Photosynthetic Light Harvesting by Carotenoids: Detection of an Intermediate Excited State

Abstract
We present the first direct evidence of the presence of an intermediate singlet excited state (Sx) mediating the internal conversion from S2 to S1in carotenoids. The S2 to Sx transition is extremely fast and is completed within approximately 50 femtoseconds. These results require a reassessment of the energy transfer pathways from carotenoids to chlorophylls in the primary step of photosynthesis.