On the electrical resolution and broadening of helium lines

The translatory motions of the radiating atoms in a luminous gas, which furnish a complete and satisfactory explanation of the widths of the spectrum lines which are emitted under certain specified conditions of excitation, are unable to account for the vastly greater broadening which occurs when the spectrum of a gas at a moderately high pressure is produced by the passage of a condensed spark discharge. The suggestion of Stark that the broadening which occurs under these conditions of excitation is intimately connected with the resolution of lines into components by the electric field, being in fact due to the electric field of neighbouring charged particles on the radiating atom, has been shown by the writer to be in qualitative agreement with the broadening which occurs under these conditions in the lines of the Balmer Series of hydrogen, and in a subsequent investigation with Prof. J. W. Nicholson, it was found that a quantitative analysis of the structure of the line Hα was in complete harmony with Stark’s observations of the electrical resolution of this line, on the assumption that the broadening was due to this cause. It was also found that certain effects in helium and lithium were in qualitative agreement with the hypothesis. Without entering into a detailed discussion of the results obtained in the case of hydrogen, it may be stated that the most striking phenomenon observed was that the line Hβ presented an appearance resembling that of a reversal, having a dark narrow space in the centre of the broadened line, which was symmetrical, as in the case of the lines Hα and Hγ . The existence of this dark space is in harmony with the fact that when the line Hβ is resolved by an electric field, there is no component (or possibly a very weak component) occupying the position of the unresolved line in planes polarised either parallel or at right angles to the electric field. On the other hand, both Hα and Hγ showed maxima in the position of the unresolved lines, which is in agreement with the presence of strong undisplaced components in the electrical resolution of these lines. In the case of helium, as Stark has pointed out (loc. cit. ), both the electrical resolution and the broadening are in certain cases unsymmetrical. The phenomena observed are thus concordant, but some recent results of Takamine and Yoshida have provided the material for a closer and more rigorous enquiry into the connection between the two effects.