Molecular Beam Sources Fabricated from Multichannel Arrays. IV. Speed Distribution in the Centerline Beam

Abstract

The collision model utilized by Giordmaine and Wang to describe the centerline beam intensity from long tubes has been extended to account for molecular‐speed effects. The modification was accomplished by utilizing the velocity‐dependent mean‐free path instead of the Maxwellian averaged value. The analysis shows that the centerline beam intensity (or peaking factor) is practically unaffected by explicitly accounting for the distribution of molecular speeds. The speed distribution on the axis of the beam, however, is depleted of slow‐speed molecules. The perturbation of the Maxwellian spectrum begins at a Knudsen number based on tube length of ∼10. For Knudsen numbers less than ∼0.1, the spectrum is no longer dependent on the Knudsen number. The average translational energy of the centerline beam in the low Knudsen number limit is 5.5% greater than that of a Maxwellian beam.