Relaxation dynamics of inhomogeneous spectral width in binary solvents studied by transient hole-burning spectroscopy

We have performed transient hole-burning spectroscopy of cresyl violet in binary solvents of different polarity components in order to detect the relaxation of inhomogeneous spectral bandwidths (relaxation of dispersion around average energy) in addition to that of spectral peak shift (relaxation of average energy). We proposed in our previous paper (J. Phys. Chem. A 1997, 101, 5729) that the relaxation process of spectral width and that of average energy of the system should be characterized by different relaxation modes of solvent molecules. In the binary solvents a slow relaxation process was observed especially for the spectral width, which was assigned to the translational diffusion of polar component of solvent molecules. On the other hand, the relaxation of average energy was not effectively affected by the solvent mixing, which could be attributed mainly to fast relaxation processes, the libration and/or rotational diffusion of solvent molecules in the closest solvation shell. Even if the distribution of the solvent shell would be disturbed by the slower translational diffusion mode of polar solvent molecules, the average energy could be immediately compensated by the fast relaxation of solvent surroundings. The developments of the molecular theory on the translational dynamics should be necessary for essential understanding of the results obtained here.