Four-wave mixing signals from b-carotene and its n =1 5 homologue

The third-order nonlinear optical responses of b-carotene and its homologue having a conjugation-double bond n = 15 have been investigated using sub-20 fs ultra- short optical pulses in order to clarify the dissipation pro- cesses of excess energy. Using the four-wave mixing spectroscopy, we observed a clear coherent oscillation with a period of a few tens of femtoseconds. The spectral den- sity of these molecules was estimated that allowed the theoretical linear and nonlinear optical signals to be directly compared with the experimental data. Calculations based on the Brownian oscillator model were performed under the impulsive excitation limit. We show that the memory of the vibronic coherence generated upon the excitation into the S2 state is lost via the relaxation process including the S1 state. The vibronic decoherence lifetime of the system was estimated to be 1 ps, which is about 5 times larger than the life time of the S2 state (*150 fs) deter- mined in previous studies. The role of coherence and the efficient energy transfer in the light-harvesting antenna complexes are discussed.