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Title: Quasi-elastic helium atom scattering : interpretation and instrumentation
Author: Jardine, A. P.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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Abstract:
Quasi-elastic helium atom scattering (QHAS) provides a powerful probe of surface diffusion. Unlike other techniques, QHAS is sensitive to the mechanism of diffusion and can measure processes which occur over the picosecond timescales and sub-nanometre distances which are characteristic of atomic diffusion. The work presented in this dissertation aims to widen the applicability of the QHAS technique. We introduce the helium scattering technique and perform a series of calculations which explore the origin of the observed giant helium scattering cross section at isolated defects. The presence of the giant cross section makes QHAS a powerful tool, particularly at low coverages. We develop an approach to apply exact close coupled calculations to scattering from isolated step edges and adatoms. From our calculations, we conclude that the origin of the giant cross section, for metal adatoms and steps, is different from previous suggestions for molecular adsorbates. For metals, the scattering must be predominantly from the repulsive part of the helium-surface interaction potential. We present a comprehensive review of the QHAS technique in chapter 3. We consider all the QHAS results published to date, the methods of data analysis and the conclusions drawn. Our review highlights a recent debate over the interpretation of QHAS data, in the light of recent and apparently contradicting first principles calculations. In chapter 4, we apply molecular dynamics simulations to CO diffusion on Cu(001), with the aim of resolving the debate. Our results show that the two sets of data are, in fact, in moderate agreement, given the limitations of both experiment and theory. The unusually low activation energy measured by QHAS arises from the low friction regime of the system. The remainder of the thesis is devoted to the development of a novel, high intensity, spin polarised and focussed 3He beam source. The beam source forms the first stage of a unique 'Spin-Echo' spectrometer, which will provide over 2 orders of magnitude improvement in experimental energy resolution. In particular, we develop a unique polarising hexapole magnet, which unlike existing magnets is capable of being used with the high intensity atomic beams necessary for OHAS measurements. Chapter 5 covers the construction of the apparatus, including the design of several major components; the hermetically sealed recycling system, a novel electrical control system and flexible software to run the equipment. Chapter 6 concentrates on the design and construction of the critical component; the polarising hexapole magnet. Careful optimisations of the polepiece shape, combined with vacuum characteristics simulation leads us to a design uniquely suited to use with high intensity atomic and molecular beams. Finally, the operation of the beam source and the behaviour of the hexapole magnet has been characterised in detail and is presented in chapter 7. A series of measurements, using 4He, demonstrate that the beam source is behaving normally. We explain the origin of the low temperature attenuation that is widely observed. Using 3He we characterise the behaviour of our novel hexapole magnet. The focussed beam profiles are in excellent agreement with our design specifications, demonstrating that the magnet is working correctly. Overall, our experimental results demonstrate that 'high-energy' spin echo is possible and lay the foundations for a spectrometer capable of exploiting the QHAS technique fully.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.605060  DOI: Not available
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