Study of ultra-fast relaxation processes by resonant Rayleigh-type optical mixing. II. Experiment on dye solutions

The three-wave optical mixing (${\omega }_3=2\mathrm{}{\omega }_1-{\omega }_2$) of the resonant Rayleigh type ($|{\omega }_1-{\omega }_2|\simeq \mathrm{inverse}\mathrm{relaxation}\mathrm{time}$, ${\omega }_1 \mathrm{and} {\omega }_2\simeq \mathrm{optical}\mathrm{transition}\mathrm{frequency}$) is applied to measure the subpicosecond relaxation times associated with the $S_0\to S_1$ electronic transition of 1,1'-diethyl-4,4'-quinocyanine iodide in ethanol and other dye solutions. Two independently tunable dye lasers excited by a nitrogen laser were used as light sources. The measured frequency responses of the nonlinear susceptibility $\left|{\chi }^{\mathrm{}\left(3\right)\mathrm{}}\right|$ as a function of ${\omega }_1-{\omega }_2$ were in good agreement with the theoretical formula based on an inhomogeneously broadened two-level model incorporating the phenomenological relaxation times. We also observed the interference effect between the resonant and nonresonant terms of the nonlinear susceptibility, which agree well with the theory. Thus we were able to determine the transverse relaxation time $T_2$ in the range of 0.02 ∼ 2 psec and the effective relaxation time $T_1^{\prime }$ (a combination of longitudinal- and cross-relaxation times) in the range of 0.1 ∼ 5 psec. These values were found to be largely dependent on the spectral position inside the broad absorption band and the sample preparation and deterioration, as well as on the kinds of dye materials and solvents. Details of the experimental procedure and a discussion of the interpretation of the experimental results are given.