Abstract
Scanning helium microscopy is an emerging form of microscopy using thermal energy neutral helium atoms as the probe particle. The very low energy combined with lack of charge gives the technique great potential for studying delicate systems, and the possibility of several new forms of contrast. To date, neutral helium images have been dominated by topographic contrast, relating to the height and angle of the surface. Here we present data showing contrast resulting from specular reflection and diffraction of helium atoms from an atomic lattice of lithium fluoride. The signature for diffraction is evident by varying the scattering angle and observing sharp features in the scattered distribution. The data indicates the viability of the approach for imaging with diffraction contrast and suggests application to a wide variety of other locally crystalline materials.
Highlights
There is no other known mechanism which can result in sharp peaks within a z scan, so the peak shape and spacing give an unambiguous signature for diffraction, compared to the smooth variation otherwise expected from diffuse scattering
Most images obtained to date are consistent with diffuse scattering, resulting in topographic contrast
The images and analysis presented above conclusively demonstrate that we have made the first observation of diffraction contrast in helium microscopy
Summary
We present data showing contrast resulting from specular reflection and diffraction of helium atoms from an atomic lattice of lithium fluoride. Contrast in helium images arises as a result of changes in the angular distribution of scattered helium atoms with location on the surface. Almost all SHeM images show contrast consistent with diffuse scattering combined with topographic variations in the surface profile3,7,13 – i.e. at an atomic level the surface appears disordered to the helium beam.
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