Femtosecond time-resolved impulse-stimulated Raman scattering: recent experiments and theory

Coherent terahertz-frequency optic phonon oscillations and decay are observed directly by femtosecond time-resolved impulsive stimulated Raman scattering. This time resolution permits study of vibrational dephasing and relaxation in organic molecular crystals at high temperatures (> 300 K)as well as low temperatures. The results can be used to deduce the microscopic mechanisms through which vibrational equilibrium is reached in these crystals. Current experiments aimed at direct measurement of homogeneous and inhomogeneous vibrational dephasing times as well as vibrational population lifetimes are discussed. Finally, preliminary results of similar experiments on low-frequency vibrations of molecules in solution are presented and discussed. The possibilities for measurement of electronic excited state-phonon interactions and observation of large-amplitude vibrational motion are suggested.