Ultrafast excitation relaxation in titanylphthalocyanine thin film

Ultrafast excitation relaxation in the whole Q band of titanylphthalocyanine amorphous thin film fabricated by physical jet deposition was investigated by femtosecond time-resolved pump-probe technique. The measured relaxation dynamics was found to be strongly dependent on the wavelength of the laser beam and consists of three quite different processes: an ultrafast process with a lifetime of 0.5-5 ps, a fast and a long-lived processes with lifetimes of about S-10ps and longer than 100 ps, respectively. The initial ultrafast decay appearing to be excitation intensity dependent is suggested to represent a bimolecular exciton-exciton annihilation process with a t ^-1/2 time dependence of the excited-state population, assigned to a one-dimensional exciton diffusion. The exciton-exciton annihilation is observed in the pump intensity as low as 0.27 GW/cm².