SOLUTION DYNAMICS BY LINE-SHAPE ANALYSIS, RESONANCE LIGHT-SCATTERING AND FEMTOSECOND 4-WAVE-MIXING

The results of line shape analysis, resonance light scattering and femtosecond four-wave mixing measurements are reported on several organic molecules in solution. It is shown that a Brownian oscillator model for line broadening provides a full description for the optical dynamics in aprotic solutions. The dynamics of all systems studied is found to occur in the intermediate modulation regime, where the amplitude-DELTA and inverse correlation time-LAMBDA of the Brownian oscillator are about equal: DELTA/LAMBDA almost-equal-to 1. Isotopic substitution of either probe or solvent is shown to have no effect on the dynamics, from which it is concluded that the vibrational force field plays a minor role in determining the line shape. Four-wave mixing experiments with chirped femtosecond pulses yield intense coherent Raman scattering signals due to ground and excited-state vibrational coherences which are very effectively driven by the two instantaneous light fields whose frequency difference matches exactly a vibrational mode. It is shown that the "chirped" four-wave mixing signal near zero delay time only mimics the two-pulse photon echo if the system dynamics fall in the fast modulation limit: DELTA/LAMBDA << 1. Otherwise this signal is damped not only by dephasing but also by solvation dynamics.