Third-order nonlinear time domain probes of solvation dynamics

Abstract

Several closely related third-order nonlinear time-resolved spectroscopic techniques, pump/probe transient absorption, transient grating, and three pulse stimulated photon echo peak shift measurements, are investigated theoretically and experimentally. It is shown in detail, through the consideration of response functions and numerical simulations including both finite pulse durations and detuning from exact resonance, how the solvation dynamics are manifested in these third-order nonlinear time-resolved spectroscopies. It is shown that the three pulse stimulated photon echo peak shift measurement and the transient grating measurement can give accurate dynamical information, whereas transient absorption may not be a reliable technique for a study of solvation dynamics in some cases. The contribution of very slow or static (inhomogeneous) components to the dynamics, however, can only be obtained from the three pulse echo peak shift measurements. Comprehensive experimental measurements are presented to illustrate and corroborate the calculations. We show that it is possible to separate the intramolecular vibrational and solvent contributions to the dephasing (or optical lineshape). Furthermore it is shown that the solvation of polar solutes in polar protic solvents has rather universal characteristics. The initial ultrafast process, usually identified as an inertial response of solvent molecules, occurs on a ∼100 fs time scale, and is essentially identical in methanol, ethanol, and butanol. The amplitude of this ultrafast component does, however, decrease with increasing alcohol size in 1-alkanols. The diffusive (≳0.5 ps) regime of the solvation process shows a strong solvent dependence, and may be described satisfactorily by dielectric relaxation theories.