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Title: The development of quasi-elastic helium-3 spin-echo spectroscopy as a tool for the study of surface dynamics
Author: Hedgeland, H.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2007
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Abstract:
The first chapter of the work introduces the basic principles of the spin-echo technique and the spectrometer instrumentation, establishing it within the context of both neutron and helium atom scattering. The following two chapters describe the ongoing development of the spectrometer to increase its signal level and the range of surface science systems accessible to study. Chapter two details the development of the detector. The approach taken is a combination of progressive experimental improvements to the existing electron-impact ionisation detector, resulting in an order of magnitude increase in efficiency, as well as simulations modelling an alternative design for the next generation of ioniser. In chapter three, we move from the detector to the helium beam source, modelling from first principles anomalous attenuation seen in its performance. We develop an understanding of the behaviour seen which allows us to suggest improvements to the source design. The remaining chapters present studies of three surface science systems: potassium on copper, benzene on copper and benzene on graphite. Chapter four concerns the diffusion of potassium on copper, developing from previous studies of sodium. We find coverage dependent behaviour and establish an anisotropic potential energy surface with barriers varying from 80 to 40 meV and a friction coefficient of 0.25 ps-1 for low coverages. We further discuss the deviations seen from these parameters at higher coverages. In chapter five we study a more complex molecule with the diffusion of benzene on copper. A simple jump diffusion mechanism is found with diffusion barriers of the order of 125 meV. Molecular dynamics are again used in combination with the data to study the potential energy surface. The final chapter consists of detailed observations of the diffusion of benzene on a graphite substrate. From quasi-elastic helium atom scattering data we establish a continuous diffusion mechanism with a friction coefficient of 5.5 ps-1 and a maximum surface corrugation of 30 meV. Complementary neutron spin-echo measurements are also presented, enabling for the first time a direct comparison between the two techniques. Broad agreement is found with the helium spin-echo benefiting from higher signal levels.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.603936  DOI: Not available
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