SPECTRAL DIFFUSION IN LIQUIDS

Spectral diffusion of an electronic transition of solute chromophores in liquid solutions is investigated experimentally and theoretically through its influence on electronic excited-state transfer (EET). Observation of dispersive EET in liquids (the EET rate depends on the excitation wavelength) demonstrates that absorption lines are inhomogeneously broadened on a nanosecond time scale in the systems studied although the time scale for homogeneous dephasing is tens of femtoseconds. A theory is developed that relates the rate of spectral diffusion to the wavelength dependence and temperature dependence of EET. Time-resolved fluorescence depolarization measurements are used to measure EET in the systems rhodamine B (RB) in glycerol and propylene glycol as a function of wavelength and temperature from room temperature (298 K) to 200 K. Comparison with theory permits the rates of the solvent fluctuations responsible for spectral diffusion to be determined for the two solvents at several temperatures. Measurements are also made of the rates of solvent relaxation about the excited RB and of RB orientational relaxation. The results demonstrate that the mechanism for spectral diffusion is solvent orientational relaxation which causes the initial (time of optical excitation) dipolar field, produced by the solvent at the chromophore, to randomize.