The primary stages of the charge carrier photogeneration in C-60 films studied by the 100-fs laser pulse pump-probe method

The primary stages of photoinduced processes are studied in thin C-60 films by the femtosecond laser pump-probe method. The films were excited by 100- fs laser pulses with photon energies above (wavelengths 345 and 367 nm) and below (645 nm) the mobility threshold, the fraction of excited molecules b eing no more than several percent. Upon probing in the spectral range from 400 to 1100 nm, several regions with substantially different decay kinetics were observed in the difference spectrum, which is caused by the simultane ous presence of several relaxing components. The appearance of the 465- and 500-nm bleaching bands in the difference spectrum upon excitation by photo ns with energies both above and below the mobility threshold, which are typ ical for electroabsorption spectra, suggests that charge carriers are produ ced in both these cases. The observed dependence of relaxation on the oxyge n amount in the sample volume suggests that during excitation both charged (electrons and holes) and neutral (excited molecules) components are produc ed. The fraction of charged components is greater upon excitation into the fundamental band. The appearance of the 500-nm absorption band delayed by 1 0(-13)-10(-14) s, the delay being increased in the presence of oxygen, was attributed to the formation of excited anions due to the capture of electro ns by C-60 molecules. It is concluded that upon excitation of the films by photons with the energy below the mobility threshold, charge carriers are p roduced due to two-photon absorption rather than due to singlet-singlet ann ihilation. When the films are excited by photons above the mobility thresho ld, the primary charge carriers are produced by direct optical excitation. (C) 2001 MAIK "Nauka/Interperiodica".