Experiments on molecular scattering in gases. I.-The method of crossed molecular beams

The development of the technique of molecular rays has made possible a direct experimental study of collision phenomena in gases. Hitherto our knowledge of molecular fields has been derived indirectly; mainly from experiments on gaseous viscosity and diffusion, which demand inevitably a statistical interpretation. On the other hand, direct measurement of the angular distribution of the molecules scattered from a beam in traversing a gas at low pressure is now practicable; and it seemed worth while to develop a method of investigating intermolecular collisions which should make use of the new technique. The experiments described in this paper are a first step in this direction. Ideally one would wish to study the mutual scattering of two similar or dissimilar beams of nearly homogeneous velocity; but it was considered advisable, in orienting experiments such as the present, to aim at a compromise. In the first place, therefore, attention has been confined to the scattering of one beam only, which we may call the primary beam. In the second place, the choice of substances for the other beam, the scatterer, has been confined to those of which the molecules are very heavy and sluggish compared with the molecules of the primary beam; in such case, the scattering of the primary beam is sensibly symmetrical about its own direction. As constituents of the primary beam we are using in the first instance the alkali metals, mainly because there already exists for them in the. surface ionization gauge an extremely sensitive detector.