| Non-Dipolar and non-polar solvation dynamics are studied via third-order, electronically resonant, time-resolved spectroscopies. Three pulse stimulated photon echo peak shift (3PEPS) measurements were used to probe the non-dipolar solvation of a quadrupolar solute and the results were compared with the dynamics in polar solvents. Results were compared to a viscoelastic continuum solvation model previously proposed to describe non-polar solvation Also studied were the influence of intramolecular vibrational modes on nonlinear, time-domain, electronically resonant signals. Both Transient Grating (TG) and 3PEPS signals were collected and analyzed from several probe molecules at various laser frequencies. In the 3PEPS profiles, excitation on the blue edge of the absorption spectrum causes a decreased initial peak shift values and more rapid initial decays, whilst in the TG signals, the magnitude of the “coherent spike” is strongly wavelength dependent. Additional thermally activated vibronic effects were studied via temperature dependent 3PEPS profiles. Our results reveal the sensitivity of the nonlinear signals to the excitation wavelengths and to the distinct vibronic structure of different chromophores studied. Pronounced modulations in both the 3PEPS and TG signals originating from coherently excited vibrational modes were directly observed. Additional oscillations were observed that are attributed to difference frequencies and higher harmonics of the fundamental modes. By varying the excitation wavelength, it is shown that the different initial conditions for the vibrational wavepackets significantly affect the signals, especially through the contributions associated with high frequency modes, often neglected in experimental analyses. The temporal properties of both WD-TG and WD-REPS signals display sensitivities to both the excitation wavelength and the vibronic structure of the specific probe molecule used. Several mechanisms for generating vibronic modulations in the signals are discussed and their effects on the signals are described. Quantitative agreement between experiment and simulated signals requires accurate characterization of the laser pulses, specifically the magnitude and sign of chirp has a significant effect on the initial temporal properties of the signals. Non-dipolar solvation dynamics is further investigated with molecular dynamics simulations to highlight the role of the spatial properties of the solvent on the resulting dynamics. |
