Wave packet dynamics in different electronic states investigated by femtosecond time-resolved four-wave-mixing spectroscopy

This paper reviews results on wave packet dynamics investigated by means of femtosecond time-resolved four-wave-mixing (FWM) spectroscopy. First, it i s shown that by making use of the various degrees of freedom which are offe red by this technique information about molecular dynamics on different pot ential-energy surfaces can be accessed and separated from each other. By va rying the timing, polarization, and wavelengths of the laser pulses as well as the wavelength of the detection window for the FWM signal, different dy namics are coherently excited and probed by the nonlinear spectroscopy. As a model system we use iodine in the gas phase. These techniques are then ap plied to more-complex molecules (gas phase: benzene, toluene, a binary mixt ure of benzene and toluene; solid state: polymers of diacetylene matrix-iso lated in single crystals of monomer molecules). Here, ground-state dynamics are investigated first without any involvement of electronically excited s tates and then in electronic resonance to an absorption transition in the i nvestigated molecules. Signal modulations result which are due to wave pack et motion as well as polarization beats between modes in different molecule s. Phase and intensity changes yield information about intramolecular vibra tional energy redistribution, population decay (T-1), phase relaxation (T-2 ), and coherence times.