Absorption spectra of a cis-trans isomer pair

The isomerization of olefins is believed to take place via an excited state in which the two sp -carbons have been twisted 90° relative to the ground state. This state is referred to as the p (perpendicular) geometry  

It is easy to see that if this twisted geometry were attained, there would be a possibility of returning to either cis or trans ground-state compound. Quantum chemical calculations indicate that the triplet excited state of olefins would have this perpendicular geometry. 

Especially detailed study of the mechanism of configurational isomerism has 

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