Velocity distribution and angular distribution of molecular beams from multichannel arrays

Three stainless-steel multichannel arrays with channels of 16, 50, and 140 μm diameter have been investigated experimentally. The whole flow pattern is measured, including the angular dependence of the velocity distribution. All measurements concern the opaque mode, with λ≤L and λ≥α, where λ is the mean free path in the source and α and L are the radius and length of the channel, respectively. The results are compared with the predictions for free molecular flow in the same channel, as a function of the reduced source density n*=L/λ. The peaking factor is 30% lower than predicted by the Giordmaine-Wang model. At angles cursive-theta sign larger than the half-width-half-maximum cursive-theta sign_1/2 the angular distribution is not perturbed by the opaque conditions; for cursive-theta sign<cursive-theta sign_1/2 it levels off to a lower peaking factor due to operation in the opaque mode. The center-line beam shows an increasing loss of slow molecules for increasing n*. At n*=10 the gain in mean translational energy is 15%, much larger than predicted by the model of Olander. The deformation of the velocity distribution decreases with increasing angle, and for cursive-theta sign≳cursive-theta sign_1/2 the molecules again have an unperturbed Maxwell-Boltzmann distribution.