The picture of the photochemically allowed pathways was subsequently further clarified by the work of van der Lugt and Oosterhoff (6; 7). They observed the photoexcited ππ * state to be essentially unreactive with respect to both thermally and photochemically allowed (i.e., conrotatory or disrotatory) distortions. Instead, the doubly excited π * π * state, which is higher in energy at the reactant geometry, crosses with the ππ * state on the photochemically allowed pathway and forms a minimum in the vicinity of the barrier for the thermally forbidden pathway. This minimum is lower in energy than the ππ * state at the reactant geometry (the Franck-Condon region). Due to the structural similarity with the barrier geometry of the thermally forbidden pathway and the similar mechanistic role to the pericyclic transition state of the thermally allowed pathway, this excited state minimum was later called the "pericyclic minimum" . This picture of the photochemical reaction pathway additionally firmly confines the Woodward-Hoffmann rules to predictions of the photochemistry triggered by excitation into low-lying ππ * states, which are characterized by the excitation of one electron from the highest occupied to the lowest unoccupied molecular orbital, both with π character.