| Full characterization of a chemical reaction requires time resolution to resolve the continuous motion along the reaction path and frequency resolution to distinguish between reacting species. Standard time-resolved experiments used to elucidate the early motions in chemical reactions require a compromise between time and frequency resolution since the two are linked by the time-bandwidth product of the excitation pulses. We have measured both the phases and amplitudes of femtosecond four-wave mixing signals and used this ability to perform direct optical analogs of the simplest two-dimensional nuclear magnetic resonance (NMR) experiments. This two-dimensional technique provides frequency resolution within the excitation pulse spectrum without sacrificing any time resolution. As in 2D NMR, information hidden in linear 1D spectra is revealed by 2D spectroscopy's ability to disentangle complicated spectra by spreading them in a second dimension, reveal connections within the energy level structure of molecules, and illuminate the flow of energy within or between molecules. Our optical 2D method provides the basis for a truly universal femtosecond spectrometer.; We have used 2D spectroscopy to follow the femtosecond dynamics of two structurally related cyanine dyes in methanol. The dye HDITCP is used to establish the vibrational and nonpolar solvation dynamics common to both dyes. A model that consistently treats all dephasing dynamics as arising from nuclear motion is developed to simulate the linear spectra, pump-probe transients, and 2D spectra of HDITCP.; The speed with which polar solvent molecules can respond to a new charge distribution plays a crucial role in determining the rate of charge transfer reactions in liquids. We have used the solvent-polarity-sensitive dye IR144 to measure this polar solvation in methanol. Using the vibrational and mechanical model of HDITCP as a foundation, we have been able to isolate the polar aspects of methanol's response and obtain an estimate for the timescale and magnitude of polar inertial solvation in methanol. |
