Early events of energy relaxation in all-trans-β-carotene following sub-10 fs optical-pulse excitation

The rapid internal conversion to the lowest lying $2^1{A}_g^{-}$ state following photoexcitation is studied in all-trans-β-carotene with unprecedented time resolution. Visible optical pulses with time duration of 7 fs are used to resonantly excite and probe the optical dynamics of the molecule dissolved in solution. A picture for the relaxation dynamics during the first 100 fs following photoexcitation is proposed based on time-resolved transmission difference spectra and kinetics. Coherent nuclear motion is observed, assigned to impulsive stimulated resonant Raman scattering. Thermalization of the $2^1{A}_g^{-}$ state, evident from excited-state spectral evolution, is rather slow, indicating that the higher lying vibrational levels are populated during energy transfer within the photosynthetic systems.