Chain-length dependence of ultrafast internal conversion dynamics in carotenoids: role of the intermediate state

This work presents a full characterization of the IC₂₁ dynamics in carotenoids with different values of N ranging from 5 to 15. To this purpose, a specially developed spectroscopic system is used, combining very high temporal resolution (pulse width less than 20 fs) and broad spectral tunability. For the longer carotenoids (N 9), the measurements reveal the existence of an additional intermediate excited state lying between S₂ and S₁ states. This state enables the explanation of the nonmonotonic dependence of the S₂-S₁ conversion rate on N.