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 ϑ larger than the half-width–half-maximum ϑ1/2 the angular distribution is not perturbed by the opaque conditions; for ϑ<ϑ1/2 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 ϑ≳ϑ1/2 the molecules again have an unperturbed Maxwell-Boltzmann distribution.
This paper describes a method for the compensation of the spherical aberration of a polar diatomic molecule in an electrostatic quadrupole field. This perturbation, caused by the higher order terms in the Stark effect, limits the opening angle of the lens and is compensated by an electrostatic hexapole field placed in the middle of the quadrupole field where the influence is the strongest. A detailed mathematical treatment of the motion of a molecule in a composite lens is given and the calculations show that the transmission of the lens increases by a factor 5. Preliminary experiments confirm the theoretical predictions.
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