The Spectra of Argon in the Extreme Ultraviolet

The term tables of A II, A III and A IV have been revised or extended upon the basis of new wavelength measurements in the extreme ultraviolet. A few lines of A I previously known have been remeasured. In the other stages the known lines have been remeasured and new lines added as follows: 23 in A II, 57 in A III, 23 (including some intersystem combinations) in A IV, and 10 in A V (where, none were previously known). These results confirm the identification of the nebular lines $\lambda 4711.4$ and $\lambda 4740.2$ as due to "forbidden" transitions in A IV. The intervals in possible pairs of "forbidden" lines from A III and A V are discussed. Accurate ionization potentials are given, 15.69 volts for A I, 27.80 volts for A II and 40.78 volts for A III, while rough estimates yield 61 volts for A IV and 78 volts for A V.