PICOSECOND TIME EVOLUTION OF PHOTOEXCITATIONS AT 2.33 EV IN ALPHA-SEXITHYENIL THIN-FILMS

Picosecond time dynamics of the isotropic as well as the anisotropic t ransient absorption saturation have been measured in an alpha-sexithye nil (T-6) thin film at room temperature by photoinduced absorption and resonant Kerr-gate experiments. Strongly damped photoacoustical oscil lations have been detected superimposed on the photobleaching signal, providing a way to evaluate the sound speed in T-6 films along the nor mal to the substrate; V-s similar or equal to 16 Angstrom/ps. A stretc hed exponential function has been used to describe the time recovery o f the total photoexcited population yielding a time constant tau=3.5 n s with an exponent alpha=0.5. This is indicative of dispersive diffusi on towards recombination centers via intermolecular hopping; the Kerr- gate transmission decay provides an estimate of the photoexcitation di ffusion constant, namely, D=6X10(-3) cm(2)/s.