The spectrum of ionised oxygen (O II)

Two different line spectra of oxygen have long been known under the names "compound line" elementary line” spectra, assigned by Schuster in 1879 on the supposition that complex and simplified molecules or “molcular groupings” were respectively involved in their production. The possibility of a further modification of the line spectrum was also foreshadowed by Schuster, in collaboration with Roscoe, in the observation of a line at λ 5592 when a condensed discharge was passed through oxygen in a short and narrow capillary tube. Additional lines of this third type were observed later by Lunt, and further investigated by Fowler and Brooksbank. In accordance with present views as to the origin of spectra, the three spectra are attributed to neutral, singly-ionised, and doubly-ionised atoms, are designated O I, O II, O III, or O, O+, O++. Other spectra representing higher degrees of ionisation are theoretically possible, and evidence of the production of O IV and O V in the spectra of vacuum sparks has been obtained by Millikan and Bowen. The first three spectra are of considerable interest to astrophysicists on account of their occurrence in stars of successively higher temperatures, in strict accordance with the results of laboratory experiments. Among other points of interest, the analysis of these spectra may eventually lead to trustworthy values of the successive ionisation potentials, and thence to important deductions as to stellar temperatures. This result, however, has only at present been finally attained with respect to O I, through the extension of the observations into the extreme ultra-violet by Hopfield.