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Free-jet atomic, cluster & molecular sources are used to produce beams of low-energy, neutral particles and find application in a wide array of technologies, from neutral atom microscopes to instruments for surface processing. However, understanding and maximising the intensity derived from such sources has proven challenging, partly because of the interference of gas in the vacuum chamber with the beam. Here, we describe a semi-analytic method to estimate the resulting attenuation in low-pressure free-jet sources. We explicitly calculate the two contributions to the interference, which arise from the interaction of the beam with (i) background gas and (ii) atoms backscattered from the 'skimmer', a conical aperture designed to extract the centre of the expansion. We demonstrate that at high stagnation temperatures the former dominates, suggesting that the design of the skimmer has an insignificant impact at around room temperature. We also determine a number of scaling relationships and analytic formulae to make it easier to include 'skimmer interference' in future calculations of the centre-line intensity. We present experimental evidence to support our approach. Finally, we apply the theory to develop a number of design recommendations for the skimmer and vacuum chamber to enhance desirable beam properties for spectroscopy and microscopy when using cryogenic-temperature atom or molecular beams., Comment: 18 pages, 9 figures main text, 7 figures appendices

Three-dimensional mapping of surface structures is important in a wide range of biological, technological, healthcare and research applications, including taxonomy, microfluidics and fabrication. Neutral helium atom beams have been established as a sensitive probe of topography and have already enabled structural information to be obtained from delicate samples, where conventional probes would cause damage. Here, we demonstrate empirical reconstruction of a complete surface profile using measurements from a scanning helium microscope (SHeM), using the heliometric stereo method and a single detector instrument geometry. Results for the surface profile of tetrahedral aluminum potassium sulphate crystals demonstrate the areas of surfaces and facet orientations can be recovered to within 5% of the expected values., Comment: 7 pages (including bibliography), 5 figures (including Appendix). Published at Applied Physics Letters on 14th May 2024 (doi.org/10.1063/5.0206374)

In studies of dynamical systems, helium atoms scatter coherently from an ensemble of adsorbates as they diffuse on the surface. The results give information on the co-operative behaviour of interacting adsorbates and thus include the effects of both adsorbate-substrate and adsorbate-adsorbate interactions. Here, we discuss a method to disentangle the effects of interactions between adsorbates from those with the substrate. The result gives an approximation to observations that would be obtained if the scattering was incoherent. Information from the experiment can therefore be used to distinguish more clearly between long-range inter-adsorbate forces and the short range effects arising from the local lattice potential and associated thermal excitations. The method is discussed in the context of a system with strong inter-adsorbate interactions, sodium atoms diffusing on a copper (111) surface.

The interfacial behaviour of water remains a central question to fields as diverse as protein folding, friction and ice formation[1,2]. While the structural and dynamical properties of water at interfaces differ strongly from those in the bulk, major gaps in our knowledge at the molecular level still prevent us from understanding these ubiquitous chemical processes. Information concerning the microscopic motion of water comes mostly from computational simulation[3,4] but the dynamics of molecules, on the atomic scale, is largely unexplored by experiment. Here we present experimental results combined with ab initio calculations to provide a detailed insight into the behaviour of water monomers on a graphene surface. We show that motion occurs by activated hopping on the graphene lattice. The dynamics of water diffusion displays remarkably strong signatures of cooperative behaviour due to repulsive forces between the monomers. The repulsive forces enhance the monomer lifetime ($t_m \approx 3$ s at $T_S = 125$ K) in a $\textit{free-gas}$ phase that precedes the nucleation of ice islands and, in turn, provides the opportunity for our experiments to be performed. Our results give a unique molecular perspective of barriers to ice nucleation on material surfaces, providing new routes to understand and potentially control the more general process of ice formation.

Spin-echo instruments are typically used to measure diffusive processes and the dynamics and motion in samples on ps and ns timescales. A key aspect of the spin-echo technique is to determine the polarisation of a particle beam. We present two methods for measuring the spin polarisation in spin-echo experiments. The current method in use is based on taking a number of discrete readings. The implementation of a new method involves continuously rotating the spin and measuring its polarisation after being scattered from the sample. A control system running on a microcontroller is used to perform the spin rotation and to calculate the polarisation of the scattered beam based on a lock-in amplifier. First experimental tests of the method on a helium spin-echo spectrometer, show that it is clearly working and that it has advantages over the discrete approach i.e. it can track changes of the beam properties throughout the experiment. Moreover, we show that real-time numerical simulations can perfectly describe a complex experiment and can be easily used to develop improved experimental methods prior to a first hardware implementation.

Helium spin echo experiments combined with ab initio-based Langevin molecular dynamics simulations are used to quantify the adsorbate-substrate coupling during the thermal diffusion of Na atoms on Cu(111). An analysis of trajectories within the local density friction approximation allows the contribution from electron-hole pair excitations to be separated from the total energy dissipation. Despite the minimal electronic friction coefficient of Na and the relatively small mass mismatch to Cu promoting efficient phononic dissipation, about $(20\pm5)\%$ of the total energy loss is attributable to electronic friction. The results suggest a significant role of electronic non-adiabaticity in the rapid thermalization generally relied upon in adiabatic diffusion theories., Comment: online abstract: https://journals.aps.org/prl/accepted/d407dYecI5d1684758993a12fec0464ce50191245

We describe a method for text entry based on inverse arithmetic coding that relies on gaze direction and which is faster and more accurate than using an on-screen keyboard. These benefits are derived from two innovations: the writing task is matched to the capabilities of the eye, and a language model is used to make predictable words and phrases easier to write., Comment: 3 pages. Final version