Spectral dependence of femtosecond relaxation and coherent phonon excitation in C-60 films

The ultrafast relaxation of excitations in a C-60 thin film was probed in the energy range 1.6-3 eV with femtosecond laser spectroscopy. The relaxation time has minima at 2 eV in the region of photoinduced darkening and 2.4 eV in the region of photoinduced bleaching. The shape of the optical density variation at zero time delay in the region of interband transitions at 2.3-3 eV is similar to the shape of the second derivative of the stationary absorption spectrum. We propose that this may indicate the creation of a random electric field in the sample during the absorption of the pump pulse. Coherent excitation of phonons in the 60-300-cm(-1) frequency range were detected over a wide spectral probe range. An oscillation with a frequency of 118 cm(-1) indicates that the nonequilibrium dimerization of C-60 molecules takes place following optical excitation. The full splitting of the H-g(1) intramolecular oscillation mode is observed clearly, which demonstrates that a strong deformation of the molecules upon photon absorption takes place.