Molecular ray

O. Stern (Pittsburgh) development of the molecular ray method and discovery of the magnetic moment of the proton. 

When gas molecules hit a solid surface, they may be either reflected, or captured by the surface field of force and condense, to evaporate again later. In Langmuir s adsorption theory the factor a expresses the fraction of the impinging molecules which do condense on the surface. In recent years much information as to the manner of reflection or condensation has been obtained experimentally by the use of molecular rays, and theoretically by wave mechanics. Molecular rays are beams of molecules on which a particular direction has been imposed by slits, moving in a vacuum so high that there are practically no collisions between molecules, and the direction of the beams is maintained for long distances. 

Stern, foreword to Ronald G. J. Fraser, Molecular Rays (Cambridge Cambridge University Press, 1931), x. 

R. G. J. Fraser, Molecular Rays, Cambridge University Press, Cambridge, 1931 N. F. Ramsey, Molecular Beams, Oxford University Press, Oxford, 1956 K. F. Smith, Molecular Beams, Methuen, London, 1955. 

Fig. 6.—Diagrammatic representation of the production of a molecular ray. The furnace 0, which contains the gas, is heated from outside.

We now proceed to explain in detail how the molecular constants enumerated in the preceding section can be determined experimentally. There is first the molecular volume, the determination of which, when neutral molecules are in question, can be effected by the methods of the kinetic theory of gases, already referred to in Chapter I (viscosity, free path, diffusion and direct measurement by molecular rays). The following table shows some molecular diameters so determined, in Angstroms ... 

Recently, Mayer has succeeded in confirming these results by direct measurement, by means of molecular rays. It may be mentioned further, that for ions in solutions of salts there exists an absorption spectrum, which is referable to this process of liberation of the electron from the halogen ion by taking into consideration the action of the surrounding medium (water), the electronic affinity is verified to a rough approximation (Franck and Scheibe). 

Devonshire, A. F. 1936a. The interaction of atoms and molecules with solid surfaces V. The diffraction and reflection of molecular rays. Proceedings of the Royal Society of London. Series A 156 37-44. 

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