A practical consideration of scanning helium microscopy

The established field of Helium Atom Scattering (HAS) has long made use of neutral helium to offer unique opportunities with regards to surface characterisation. A thermal helium atom is an ideal probe particle: strictly surface sensitive, totally inert, a wavelength of the order of typical crystallographic dimensions, and well matched in both energy and momentum to dynamic surface processes. Technological limitations have restricted HAS to broad illumination of a sample surface. The development of a spatially resolved version of the technique - a Scanning Helium Microscope or SHeM - forms the basis for the work presented in this thesis. Such an instrument would prove of great benefit to the wide range of samples (including delicate adsorbate structures, organic molecules and biological materials) which suffer damage under the energetic probes of traditional microscopies. Chapter 1 first reviews the nature of the helium atom-surface interaction (and the possible contrast mechanisms that arise as a result), before looking at the intensity constraints that have prevented the manufacture of a SHeM previously. Chapters 2 and 3 concern the development of a prototype instrument – the Mark I SHeM. A detailed discussion of the design decisions is included, followed by experimental studies conducted with the new instrument. With the successes found with the prototype, progress then began on creating an instrument from the ground up. Chapter 4 covers the design of the Mark II SHeM, as well as the performance improvements as compared to its predecessor. The experimental investigations into not only samples but the technique itself are explored in Chapter 5. These include studies of image formation, secondary beam effects, contrast mechanisms, and fundamental instrument optics. Finally, Chapter 6 comprises a review of the state of the emerging field with a particular focus on the technical requirements to more fully harness each of the available contrast mechanisms.