The arc spectrum of iodine

Abstract

Although a great deal of work has been done on the spectra associated with iodine, very little information concerning the arc spectrum is available. Wood and Kimura give the wave-lengths of a large number of iodine lines in the visible, and separate arc from spark lines. Turner has measured the wave­ lengths of arc lines in the Schumann region and Robertson and Findlay and Landau-Ziemecki have observed iodine spectra in the near ultra-violet under various conditions of excitation. MM. Bloch and Kerris have extended accurate measurements up to λ 7500, but their lists contain almost entirely spark lines. Very recently Iwama has extended observations of the arc spectrum into the infra-red, but his wave-lengths are only approximate and it has been found that many of his lines are spark lines. The only progress that has been made with the analysis is the identification by Turner ( loc. cit. ) of the fundamental 2 P 1/2 , 3/2 difference from the Schumann lines and other papers by the same author, in which suggestions are made as to the origin of some of the Schumann lines. The work of analysis has pre­sumably been impeded by the uncertain classification of arc and spark lines and the absence of accurate measurements in the infra-red. In the present work it has been found that the multiplet intervals are very wide and the spacing between the electronic levels is relatively small, so that many significant lines lie in the infra-red, and there is reason to believe that there are many more lines of still higher wave-length, beyond the reach of the sensitised photographic plate. The uncertainty regarding the classification of arc and spark lines is, in all probability, due to the ease with which iodine vapour may be excited to give complex spark spectra and the resultant difficulty of obtaining a reasonably pure arc spectrum of sufficient intensity to photograph on high dispersion.