A two-dimensional Fourier transform spectroscopic study of the nonequilibrium solvation dynamics of IR144 in methanol

Although polar solvation dynamics have been studied extensively both theoretically and experimentally, the fastest and most important non-equilibrium processes have not been clearly observed. Molecular dynamics simulations predicted the "inertial" component of solvation which consists mainly of small solvent rotations and dominates the non-equilibrium solute-solvent energetics for charge transfer reactions in polar solvents. The first experiments found that inertial solvation was as fast or faster than the 100 fs instrument response. Femtosecond four-wave mixing experiments with 20 fs time resolution were used to estimate the timescale of inertial solvation but found to greatly distort the dynamics. Two-dimensional Fourier transform (2D FT) optical spectroscopy is presented as a way to observe nonequilibrium solvation dynamics.;The first method for measuring the complete electric field of four-wave mixing signals at their point of origin is presented. Three pulse photon echo and transient grating signals from IR144 in methanol are time and frequency resolved. The measurement of the complete electric field of the three pulse photon echo makes it possible to obtain optical 2D FT spectra with separable absorptive and dispersive parts.;Calculations of 2D FT spectra using the Brownian oscillator model of solvation dynamics show how information such as the inertial solvation time can be obtained directly from these spectra. Several limiting cases of the Brownian oscillator are considered in order to understand how in formation is manifested in the 2D FT spectrum. The calculations are used to model vibrationally under damped systems and show how correlations between energy levels are seen in 2D spectra and how the spectra can be used to follow vibrational wave packet motion.;The calculated 2D FT spectra are compared with experimentally measured spectra of IR144 in methanol. They are fit to the experimental spectra to obtain the value for the inertial solvation time of IR144 in methanol of 125 fs +/- 10 fs. A slightly different experimental method is suggested for the measurement of 2D FT spectra and calculations of the expected signals are presented.