3-PULSE ECHO PEAK SHIFT STUDIES OF POLAR SOLVATION DYNAMICS

The three-pulse photon echo peak shift technique is used to study room temperature solvation dynamics in acetonitrile, benzonitrile, methano l, ethanol, ethylene glycol, and chloroform utilizing the tricarbocyan ine dye, IR144 as the probe molecule. In all cases, an ultrafast compo nent, ascribed to inertial solvent motion, was evident after the very rapid dephasing of the intramolecular vibrational modes. All the solve nts except acetonitrile show biexponential behavior on the picosecond (diffusive) time scale. In the case of acetonitrile, the spectral dens ity governing the solute-solvent interaction is recovered by numerical fitting of peak shift data. The inertial component of the spectral de nsity is compared with the solvation spectral density obtained from an instantaneous normal mode calculation. The two spectra agree well. Th e fitting parameters allow the absorption spectrum to be well reproduc ed. For ethylene glycol, data are also presented for 397 K; the ultraf ast component of solvation is quite independent of temperature, while the diffusive portion speeds up dramatically at high temperature.