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Solids interact with their surroundings through their surfaces, so it is the surface properties which dominate the reactivity of solids as catalysts, their response to corrosive environments, and in the way they can bond to other solids to make electronic devices. Knowledge of the structure of these surfaces (the relative positions of the atoms and molecules) is the key to most attempts to understand the chemical and electronic properties. This project seeks to use one particular specialised method, scanned-energy mode photoelectron diffraction (PhD), to investigate a series of specific structural problems selected to give insight into distinct classes of surface processes. The special features of this method are that it can provide the local geometry of specific atoms within a molecule bonded to a surface, distinguishing atoms of different elements and atoms of the same element in different local bonding environments. This means that one can tackle problems of increasing complexity and thus of increasing relevance to 'real world' surface processes. For example, the interaction with surfaces of large biological molecules, such as proteins and DNA, is potentially important in medical screening for disease, and in ensuring the body does not reject medical implants (e.g. artificial hip joints).These molecules are far too large for their surface interaction to be probed at an atomic scale with any available methods. However, these molecules interact with surfaces through key molecular components, and while even these 'small' molecules present a major challenge for surface structure determination, use of the PhD method should allow the local bonding structure to be determined; one objective of this work is thus to understand the interaction of surfaces with the simplest of these component molecules. Other problems to be addressed mostly involve simpler molecular and atomic adsorbates, but more complex surfaces. In particular, while most surface science experiments have been performed on metal and semiconductor surfaces, a very important class of materials with surface chemical properties of practical importance are oxides, but these are mostly insulators and prove difficult to study by standard methods. In particular, there is a dearth of structural information on oxide surfaces. Recent work using the PhD technique to probe the surfaces of ultra-thin oxide films has proved very successful in obtaining adsorption structures on model surfaces, some of the results highlighting failures in current theoretical understanding of these materials. A significant extension of this work to make inroads into our understanding of molecule-oxide surface interactions is envisaged as a key ingredient of this research programme.The project also aims to explore an important novel extension of the technique to allow structure determination under 'real' chemical reaction conditions. The way atoms and molecules interact with surfaces underpins the hugely important area of heterogeneous catalysis, but the practical application of this technology, such as the clean-up of car exhaust gases in the car's catalytic converter, operate around one atmosphere of pressure. Most methods to probe surface properties, however, have been developed to work only in very good ('ultra-high') vacuum conditions; lowering the pressure of the gas (air) surrounding a surface lowers the rate at which molecules arrive from the air and contaminate the surface, and to ensure this process is slow enough to keep a sample clean for an hour or so, one needs to work in a pressure of about one million-billionth of an atmosphere (comparable to, but somewhat higher than, the pressure in outer space). Finding a way to obtain local structural information at pressures closer to those of 'real' reactions is thus an important goal, and this proposal seeks to explore one possible method to achieve this in by a method capable of unravelling the complexity of such a surface.
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