The spectrum of doubly ionised oxygen (O 111)

Introductory . In continuation of previous investigations of the spectra of the lighter elements, the present paper gives an account of the spectrum of doubly ionised oxygen. The lines have been observed mainly with suitably strong discharges in vacuum tubes, and have been distinguished from those of O II by the usual method of comparing the intensities of the lines under the action of different discharges. Most of the stronger lines have been measured with considerable accuracy on plates taken with a 10-ft. concave grating, or with a large quartz spectrograph. As was expected, the spectrum is generally similar to that of N II, and the terms so far determined are in entire accordance with Hund’s theory.Notation. It has been found that the system of numeration used for the terms of N II is not conveniently adaptable to certain other spectra, and in the present communication a modified form which seems likely to be of more general application, has been adopted. In particular, it has seemed desirable to assign the same prefix to all terms which arise from the same electron configuration. A convenient procedure, which has already been followed to some extent by others, is to use the orbital designation of the “series electron” as a prefix, with small italic letters for electron orbits, as distinct from the capital letters used for terms. Thus, 3s 3 P2 will indicate that the3 P term in question is due to a configuration in which the series electron occupies a 31 orbit. Combination possibilities are then shown by the application of the ordinary selection rule for azimuthal quantum numbers to the electron symbols, in addition to the inner quantum number rule applicable to the term symbols. For example, a 2s 1p1 might combine with a 3p1 P 1 , but not with a 3s 1 P1 or 3d 1 D2 term. In this way it becomes unnecessary to “dash” any of the term or electron symbols in connection with the simpler spectra.