We present time-resolved A two-photon pump-probe data measured with photosystem I (PS I) of Thermosynechococcus elongatus. Two-photon excitation (lambda(exc)/2 = 575 nm) in the spectral region of the optically forbidden first excited singlet state of the carotenoids, Car S-1, gives rise to a 800 fs and a 9 ps decay component of the Car S-1 -> S-n excited-state absorption with an amplitude of about 47 +/- 16% and 53 +/- 10%, respectively. By measuring a solution of pure beta-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 (lambda(exc) = 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% beta-carotene states that can potentially transfer their energy efficiently to chlorophyll pigments via the optically dark Car S-1 state. We compared these data with conventional one-photon pump-probe data, exciting the optically allowed second excited state, Car S-2. 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 S-1 populations are too small after Car S-2 -> Car S-1 internal conversion. A direct comparison of the amplitudes of the Car S-1 -> S-n, excited-state absorption of PS I and beta-carotene observed after Car S, excitation allows determination of a quantum yield for the Car S, formation in PS I of 44 +/- 5%. In conclusion, an overall Car S-2 -> Chl energy-transfer efficiency of similar to 69 +/- 5% is observed at room temperature with 56 5% being transferred via Car S2 and probably very hot Car S, states and 13 5% being transferred via hot and "cold" Car S-1 states.