The formation of helium molecules. II

Abstract
The occurrence of band spectra in the chemically monatomic gases leads to the belief that molecules are formed in the gas by the electrons or photons producing the spectra. Arnot and Milligan (1936) and Arnot and M’Ewen (1938 a ) have shown that in mercury vapour diatomic molecular ions are formed by the attachment of excited atoms in P-states to each other and to normal mercury atoms. Using a balanced space-charge apparatus we obtained results (1938 b ) which led to the conclusion that diatomic helium molecular ions are formed by attachment of metastable atoms of 19.77 eV energy to normal atoms. Although the results given in this first paper on helium appear to admit of no other conclusion, it was desirable to check this conclusion by the far more definite evidence that can be obtained with a mass spectrograph.