Solvent-Solute Interaction in Liquid and Solid Phases of Ethanol Probed by Photon Echo Spectroscopy

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The optical dephasing of dye molecules in solution has been extensively investigated by various experimental methods and it is shown that the dephasing occurs on the time scale of femtoseconds with a non-Markovian nature. Since photon echo spectroscopy is a powerful tool for investigating solvation dynamics, several groups report echo responses measured at room temperature utilizing femtoseconds laser pulses. However reports on the temperature dependence of solvation dynamics are quite limited. The investigation of temperature dependence is important because it gives detailed information about the solvation dynamics.In this research, the solvation dynamics of beta-carotene molecules in ethanol has been investigated by incoherent photon echo spectroscopy at various temperatures from 300 K to 50 K.Experimental results are analyzed based on a dynamical model for a dephasing in femtoseconds and a stochastic model for a slow energy fluctuation. Using parameter values for the dynamical model determined from fluorescence spectra, we obtain amplitude and correlation time of the slow energy fluctuation in picoseconds time scale in solid phase of ethanol. It is shown that the fast response of echo signals in femtoseconds time scale essentially comes from the relatively strong electron-phonon coupling in this system and that the slow energy fluctuation decreases the rephasing process resulting in much faster response. The incoherent photon echo probes the slow energy fluctuation in picosecond order, Which is longer than the delay time between two excitation pulses.