Random electric fields and coherent phonon excitation in C60 films by femtosecond laser pulses

The ultrafast relaxation of excitations in a C60 thin film was probed in the energy range 1.6 - 3.4 eV by pump- supercontinuum probe technique with 40 fs time resolution. The relaxation rate shows pronounced spectral dependence with maximum at 2 eV in the region of photoinduced darkening and at 2.4 eV in the region of photoinduced bleaching. It is found that the ultrafast relaxation rate decreases with increasing pumping pulse intensity. The shape of the optical density variation at zero time delay in the region of interband transitions at 2.3 - 3.4 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. We suggest that the decrease of the relaxation rate with the increase of pumping pulse intensity results from extra-heating of the carriers in hu and t1u bands due to internal conversion from higher excited states, which are populated by two-step photon absorption of the intense 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 C60 molecules takes place following optical excitation. The full splitting of the Hg(1) intramolecular oscillation mode is observed clearly, which demonstrates that a strong deformation of the molecules upon photon absorption takes place.