The spectrum of neutral helium.

It is well known that the spectral terms of helium can be divided into two sets such that no term of the one will combine with a term of the other to produce a spectral line. Both sets are approximately like hydrogen terms.

One set by its transitions gives the ‘para helium’ lines and consists of singulet terms and to it belongs the ‘normal state’ 1s; the other set gives the ‘ortho helium’ lines and (apart from the singulet terms) consists of very narrow doublets (theory suggests that these are probably triplets). The energies of the ortho terms are slightly higher than those of the corresponding para terms.

On account of these two sets of lines, helium had been thought to be a mixture of two gases, para and ortho helium.

Many attempts were made to give the theory of these spectra on the earlier quantum theory, both for normal and excited helium, but all failed to give results even in tolerable agreement with experiment.

The obvious failure of the classical mechanics and the correspondence principle to solve the problem of a nucleus with two outer electrons was one of the factors which compelled Heisenberg to seek for a new quantum mechanics of discontinuous processes to replace the older theory.