Picosecond time evolution of photoexcitations at 2.33 eV in α-sexithyenil thin films

Picosecond time dynamics of the isotropic as well as the anisotropic transient absorption saturation have been measured in an {α}-sexithyenil (T₆) thin film at room temperature by photoinduced absorption and resonant Kerr-gate experiments. Strongly damped photoacoustical oscillations have been detected superimposed on the photobleaching signal, providing a way to evaluate the sound speed in T₆ films along the normal to the substrate; VS{}16 {}/ps. A stretched exponential function has been used to describe the time recovery of the total photoexcited population yielding a time constant {τ}=3.5 ns with an exponent {α}=0.5. This is indicative of dispersive diffusion towards recombination centers via intermolecular hopping; the Kerr-gate transmission decay provides an estimate of the photoexcitation diffusion constant, namely, D=6×{}10^-3 cm²/s.