Following a brief historical introduction an overview is given relating the most recent studies of rarefied gas flow to the early work of Knudsen. The first paper submitted in October 1908 (published in 1909) initiated a period of intense activity by Knudsen, Smoluchowski and, a little later, by Gaede and Jangmuir. This also covered the transition to the already well established hydrodynamic flow expressed in terms of the ratio of mean free path to critical apparatus dimension-which we now refer to as the Knudsen number. The desorption, evaporation and scattering of molecules from surfaces was described in terms of the Knudsen cosine law of scattering. The Knudsen effusion method for determining vapour pressure, also introduced in 1909, has become the main tool for studies of the reiated problem of the dissociation, chemical bonding and the vaporisation process itself. Clausing developed, as an alternative to conductances, the concept of transmission probability, still referred to as the Clausing factor, and provided a procedure for their more accurate evaluation in long and short tubes. A number of misconceptions of these early efforts have found their way into the literature and current books on vacuum science and technology. However, detailed studies have clarified the problem of gas-surface interactions; the gas flow in tubes has been tackled with Clausing-type integral equations and by statistical computation techniques based on Monte Carlo analysis procedures adaptable to more complex systems. Results have been confirmed experimentally using molecular-impact pressure probe measuring techniques.

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We present an analysis of electron loss to the continuum (ELC) for 1.6-2.8 MeV He+ colliding with He, Ne and Ar using the first six terms of the multipole expansion method. We give absolute values for E,, and discuss the way in which the method of analysis influences the values of the dipole and quadrupole parameters. Our dipole parameter is slightly positive for the He target and becomes progressively more negative for Ne and Ar. Our quadrupole parameter is slightly positive for He and Ne and nearly zero for Ar. For HeC+ He the magnitude of E,, and its velocity dependence are in reasonable quantitative agreement with a recent theoretical calculation based on the second Born approximation. However our dipole and quadrupole parameters are both more positive than predicted.