Minimising Interference in Low-Pressure Supersonic Beam Sources

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