Photosynthetic light harvesting by carotenoids: Detection of an intermediate excited state

We present the first direct evidence of the presence of an intermediate singlet excited state (S-x) mediating the internal conversion from S-2 to S-1 in carotenoids. The S-2 to S-x 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.