Ultrafast nonlinear spectroscopic study of reaction dynamics in the condensed phase

Various third-order nonlinear spectroscopic methods have been applied to study fundamental aspects of reaction dynamics in several systems, including electron-transfer in electron-donating solvents, isomerization in 1,1 ′-diethyl-4,4′-cyanine (1144C) and the triphenylmethane (TPM) molecules.; The photo-induced electron-transfer reaction from electron-donating solvents dimethylaniline and diethylaniline to a dye rhodamine6G was studied using transient-grating (TG) and three-pulse photon echo peak shift (3PEPS) techniques. A three-level model, including contribution from excited state absorption, was used to model the 3PEPS and TG data. It was demonstrated that 3PEPS retrieves information about both the reaction coordinate and solvation dynamics. The interplay between solvation dynamics and ultrafast electron transfer reaction has been discussed using complementary information obtained from the two techniques.; The isomerization of 1144C was studied by 3PEPS, TG and pump-probe (PP) techniques. It was found that the ground state recovery process is dominated by the bond-twisting motion on the excited state surface. A larger isomer-product yield was obtained in less viscous solvents or with greater excitation energy. Apparently different behavior was observed in the TG and PP signals, indicating that the real and imaginary components of third-order polarization P( 3) are associated with different decay profiles.; An optical heterodyne detected transient grating (OHD-TG) experiment was carried out to separate the real and imaginary components of P( 3). We developed a simplified diffractive optics based OHD-TG arrangement, which features ease of alignment, suppression of scattering, and is free of pump-probe contamination in the detected TG signals. We applied this arrangement to the OHD-TG study in various resonant systems. OHD-TG measurements in a non-reactive system, rhodamine640 in dimethylsuloxide, showed that the real and imaginary components follow the same decay profiles, except for differences in their amplitudes and the short time behavior. The latter was found to be contaminated by interference between the resonant solute and non-resonant solvent contributions. In contrast, OHD-TG measurements in reactive TPM molecules revealed striking differences between the real and imaginary components of P(3). A model including the contribution from hot ground state absorption was proposed to account for these differences and provide a consistent picture for the underlying dynamics in these systems.