Femtosecond nonlinear coherent spectroscopy of carotenoids

In this thesis, femtosecond spectrally resolved one- and two-colour four-wave mixing coherent spectroscopy is used to study the population dynamics and coherence dynamics in two carotenoids, lycopene (with eleven conjugated carbon bonds (C=C), n = 11) and spheroidene (n = 10), in n-hexane solution. This information could play an important role in elucidating the light harvesting function of lycopene and spheroidene in the process of photosynthesis. The population dynamics of the two carotenoids are studied by measurements of transient grating-like signals. By selecting appropriate wavelengths for the three laser pulses in two-colour measurements, the vibrational relaxation times in the first optically allowed excited state 1B+u and the ground state 1A¡-g are determined for lycopene. The results suggest the active role of the conjugated carbon bond C=C in the vibrational relaxation processes of the ground state. The transient grating-like measurements were also successful in detecting the internal conversion process from the 1B+u state to the dark 2A¡-g state , from the dark 2A¡-g state to the ground state 1A¡-g , as well as vibrational relaxation in the 2A¡-g state, for both lycopene and spheroidene. These results are in agreement with those obtained from other techniques. The decoherence process of certain transitions in lycopene and spheroidene are studied by one-colour photon echo measurements. The decoherence time of the transition 1A¡-g (0) ¡ 1B+u (0) is determined for lycopene and spheroidene, and that of the transition 1A¡-g (0) ¡ 1B+u (1) is determined for lycopene. The results indicate a larger scattering rate in the higher vibrational level in the 1B+u state of lycopene. The results on the population dynamics and the molecular decoherence both indicate a slower dynamical behaviour in spheroidene compared with lycopene, which is consistent with other studies. Investigation of the coherence dynamics were successful in detecting coherent coupling between the levels 1A¡-g (1) and 1B+u (0) in lycopene and the coherence time is found to be less than 100 fs. The spectrally resolved two-colour four-wave mixing signals were able to detect transitions between the dark states 1B¡-u (1), 3A¡-g (0) and the ground state in spheroidene and indicate the presence of coherent coupling between these dark states and the ground state. The energies of the dark states 1B¡-u (1) and 3A¡-g (0) are deduced. A phase retrieval technique that allows one to obtain the phase of the four-wave mixing signal from the spectrally resolved two-colour data without the need for phase stabilised input pulses is reported. The spectrally resolved signal which is processed by a phase retrieval algorithm, yields the time resolved emission signal and Fourier transformed correlation spectra, and hence helps to identify the coherence dynamics of our samples. Preliminary results on the coherent coupling in the laser dye cresyl violet and in lycopene were obtained using this phase retrieval technique. The technique is able to provide additional information about the coherence dynamics and is a very promising tool for future studies.