Abstract
A ray tracing method for predicting contrast in atom beam imaging is presented. Bespoke computational tools have been developed to simulate the classical trajectories of atoms through the key elements of an atom beam microscope, as described using a triangulated surface mesh, using a combination of MATLAB and C code. These tools enable simulated images to be constructed that are directly analogous to the experimental images formed in a real microscope. It is then possible to understand which mechanisms contribute to contrast in images, with only a small number of base assumptions about the physics of the instrument. In particular, a key benefit of ray tracing is that multiple scattering effects can be included, which cannot be incorporated easily in analytic integral models. The approach has been applied to model the sample environment of the Cambridge scanning helium microscope (SHeM), a recently developed neutral atom pinhole microscope. We describe two applications; (i) understanding contrast and shadowing in images; and (ii) investigation of changes in image formation with pinhole-to-sample working distance. More generally the method has a broad range of potential applications with similar instruments, including understanding imaging from different sample topographies, refinement of a particular microscope geometry to enhance specific forms of contrast, and relating scattered intensity distributions to experimental measurements.
Highlights
Neutral atom beam microscopy is an emerging technique that uses a focused or collimated beam of neutral atoms, principally helium, as a microprobe
With emerging evidence for chemical contrast (Barr et al, 2016) and the potential for diffractive and interference contrast, it becomes important to understand which features in images can be explained purely by diffuse scattering through sample topography
Scanning helium microscopy uses a collimated thermal energy helium beam as a probe that is rastered over the surface of a sample, to create a spatially resolved image
Summary
Neutral atom beam microscopy is an emerging technique that uses a focused or collimated beam of neutral atoms, principally helium, as a microprobe. Presented here is the development of a computational framework to construct topographic contrast in a scanning helium microscope (SHeM) from a 3D model of the sample and signal collection environment. Beam atoms may scatter multiple times from the sample and its surroundings, while still continuing to be detected These multiple scattering processes can cause unexpected features in images, such as diffuse illumination (Witham and Sanchez, 2014); and could not be properly modelled using integral models (Hedgeland et al, 2005). In addition the approach can be used to investigate instrumental factors such as the transmission of atoms through a particular detection geometry, and the consequence in images of the effusive beam components that have been seen in previous work (Fahy et al, 2015).
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