Untersuchung der ultraschnellen Relaxationsdynamik in Carotinoiden mit zeitaufgelöster nicht-linearer Spektroskopie

Abstract

Main aspect of this work is the investigation of the ultrafast relaxation dynamics following photoexcitation in carotenoids using time-resolved nonlinear spectroscopy. Special attention is paid to the early dynamics of less than 100 fs after excitation. The chosen spectroscopic method is pump-degenerate four wave mixing (pump-DFWM), a two-dimensional technique which allows for the simultaneous observation of population and vibrational dynamics. Numerical model simulations of the signal are implemented in order to provide unambiguous interpretation of the complicated pump-DFWM signal. This permits analysis of the general characteristics of the pump-DFWM signal of complex molecules as well as the influence of experimental parameters such as pulse length and chirp. Carotenoids are natural pigments which play a crucial role in photosynthesis. The relaxation pathway after excitation with visible light of these molecules, however, is still not clear. The properties of the excited states of carotenoids depend strongly on the conjugation length N. This work starts with the investigation of lycopene (N = 11) as a typical member of the carotenoid family. It is shown that a long-living pump-DFWM signal at early delays is key to the answer of the question whether additional dark states play any role in the relaxation. This signal is assigned to a stimulated emission from the excited state to a vibrationally hot ground state (hot-S0). No hints for a participation of additional states are found for lycopene. Comparison of several carotenoids with different conjugation lengths from N = 9 to 13 as well as measurements of the closed-chain carotenoids -carotene and lutein in different solvents, however, reveal unusual characteristics in the signal around N = 10. The appearance of an additional absorption signal, a delay of the stimulated emission as well as interferences in the vibrational spectra of spheroidene and lutein point to a mixing of the initially excited state with an additional dark state. Hence, participation of a dark state in the relaxation pathway for N = 10 is shown while no hints are found for additional dark states in all other carotenoids.