Ethylene internal twisting

The above considerations lead us naturally to the question of the nature of the one or more intermediate states involved directly in the photochemical reaction. We have displayed in Figure 27 the simplest scheme consistent with the above, admittedly preliminary, experimental results. The prime superscript refers to the frans-isomer, while the processes denoted 1, 2, 3,4, and 5 describe fluorescence, Sx S0 internal conversion, Si -> X intersystem crossing, isomerization, and deactivation to the ground state, respectively. Again, X and X may represent a common state, as, for example, is the case with the twisted triplet in ethylene (cf. Mulliken and Roothaan190 and Kaldor and Shavitt191). As late as 1962, investigators were still unable to determine the source of the temperature... 

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