We present time-resolved fs two-photon pump-probe data measured with photosystem I (PS I) of Thermosynechococcus elongatus. Two-photon excitation (λexc/2) 575 nm) in the spectral region of the optically forbidden first excited singlet state of the carotenoids, Car S1, gives rise to a 800 fs and a 9 ps decay component of the Car S1f Sn excited-state absorption with an amplitude of about 47 ( 16 % and 53 ( 10%, respectively. By measuring a solution of pure "-carotene under exactly the same conditions, only a 9 ps decay component can be observed. Exciting PS I at exactly the same spectral region via one-photon excitation (λexc) 575 nm) also does not show any sub-ps component. We ascribe the observed constant of 800 fs to a portion of about 47 ( 16 % "-carotene states that can potentially transfer their energy efficiently to chlorophyll pigments via the optically dark Car S1 state. We compared these data with conventional one-photon pump-probe data, exciting the optically allowed second excited state, Car S2. This comparison demonstrates that the fast dynamics of the optically forbidden state can hardly be unravelled via conventional one-photon excitation only because the corresponding Car S1 populations are too small after Car S2 f Car S1 internal conversion. A direct comparison of the amplitudes of the Car S1 f Sn excited-state absorption of PS I and "-carotene observed after Car S2 excitation allows determination of a quantum yield for the Car S1 formation in PS I of 44 ( 5%. In conclusion, an overall Car S2f Chl energy-transfer efficiency of∼69 ( 5 % is observed at room temperature with 56 ( 5 % being transferred via Car S2 and probably very hot Car S1 states and 13 ( 5 % being transferred via hot and “cold ” Car S1 states
