Two-dimensional probing of ground-state vibrational dynamics in porphyrin molecules by fs-CARS

We report on spectrally dispersed femtosecond time-resolved coherent anti-Stokes Raman scattering (CARS) for the investigation of the electronic ground-state vibrational dynamics of porphyrin molecules in solution. The two-dimensional experimental data obtained in our measurements provide a detailed mapping of the large number of molecular vibrations in these systems, excited by broadband ultrashort laser pulses, and make a thorough interpretation of the complex signal structure possible. In particular, an analysis of the data by applying Fourier transform methods to the time domain signal allows a clear identification of the excited porphyrin normal modes when compared with results on the mode structure from spectrally resolved (cw) spectroscopy. Altogether the method is capable of yielding detailed information on the dephasing behavior and on the relative spectral positions of the excited vibrations at the same time. Three porphyrin systems, dissolved in dichloromethane, were investigated: magnesium octaethylporphyrin, magnesium tetraphenylporphyrin and the free-base porphyrin octaethylporphyrin. The measurements were performed in the spectral region of the ring vibrations of the porphyrin macrocycle. We also provide a discussion of the expected signals on a theoretical basis by deriving the CARS third-order non-linear polarization for these experiments. Copyright (C) 2001 John Wiley & Sons, Ltd.