Klaus von Haeften

Reader

Department of Physics and Astronomy,
University of Leicester
University Road, Leicester, LE1 7RH

email: kvh6@le.ac.uk
telephone: +44 (0)116 252 3525
fax: +44 (0)116 252 2770

Research

Quantum Fluids and Atomic Clusters

My research is concerned with the basic properties of quantum fluids and with clusters of atoms or molecules. Clusters consist of a few up to thousands of atoms. Their characteristics often show a distinct sensitivity to size. Adding just one atom, for example, may turn a cluster with semiconductor properties into a metal. Production of clusters with control over the size is therefore important. In addition to size-control, the role of chemical composition gains more and more attention. My current research investigates the effect of varying chemical composition for which we have developed a molecular beam-based synthesis method to control the structure of bimetallic clusters so that alloys or core-shell systems can be produced, even in the limit of one single atom: our technique is able to dope single foreign atoms either into the bulk volume or onto the surface of clusters. We investigate the clusters in vacuum where they are free of any interaction, or deposit them into liquids or onto surfaces. The distinction between pure clusters in vacuum and clusters interacting with an environment, be it a dense gas, a liquid, a solid or a surface is important for a comprehensive understanding of how clusters interact and essential for future applications. Deposition into liquids is achieved by co-depositing cluster beams in ultra high vacuum with water vapour beams onto a cold target where the mixture freezes. Melting of the ice yields clusters in liquids. A second effect of the co-deposition is that the cluster surface becomes chemically modified which we have used to create fluorescent sites. Deposition onto surfaces bears the risk that the clusters disintegrate and it is important to keep the impact as small as possible.  Our cluster beam technique has the specific advantage of a low kinetic energy per atom of approximately 50 meV which allows us to soft-land the clusters onto surfaces thereby providing their integrity.

My second research strand deals with quantum fluids. Helium is a unique and fascinating quantum fluid. It exhibits superfluidity, a macroscopic quantum effect, in its condensed phases. Many features of superfluidity can be described in terms of drastically reduced interaction and vanishing friction. The term 'nanosuperfluidity' designates unimpeded molecular rotation and can be regarded as the microscopic counterpart of superfluidity. Much of our current knowledge of nanosuperfluidity has been obtained from experiments with clusters and droplets of helium because only by using these it has been possible to place single foreign molecules into a superfluid. However, the method of using helium droplets has limitations, for example, hydrostatic pressure and temperature cannot be externally varied and not all molecules that would be good benchmark systems are accessible to ro-vibrational spectroscopy. We address these issues with a novel experimental methodology. We use time-resolved rotational spectroscopy to access a wider range of molecular probes for nanosuperfluidity and study how the helium density follows foreign rotating molecules. Furthermore, we use a bulk-helium experiment to control temperature and hydrostatic pressure. We overcome the difficulties of implanting foreign molecules as microscopic rotating probes by using short-lived helium excimers which we produce in continuous corona discharges. The lifetime of the excimers is too short to cause agglomeration and the excimer fluorescence spectrum shows well-resolved rotational lines allowing us to study nanosuperfluidity as a function of pressure and temperature.

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