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 is shown that by making use of the various degrees of freedom which are offered by this technique information about molecular dynamics on different potential-energy surfaces can be accessed and separated from each other. By varying the timing, polarization, and wavelengths of the laser pulses as well as the wavelength of the detection window for the FWM signal, different dynamics are coherently excited and probed by the nonlinear spectroscopy. As a model system we use iodine in the gas phase. These techniques are then applied to more-complex molecules (gas phase: benzene, toluene, a binary mixture of benzene and toluene; solid state: polymers of diacetylene matrix-isolated in single crystals of monomer molecules). Here, ground-state dynamics are investigated first without any involvement of electronically excited states and then in electronic resonance to an absorption transition in the investigated molecules. Signal modulations result which are due to wave packet motion as well as polarization beats between modes in different molecules. Phase and intensity changes yield information about intramolecular vibrational energy redistribution, population decay (T1), phase relaxation (T2), and coherence times.