The photo-isomerization of stilbene

The S₀ potential of stilbene has minima in the trans (t) and cis (c) configurations and a maximum in the perp (p) configuration. The S₁ potential has minima at t, p and c due to crossing of the ¹B_u and ¹A_g potentials, and it is proposed that the T₁ potential has similar minima due to crossing of the ³B_u and ³A_g potentials. Five alternative isomerization mechanisms are considered, the yields of which depend on competition between vertical radiative and radiationless transitions to lower states, and horizontal radiationless transitions between the t, p and c configurations of S₁ and T₁.