Nonexponential relaxation in solid C60 via time-dependent singlet exciton annihilation

We present measurements and detailed modeling of the dynamics of solid C60 following photoexcitation. Optical pulses 60 fs in duration centered at 620 nm are used to excite the lowest optical band, and the relaxation of the photoexcitations is measured using a time-resolved pump-probe technique. The response is shown quantitatively to follow a time-dependent singlet exciton annihilation process consistent with long-range Fِrster annihilation. This result is supported by measurements of the dependence of the relaxation on excitation density and probe wavelength, by comparison of the response in thin film, crystalline, and solution samples, and by numerical modeling.