The emission band spectrum of chlorine (Cl + 2 ). II

Abstract
In continuation of our work (Elliott and Cameron 1937 a ) on the spectrum of chlorine excited by a high-frequency electrical discharge, we have established the fact that the discrete bands which appear are due (at least in part) to the ionized molecule Cl + 2 . In the publication referred to, we have described measurements of the wave numbers of the band heads in the region 3864-5435 A, as well as of the maxima of some continua in the ultra-violet. An extensive vibrational isotope effect was identified in the discrete bands, and on the basis of this a suggestion was made that two systems were present, with origins situated at 21000 and 21340 cm. -1 respectively. The vibrational analysis of Ota and Uchida (1928) was shown to be untenable, but no alternative analysis was put forward. It was suggested that the intervals of 627 and 632 cm. -1 , which occurred frequently, might be the vibrational frequency of the chlorine molecule in either the upper or the lower state involved. The bands considered in the present paper, with the exception of that at 26166∙5 cm. -1 , were all included in the first paper, but in some cases the wave numbers of the heads have been slightly amended. About one-half of the ninety-four bands previously tabulated (excluding isotopes) have now been classified and assigned to Cl + 2 . The two systems previously mentioned are now identified as the subsystems of a doublet system; the system origins are much more accurately determined and are found to be situated at 20596∙9 and 20797∙3 cm. -1 respectively. It is found that the intervals 627 and 632 (more accurately 627∙9 and 633∙7 cm. -1 ) are the vibrational frequencies of Cl + 2 for the hypothetical levels with vibrational quantum numbers 2½ and 1½.
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