Abstract
The impingement rate of a molecular beam is expressed as the product of the density and the mean molecular velocity. Two possible interpretations of the mean velocity are considered and it is shown that, if the mean velocity of molecules “in the beam” is employed, the expression for the impingement rate either becomes erroneous or yields a molecular beam density which is devoid of any real physical significance. It is considered that the prevailing use of the “in the beam” mean velocity is attributable to the widespread belief that the expressions describing the velocity distribution of molecules in a volume in thermal equilibrium may not be applied to molecules in a beam having this volume as source. It is shown that, in most experimental cases, this belief is ill-founded.
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