The structure of the band spectrum of helium.—VI

Much progress in the analysis and interpretation of the helium band spectrum has been made since the publication of Part V of this series about a year ago. Nearly a hundred new branches have been identified, many of them relatively weak and arising from vibrational states of the molecule, and all derived from electronic levels belonging to known sequences. The most important development has been in connection with the interpretation of these levels. It was pointed out by one of us that the He2 spectrum affords the first clear evidence of the existence of electronic levels additional to those characteristic of atoms, and suggestions as to a possible correlation of these with the predictions of Hund were put forward in V. Weizel independently arrived at the same conclusion at about the same time, and has succeeded in accounting for all the known types of He2 term on the basis of Hund's theory. He has also given an explanation of certain peculiarities of the rotation term differences by postulating a change of coupling of the electronic orbital angular momentum. Dieke, working on similar lines, has accounted for the anomalous appearance and intensities of the branches of some of the bands. There have been several other publications, to which reference will be made later, giving details of a number of new bands and remeasurements of some already known. A brief résumé of the above results may help to make what follows more intelligible.