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On the basis of a review of existing experimental evidence, theoretical calculations, and other theoretical considerations a table of the observed excited states of ethylene is constructed, with assignments of essentially all lower-energy excited states to definite electron configurations, provided the observed V←N spectrum corresponds to a single transition. The probable alternative that it corresponds to an overlapping of two transitions mixed by twisting leads to difficulties in the assignments.
Robert S. Mulliken (Tue,) studied this question.