Femtosecond Dynamics of Molecules in the Condensed Phase

A variety of spectroscopic methods are used to study the electronic dephasing of molecules in the condensed phase. The dephasing process is intimately connected to the process of solvation, which in turn plays an important role in chemical reactions in liquid solution. A quantitative understanding of the dephasing on the femtosecond timescale should provide insight into the ultrafast dynamics of solvation and thus chemical processes in solution. The experiments are analyzed using a well-established theoretical description of the nonlinear optical response. The femtosecond two and three pulse photon echo measurements are analyzed in detail, with special emphasis on the roles played by realistic pulse shapes and molecular vibronic structure in the observed signals. The three pulse photon echo is shown to be effective in suppressing the effect of the dominant vibrational mode in the molecule LD690 and allows us to resolve the dephasing. The dephasing of LD690 in a series of n-alcohols is analyzed and shown to be non-Markovian, driven by a process with a correlation time on the order of 25 fs and a prefactor depending on the solvent number density. The dephasing of LD690 in a variety of other solvents is investigated and the solvent fluctuation function is found to have correlation times ranging from 20 to 70 fs and variable prefactors. The data are inconsistent with a proposed model of solvation where the initial dynamics are determined by inertial rotational motion of the solvent dipoles and suggest that intermolecular vibrational coupling may be responsible for the solvent dependence of the dephasing rate. The temperature dependence of the electronic dephasing of LD690 in polymers has been measured, and the dephasing appears to be the result of a very low frequency mode, although the data cannot be fit with a simple Brownian oscillator model. Finally, we show that chirped ultrashort pulses can help discriminate between molecular vibrational dynamics on the ground and excited electronic states and provide a new way to probe the coherent wavepacket dynamics on those states.