About 200 additional lines have been classified in Cl III. 21 of these are intercombination lines. This analysis fixes most of the doublet and quartet terms arising from the addition of a $4s$, $5s$, $4p$, $3d$ or $4d$ electron to either the ³P or ¹D state of the core. Approximately 50 new lines have been classified in Cl IV and 20 in Cl V.
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I. S. Bowen (1934) studied this question.