On the nature of initial solvation in bulk polar liquids: Gaussian or exponential?

Measurement of time evolution of fluorescence of a probe solute has been a quintessential technique to quantify how dipolar solvent molecules dynamically minimize the free energy of an electronically excited probe. During such solvation dynamics in bulk liquids, a substantial part of relaxation was shown to complete within sub-100 fs from time-gated fluorescence measurements, as also predicted by molecular dynamics simulation studies. However, equivalent quantification of solvation timescales by femtosecond pump-probe and broadband fluorescence measurements revealed an exponential nature of this initial relaxation having quite different timescales. Here, we set out to unveil the reason behind these puzzling contradictions. We introduce a method for estimating probe wavelength-dependent instrument response and demonstrate that the observation of the Gaussian vs exponential nature of initial relaxation is indeed dependent on the method of data analysis. These findings call for further experimental investigation and parallel development of theoretical models to elucidate the molecular-level mechanism accounting for different types of early time solvation.