Femtosecond time-resolved coherent Raman spectroscopy for probing molecular dynamics

This paper reviews results on wavepacket 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 these nonlinear Raman or transient grating techniques information about molecular dynamics on different potential energy surfaces can be accessed and separated front each other. By varying the timing, and wavelengths of the laser pulses as well as the wavelength of the detection window for the FWM signal, different dynamics are coherentely excited and probed by the nonlinear spectroscopy. As a model system iodine in the gas phase was used. Second we report about selective excitation of the vibrational modes in the electronic ground state of porphyrin systems by means of a femtosecond time-resolved CARS (coherent anti-Stokes Raman scattering) scheme. By detecting the spectrum of the transient CARS signal, a detailed mapping of the dynamics initiated by the stimulated Raman pump process is achieved. The method is able to yield the dephasing behaviour and spectral information of the investigated system at the same time. The different contributions to the ground state vibrational dynamics are selected by changing the direction of the CARS signal analyzer in the polarization arrangement used.