Ab initio molecular dynamics study of cis–trans photoisomerization in ethylene

We have used ab initio multi-electronic state molecular dynamics to study the photoinduced cis–trans isomerization of ethylene. The initial motion on the excited state is a stretching of the CC bond and the photoisomerization begins within ∼50 fs of optical excitation. Quenching to the ground electronic state is found to be ultrafast and proceeds from an ionic state via a conical intersection. Accessing the conical intersection requires pyramidalization of one of the methylene groups and this can happen only after energy is funneled from the twisting mode into the pyramidalization mode.