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| - Coherent Electronic and Nuclear Dynamics for Charge Transfer in1-Ethyl-4-(carbomethoxy)pyridinium Iodide
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| - Although polaronic interactions and states abound in charge transfer processes and reactions, quantitativeand separable determination of electronic and nuclear relaxation is still challenging. The present paper employsthe amplitudes, polarizations, and phases of four-wave mixing signals to obtain unique dynamical informationon relaxation processes following photoinduced charge transfer between iodide and 1-ethyl-4-(carbomethoxy)pyridinium ions. Pump−probe signal amplitudes reveal the coherent coupling of an underdamped 115 cm-1nuclear mode to the charge transfer excitation. Assignments of this recurrence to intramolecular vibrationalmodes of the acceptor and to modulation of the intermolecular donor−acceptor distance are discussed on thebasis of a high-level density functional theory normal-mode analysis and previously observed wave packetdynamics of solvated molecular iodine. Nuclear relaxation of the acceptor induces sub-picosecond decay ofthe pump−probe polarization anisotropy from an initial value of 0.4 to an asymptotic value of −0.05. Electronicstructure calculations suggest that relaxation along the torsional coordinate of the ethyl group is the origin ofthe anisotropy decay. Electric-field-resolved transient grating (EFR-TG) signal fields are obtained by spectralinterferometry with a diffractive optic based interferometer. These measurements show that the signal phaseand amplitude possess similar dynamics. Model calculations are used to demonstrate how the EFR-TG signalphase yields unique information on transient material resonances located outside the laser pulse spectrum.This effect can be rationalized in that the real and imaginary parts of the nonlinear polarization are related bythe Kramers−Kronig transformation, which allows the dispersive component of the polarization response toexhibit spectral sensitivity over a larger frequency range than that defined by the absorption bandwidth.
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