, will study the aggregation of atomic and molecular species in ultracold, superfluid helium droplets. Species to be studied include metal atoms, methane, water, ammonia, hydrogen chloride and parahydrogen clusters, among others. Cluster sizes span the range from a few atoms/molecules to tens of millions of solute species. The goal of these studies is to investigate how molecules interact with each other and with this rather unusual solvent over a wide range of size. The experiments will be carried out in the gas phase with mass spectrometric and laser spectroscopic methods.
The ultimate aim of research like this is to learn about new ways in which matter can interact in a highly unusual environment. The large clusters to be studied in this work are intermediate between molecular and bulk (mesoscale) and are predicted to have unusual properties. The very large helium droplets that Vilesov proposes studying may be useful vehicles for making interesting kinds of new materials. Students working with Prof. Vilesov will receive excellent training in a number of areas of high-technology -- including lasers, vacuum science and electronics. Prof. Vilesov has a strong history of international collaboration, and this will likely continue with this work.
Arrays of nanometer-sized metal clusters attached to an inert surface are of widespread use in chemical catalysis, solar energy harvesting, and in spectroscopic studies of single molecules. Some obstacles often encountered in such areas, however, are the production of size-selected clusters, their gentle deposition onto a surface, and the stability of such arrays of clusters which often uncontrollably agglomerate. Therefore, through an award funded by the Chemical Structure, Dynamics, and Mechanisms Program of the Division of Chemistry, Prof. Andrey Vilesov from the University of Southern California, along with his group of graduate and undergraduate student researchers, attempted to address these outstanding obstacles. Their strategy was to study the controlled assembly of atoms and molecules, in nanometer-sized droplets of superfluid helium at the ultra-low temperature of about -460 °F, into such clusters. Specifically, they focused on the assembly of silver atoms into silver clusters and even the formation of mixed clusters of silver with hydrocarbon molecules. The cluster size range was inclusively wide spanning from clusters comprised of just a few atoms and/or molecules to tens of millions. By employing laser methods in conjunction with mass spectroscopy, the experiments were carried out in the droplets themselves, which were used to grow the clusters. In addition, the silver clusters grown in the droplets were gently deposited onto a surface and imaged using an electron microscope. The results of this study show that the helium droplet technique offers a way to produce clusters of predetermined size, and that it is amenable toward the formation of clusters of mixed composition – as are often encountered in catalytic applications. Laser spectroscopic study of the light-absorbing properties of the silver clusters also revealed different pathways to cluster formation in the droplets and helped elucidate the resulting cluster structures along with their corresponding optical excitations. Furthermore, it was shown that such clusters can be deposited onto a substrate, and that they are stable throughout the imaging process. The images revealed insight into the silver cluster size distributions and confirmed visually the efficacy of the helium droplet technique for controlled cluster growth. The broader impacts to the scientific community include the offer of a unique approach to cluster production and an extension of the variety of nano-clusters currently available to researchers. This NSF-funded work was a collaborative effort between groups at the University of Southern California from different disciplines. It actively involved both undergraduate and graduate students, including those from historically underrepresented groups, allowing them to develop their scientific and research potential.
