Spectrally resolved femtosecond nonlinear spectroscopy of vibrational coherence relaxation in molecules

We investigate the potential of spectrally resolved femtosecond nonlinear spectroscopy for studying the dynamics of vibrational relaxation of molecules on a time scale of vibrational coherences in the specific case of the transient grating. Using a three-pulse two-colour spectrally resolved nonlinear technique we have carried out a detailed study of the dependence of the observed spectra on the wavelength of the excitation laser pulses for different dye molecules. The influence of molecular structure on the transfer of the coherence ensemble, which can be observed in the nonlinear spectrum and could be related to homogeneous and inhomogeneous broadening of molecular energy states, is discussed. The dynamics of dephasing and vibrational relaxation (80 fs/10 nm) can be deduced from the spectrally resolved measurement.