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The dynamics of a liquid and its coupling to a solute are crucial for a better understanding of chemical processes in the liquid phase. In isotropic and homogeneous solutions, the time-correlation function of a solute is expected to vanish over time due to the translational and diffusive motions of the solvent. The three-pulse photon echo peak shift (3PEPS) is a third-order nonlinear spectroscopy technique that records the time-correlation function of a solute molecule in a solution, including an offset (inhomogeneity). In this work, we utilized a diffractive optics-based 3PEPS apparatus to fully resolve the dynamics in liquids from femtoseconds to nanoseconds while varying the temperature in the range of 80-298 K and the probe solute molecules. Our observations reveal dynamics slower than the dielectric relaxation of n-alcohols, even at room temperature, consisting of a ∼0.5 ns time constant that persists below the melting points and a static component (offset) on a nanosecond timescale. Based on the experiments, we suggest that locally formed glass-like clusters in liquids can be responsible for the slow dynamics. Our results may provide new insights into the dynamics of liquids and related phenomena such as liquid-glass and liquid-liquid phase transitions., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

Polar solvation dynamics of coumarin 153 in acetonitrile, methanol, and butanol are investigated by dynamic Stokes shift function, S(t). In small protic solvents, it is known that an initial ultrafast component below 50 fs constitutes more than half of the total solvation process. We use fluorescence up-conversion technique via two-photon absorption process, which can provide 40 fs time resolution for the whole emission wavelength range. Moreover, time-resolved fluorescence spectra are recorded directly without the spectral reconstruction. We observe a temporal oscillation in frequency of whole emission spectrum in the solvation curve. In the obtained S(t), initial solvation time scale is 120 fs, invariant to solvents used in this experiment, although its amplitude varies in different solvents.

We report a three-pulse photon echo peak shift (3PEPS) apparatus adopting a diffractive beam splitter to generate triplets of femtosecond pulses with tilted wave front, which enhances the overlap over the entire aperture in a noncollinear geometry while preserving femtosecond pulse duration. The apparatus provides highly reproducible 3PEPS data including the long time peak shift, a critical parameter to recognize the spectral inhomogeneity in condensed phases. To demonstrate the high performance of our setup and to examine the inhomogeneous line broadening in liquids, we measured 3PEPS of a carbocyanine dye IR125 in several solvents. The solvation dynamics becomes slower for IR125 compared to the typical values reported previously probably due to the multipolar nature of the excitation in this large and flexible probe molecule. More importantly, inhomogeneity was observed unambiguously in all solvents, and it is excitation energy dependent. It was speculated that slow dielectric relaxation and conformational heterogeneity may be responsible for the inhomogeneity.

We present highly time-resolved spontaneous fluorescence spectra of a porphyrin array system that consists of an energy donor and an acceptor linked by a phenyl group. The donors are meso-meso directly linked zinc(II) porphyrin arrays and the acceptor is a zinc(II) 5,15-di(phenylethynyl)porphyrin. The spectra over the entire Q (S1) emission band following the excitation of the donor B (S2) state have been measured directly without the conventional spectral reconstruction method. The time-resolved fluorescence spectra revealed detailed energy relaxation processes within the donor and subsequent energy transfer to the acceptor. The observed energy transfer rates to the acceptor are consistent with the Forster energy transfer rates calculated on the assumption that the energy is localized in the Q state of each porphyrin unit of the donor prior to the energy transfer. The passage of the energy deposited initially on one porphyrin unit of the donor to the acceptor illustrates a sequence of energy delocalization and localization processes before it finally reaches the acceptor.

Transient grating of a dye in liquid has been measured as a function of the electronic coherence period. A diffractive beam splitter and a pair of wedge prisms are implemented to achieve precise spatial phase overlap and interferometrically accurate control of the time delay between the pump pulses. As the electronic coherence period is varied, coherent interactions lead to an enhancement or loss of the sharp feature in the transient grating signal near time zero, which is usually called coherent spike. Sensitivity of the transient grating signal to the solvation process also changes by the coherence time delay. All the features can be accounted for by invoking third-order nonlinear response functions. Numerical simulations have been performed to corroborate our description. This work identifies a major source of the coherent spike in the transient grating and transient absorption experiments. In addition, it allows us to propose a method that measures the solvation function more efficiently than conventional transient grating technique does., ((c) 2004 American Institute of Physics.)