Strong coherent coupling of vibronic oscillations in spheroidene

The four-wave mixing signals from spheroidene have been systematically investigated at various coherent periods τ. The pronounced vibronic oscillations of the fundamental modes in the ground state are clearly observed in the transient grating configuration (τ=0). Interestingly, it was found that the overtones and coupled modes become very strong compared to the fundamental modes when the signal is measured in the virtual photon-echo configuration (τ<0). The experimental results have been analyzed using a Brownian oscillator model. The intensity change with respect to the coherent period is qualitatively reproduced by the calculations. We discuss a plausible model that allows a quantitative agreement between experiment and calculation.