The slowing-down of ultrafast relaxation in C60 films at high femtosecond pump intensities

60

thin film was investigated in the energy range between 1.6 eV and 3.4 eV by a pump-supercontinuum probe (PSCP) technique with a 40 fs time resolution. The relaxation rate showed pronounced spectral dependence, with a maximum at 2 eV in the region of photo-induced darkening and at 2.4 eV in the region of photo-induced bleaching. The ultrafast relaxation rate decreased with increasing pump-pulse intensity. We suggest that the observed decrease results from extraheating of the carriers in the h_u and t_1u bands due to internal conversion from higher excited states, which are populated by two-step photon absorption of the intense pump pulse.