Spectral dependence of ultrafast relaxation in solid C-60

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 a minimum at 2 eV in the region of photoinduced darkening and at 2.4 eV in the region of photoinduced bleaching. The shape of the optical density variation at zero time delays in the region of interband transitions at 2.3-3 eV suggests that creation of a random electric field in the sample occurs during the absorption of the pump pulse. Excitation of coherent phonons in the 60-300 cm(-1) frequency range was investigated. The Fourier spectrum of oscillations indicates that the nonequilibrium dimerization of C-60 molecules and strong deformation of the molecules takes place following optical excitation.