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.
Highlights
A great deal of work has been done on the spectra associated with iodine, very little information concerning the arc spectrum is available
Turnerf 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
The high frequency discharge in iodine vapour was used as source in view of the ease with which the conditions of the discharge may be controlled to give a pure arc spectrum ; a further advantage is the absence of internal electrodes
Summary
A great deal of work has been done on the spectra associated with iodine, very little information concerning the arc spectrum is available. In the red and far-red there are a number of lines which, with high dispersion are seen to be close doublets or groups of lines having separations of the order of one or two wave-numbers Their behaviour in the discharge suggests that they belong to the arc spectrum, but they do not appear to arise from combinations with any of the terms which have been identified in this analysis. From consideration of the structures of the arc spectra of chlorine and bromine the transition 6p -> 6s may be expected to give a number of strong lines in the infra-red and 7p 6s another set of strong lines of higher frequency. Spectra the absolute values of the terms 6p &3/2, 7 63/2 may be estimated
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