The third spark spectrum of arsenic (as IV)

Abstract

The third spark spectrum of Arsenic has been investigated in the extreme ultra-violet by Sawyer and Humphreys, who have found the three triplet combinations of the deep term 4 p 3 P with 5 s 3 S, 4 d 3 D and 4 p 2 3 P and the singlet resonance line 4 s 1 S 0 — 4 p 1 P 1 . Two more triplets involving the term 5 p 3 P were discovered later by the writer. The present paper deals with the identification of the higher members of the triplet system as well as the singlets and inter-combination lines of the spectrum. Most of the lines which could be assigned to the trebly-ionised atom of Arsenic have been classified and it is now possible to estimate the absolute values of the characteristic terms. Experimental . The general experimental methods employed in the present investigation on the spectrum of Arsenic have been briefly described already in two previous communications dealing with the arc and the second spark spectrum of Arsenic. For the excitation of the third spark spectrum, the chief source which has proved useful is a condensed discharge through Arsenic vapour contained in a capillary tube of about ½ or 1 mm. bore. Constant heating of the tube was found necessary, in order to have sufficient pressure of the vapour in the capillary and to maintain a steady discharge. After running the tube for a few hours, however, it was found to require very little attention. The lines of As IV are but feebly represented in a spark in hydrogen and are absent altogether in a spark in air between two poles of the metal. This has made possible the selection of the lines belonging to the fourth stage of ionisa­tion of the atom. In the extreme ultra-violet below λ 1400, the chief source employed was a highly condensed spark in vacuo between aluminium electrodes containing arsenic. These photographs were taken with a Siegbahn vacuum spectrograph at Upsala; the method has been described fully elsewhere. As in the case of selenium, a coil of self-inductance consisting of 31 turns of copper wire 0·4 mm. thick, 60 cms. long and 73 cms. in diameter, served the purpose of distinguishing the lines due to the different stages. Lines of As II and As III were present in this inductance spectrum strongly, while those of As IV were considerably weakened and those of As V absent altogether.