Velocity distributions in microskimmer supersonic expansion helium beams: High precision measurements and modeling

S D Eder; A Salvador Palau; T Kaltenbacher; G Bracco; B Holst

doi:10.1063/1.5044203

Velocity distributions in microskimmer supersonic expansion helium beams: High precision measurements and modeling

Rev Sci Instrum. 2018 Nov;89(11):113301. doi: 10.1063/1.5044203.

Authors

S D Eder¹, A Salvador Palau², T Kaltenbacher¹, G Bracco¹, B Holst¹

Affiliations

¹ Department of Physics and Technology, University of Bergen, Allégaten 55, 5007 Bergen, Norway.
² Department of Engineering, Institute for Manufacturing, University of Cambridge, Cambridge CB3 0FS, United Kingdom.

PMID: 30501316
DOI: 10.1063/1.5044203

Abstract

Supersonic molecular beams are used in many applications ranging from spectroscopy and matter wave optics to surface science. The experimental setup typically includes a conically shaped, collimating aperture, the skimmer. It has been reported that microskimmers with diameters below 10 μm produce beams with significantly broader velocity distributions (smaller speed ratios) than larger skimmers. Various explanations for this phenomenon have been proposed, but up till now, only a limited amount of data has been available. Here we present a systematic study of the velocity distribution in microskimmer supersonic expansion helium beams. We compare a 4 μm diameter skimmer with a 390 μm diameter skimmer for room temperature and cooled beams in the pressure range 11-181 bars. Our measurements show that for properly aligned skimmers, the only difference is that the most probable velocity for a given pressure and temperature is slightly lower for a microskimmed beam. We ascribed this to the comparatively narrow and long geometry of the microskimmers which can lead to local pressure variations along the skimmer channel. We compare our measurements to a model for the supersonic expansion and obtain good agreement between the experiments and simulations.