The structure of the band spectrum of helium. —III. The doublet bands

Abstract

The discovery by Fowler, in 1915, that certain of the helium bands, named by him the “doublet bands,” were distributed according to a line-series law, presented several features of interest. In the first place, no other case of the kind was known, nor, indeed, has any been recognised since. Yet on the accepted view of the origin of band spectra it would appear to be a fairly probable occurrence. Secondly, the theory of the production of line series is now so far advanced that the above-mentioned peculiarity of these bands may be expected to afford valuable assistance in their analysis and interpretation, and thus, in spite of its unique character, contribute something to the solution of the general problems of band spectra. In a previous communi­cation the results were given of a preliminary study of these bands, from which it appeared that the broad features of their structure were in conformity with the quantum theory. But in matters of detail the agreement with theory was far from complete. Not only were the theoretical formulæ incapable of accurately representing the separate branches, but the positions of the latter were not as expected, one of them being apparently displaced by a very con­siderable amount. Further, although a combination law was discovered, there remained some doubt as to whether it was strictly or only approximately valid; this is a question of some importance, since such relationships are in general the key to the interpretation of band spectra. Since the previous paper was written, appreciable progress has been made in the theory of the subject, and it seemed possible that some of the results might aid in clearing up the points at issue, particularly if further data could be secured. On the grating plates originally used only seven of the bands appeared, three of the “main series” and four of the “second series,” and although further members had been photographed with a small quartz spectro­graph, the dispersion was insufficient to reveal their structure. A large quartz Littrow type instrument having recently become available, there seemed to be a prospect of making a valuable extension of the series, and this was accord­ingly attempted.