Femtosecond laser spectroscopy and dynamics of solvation in liquids and electrolytes

Abstract

We report femtosecond and picosecond laser spectroscopy studies on the dynamics of molecules in weakly and strongly solvated systems. Femtosecond optical Kerr studies on CS₂ at 298 K reveal the central role of transitory, local liquid structures in which CS₂ librations rapidly dephase (< 170 fs), and the molecule executes small linear translational displacements on the timescale of ca. 400 fs and reorientationally relaxes with τ_rot= 1.61 ps. Upon dilution in alkanes to simulate weak interactions, the frequency of the CS₂ librational responses decreases, translational displacements become more important and reflect changing interaction-induced distortion of the molecular polarizabilities, and the reorientational motion varies directly with the shear solution viscosity. In strongly solvated resorufin, an anion probe molecule in aqueous and methanolic electrolyte solutions, τ_rot dominates the relaxation process and varies from 75 ps in pure water to 275 ps in 7 mol dm^–3 LiCl(aq); an from 79 ps in MeOH to 356 ps in 2.5 mol dm^–3 LiCl–MeOH. While the viscosity increases significantly with increasing LiCl, the reduced rotational times (τ_rot/η) differ notably from predictions based upon Debye-Stokes-Einstein and dielectric friction theories. The change in the number of solvating molecules about the probe ion is proposed to explain the data, once again focussing on the transitory solvation structure as the principal influence on τ_rot.