The effective cross section of the oriented hydrogen atom

1. Evidence has recently been accumulating that atoms may show an isotropy far greater than would be anticipated on the basis of the atom model developed by Bohr and Sommerfeld; a suggestion which is thrown into strong relief by the examination of matter under conditions of space quantisation in a magnetic field. Evidently, the possibility of orienting at will the momentum axes of certain atoms in a prescribed direction in space offers an ideal means of investigating atomic symmetry. One must be careful, however, in drawing deductions from observations of this kind. Sommerfeld has clearly shown that those atoms which possess a closed electron group in the sense of the Stoner classification of electron levels cannot orient when in the normal state. The experiments of Stern and Gerlach on zinc, cadmium, mercury, tin and lead are in excellent agreement with this idea. In this regard, the observations of, for example, Dymond on the excitation of helium by electron impacts, Rusch§ on the cross section of argon, and of Weatherby and Wolf|| on the dielectric constant of helium cannot be interpreted as indicating marked atomic isotropy; because the inert gases possess a closed group.