The spectrum of doubly ionised fluorine (F III)

When discharges of gradually increasing intensity are passed through a vacuum tube containing silicon tetrafluoride (SiF4 ), the lines attributable to fluorine appear successively in four fairly compact groups. Between the first three of these groups it is possible, by careful control of the conditions of discharge, to make a clear separation: the fourth, though distinguishable from the third, is less easily isolated. The first group has already been identified as the spectrum of the neutral atom of fluorine (F I). It is probable, therefore, that the others are assignable to F II, F III, and F IV, respectively. The present paper contains an account of the third group, with a partial analysis. The resemblance between the spectrum and that of singly ionised oxygen (O+ ), which has the same electron structure as doubly ionised fluorine, confirms its assignment to F++ .Experimental Arrangements The lines of the spectrum in question appear when strongly condensed discharges are passed through silicon tetrafluoride at very low pressure. The intensity of discharge necessary is indicated by the fact that the lines do not appear in the photographs which accompany Prof. Fowler’s study of the silicon spectra, although Si III and Si IV are shown there in considerable strength. The method of preparation and purification of the gas has been described in the first paper on the spectrum F I (loc. cit. ). With the stronger discharges, however, the lines of impurities arising from the material of the tube and electrodes were considerably increased in number, and great care was necessary to identify them as completely as possible. Boron, carbon, oxygen, sodium, magnesium and aluminium were the chief impurities found; in particular, the B III pair at λλ 2068, 2066 occurred in surprising strength, and formed the most intense lines in the spectrum. A few lines which have not been identified have been removed from the list of F III lines on account of their appearance in the spectrum, kindly lent by Prof. Fowler, of an "empty” tube containing the minimum amount of air necessary to permit the discharge to pass. The lines of silicon were identified by means of Fowler’s tables.