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Chemical Physics Letters

Photon echoes in a nonpolar fluid

Alan M. Walsh, Roger F. Loring

Published Nov 1, 1991• 55 citations
Publisher

Abstract

We calculate the photon echo signal for a nonpolar chromophore dissolved in a nonpolar solvent. Results of molecular dynamics simulations are compared to calculations in which molecular motions are taken to be ballistic, to assess the contribution of such motions to the echo decay. The validity of the conventional assumption that fluctuations in the electronic transition frequency obey Gaussian statistics is investigated for this model. We consider the role of fluctuations in the repulsive portion of the intermolecular potential in determining the echo decay. The calculations suggest that electronic dephasing in liquids may arise primarily from a slight rearrangement of the first solvation shell about the chromophore.

DOI
10.1016/0009-2614(91)80195-4

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We calculate the photon echo signal for a nonpolar chromophore dissolved in a nonpolar solvent. Results of molecular dynamics simulations are compared to calculations in which molecular motions are taken to be ballistic, to assess the contribution of such motions to the echo decay. The validity of the conventional assumption that fluctuations in the electronic transition frequency obey Gaussian statistics is investigated for this model. We consider the role of fluctuations in the repulsive portion of the intermolecular potential in determining the echo decay. The calculations suggest that electronic dephasing in liquids may arise primarily from a slight rearrangement of the first solvation shell about the chromophore.
Two- and three-pulse photon echo signals are calculated for various model systems. The use of an experimental solvation correlation function as the solvent fluctuation correlation function leads to two conclusions. First, inertial solvent motion plays a major role in the electronic dephasing process. Second, simple models such as Markovian or exponential models for the solvent fluctuation correlation function may not provide an adequate description of the echo signal. The real and imaginary parts of the echo response, which may be measured via heterodyne detected stimulated photon echoes, are compared with conventional photon echo signals.
We calculate the signal in a time-resolved, coherent light scattering experiment on a dilute solution of a nonpolar solute dissolved in a nonpolar solvent. We model the solute and solvent molecules as electronic two level systems, and assume that only the solute interacts with radiation. The electronic degree of freedom is treated perturbatively within the Born–Oppenheimer approximation. The nuclear degrees of freedom are treated classically, and their effects are modeled by a molecular dynamics simulation. We assume the electronic absorption spectrum is dominated by inhomogeneous broadening. The validity of this assumption for our model is verified through the calculation of the absorption line shape with and without the inhomogeneous broadening assumption. A quantitative measure of the homogeneous dephasing time is obtained from a calculation of the photon echo signal for our model. The time dependence of the light scattering signal is shown to reflect both dynamics of the fluid in its electronic ground state and in its electronic excited state. We analyze the relative importance of contributions to the signal from these two types of dynamics as a function of experimental time scale and laser frequency.
A rigorous theoretical connection between the polarizability spectral density obtained from optical Kerr effect (OKE) measurement with the correlation function describing solvent-induced optical dephasing detected in photon echo measurements is given. The experimentally obtained spectral density has a more reasonable physical basis than model correlation function descriptions of solvent fluctuations. The experimental OKE spectrum is demonstrated to provide a natural description of solvent motions that modulate the chromophore electronic states in the case of weak induced-dipolar interactions. The chromophore optically active vibrational modes are obtained from pump-probe spectra and are employed in the calculation of echo signals. It is found that the fast decays of the two-pulse echo signals result from both solvent and solute intramolecular motions while the echo peak shifts are dominated by the solvent intermolecular modes.
The electronic dephasing dynamics of a solvated chromophore is formulated in terms of a non-Markovian master equation. Within this formulation, one describes the effect of the nuclear degrees of freedom on the electronic degrees of freedom in terms of a memory kernel function, which is explicitly dependent on the initial solvent configuration. In the case of homogeneous dynamics, this memory kernel becomes independent of the initial configuration. The Markovity of the dephasing process is also the most conveniently explored by comparing the results obtained via the non-Markovian master equation to these obtained via its Markovian counterpart. The homogeneous memory kernel is calculated for a two-state chromophore in liquid solution, and used to explore the sensitivity of photon echo signals to the heterogeneity and non-Markovity of the underlying solvation dynamics.
Optical dephasing of a cyanine dye chromophore in solution is investigated by time-gated photon echo spectroscopy in a three-pulse scattering geometry. The third-order polarization response is gated by up-conversion with a reference pulse in a nonlinear optical crystal. Time gating establishes two controllable periods of system evolution in an optical coherence. The measurements are compared with numerical simulations employing the measured OKE spectrum of solvent (butanol, DMSO) fluctuations. The time-shift of the time-gated echo signals for several gating forming pulse time delays agrees with simulation while the narrower simulated widths suggest a frequency-dependent solute-solvent interaction and the consideration of chromophore vibronic transitions.
Abstract This account summarizes our recent work on measurements of the correlation function of the transition frequency fluctuations by a three-pulse photon echo method. The correlation function of the transition frequency fluctuations is a sensitive measure for the magnitudes and time scales of dynamical interactions between solute and solvent. We describe the results of the solvation dynamics of small ions in protic solvents such as methanol and water obtained by an infrared photon echo method.