Vibrational relaxation in excited states of all-trans-carotenoids has been investigated using femtosecond stimulated Raman spectrosocpy with resolutions of 250 fs and 25 cm-1. The initially photoexcited 1B+u state in carotenoids relaxes to the 2A-g excited state within 1 ps. The 1B-u excited state is an intermediate state of the relaxation. After the relaxation to the 1A-g state, the excess energy is held in a C=C stretching mode (ν1) and remains longer than several picoseconds in short (mini) carotenes. Vibrational feature of carotenoids is of importance in energy transfer of phtoosynthesis. Femtosecond time-resolved vibrational spectroscopy is necessary to investigate the initial ultrafast kinetics.
Initial relaxation kinetics in a series of carotenes has been investigated using femtosecond absorption and Raman spectroscopy. The internal conversion from the photoexcited 1Bu+ state to the 2Ag- state is faster in the carotenes with middle polyene lengths. The vibrational relaxation of the 2Ag- state has fast and slow components. The time constant of the slow component is longer than several picoseconds. The 2Ag- state in the short carotenes remains in the vibrational excited levels because of the slow vibrational relaxation. These properties can be explained in terms of the 1Bu+ state which assists the internal conversion to the 2Ag- state and the fast vibrational relaxation of the 2Ag- state. The vibrational features of the excited states in carotenoids is very important in energy transfer mechanism of acterial photosynthesis.
Ultrafast optical response in several polydiacetylenes (PDAs) with different side-groups and morphologies has been investigated by femtosecond absorption spectroscopy. Several nonlinear optical processes, i.e., excitonic absorption saturation, hole burning, Raman gain, inverse Raman scattering, optical Stark effect, and induced-phase modulation, have been observed and the mechanisms are discussed. The relaxation from photoexcited free excitons to self-trapped excitons (STEs) has been observed in both blue- and red-phase PDAs. The time constant is estimated as 100 - 150 fs. The decay of STEs in the blue-phase PDAs is nearly exponential with time constant of about 1.5 ps at 290 K and about 2.0 ps at 10 K. The decay curve in the red-phase PDAs substantially deviates from exponential function. It is fitted phenomenologically to biexponential functions with time constants of slightly shorter than 1 ps and about 5 - 10 ps. These two time constants correspond to relaxations to the ground state, respectively, from the unthermalized (hot) STE and from the thermalized STE.
The femtosecond absorption spectroscopy of bacteriorhodopsin was studied in the wide spectral region (450 - 900 nm) and the primary photoprocesses of the neutral purple form and the acidic blue form were compared. At neutral pH the stimulated emission at 860 urn and the excited state absorption at 480 tim decayed synchronously with a time constant of 500 fs. Thus we obtained a conclusive support for previous assignments of the transient species ''460'' being the S1 state in hR568. At acidic pH the decay kinetics of the stimulated emission at 6O nm and the excited state absorption at 480 nm were described with two time constants 1. 5 0. 2 ps and 8. 6 0. 9 ps. These two components are presumably due to the two bR isomers that exist in the blue form. Even at acidic pH no clear time-dependent oscillatory behavior was found in the stimulated emission or excited state absorption. This result shows that the isomerization around C1C14 of the retinal molecule in the S1 state is described by the over-damped oscillation. That is the isomerization angle varies only monotonously in the S1 state. 1.