Femtosecond dynamics in thin organic films of fluoroaluminum phthalocyanine.

Fluoroaluminum phthalocyanine (AlPc-F), which contains a 2-D π-electron system, has shown a relatively large optical nonlinearity [X⁽³⁾ = 5 × 10^-11 esu] and very high laser damage threshold (>20 GW/cm²). The measurement of the electronic dynamics of polycrystalline AlPc-F at room temperature is reported. Femtosecond laser pulses of 150-fs duration at an 8.5-kHz repetition rate are used in this experiment. The more energetic narrowband pump pulse centered at 6200 angstrom creates an excited-state population. The differential transmission spectrum (DTS) of the weaker broadband probe beam is then monitored at the electronic band edge between 7400 and 8600 angstrom. The DTS of the probe pulse at various time delays between the pump and probe pulses, the evolution of the DTS signal at longer time delays up to 15 ps, and the decay of the DTS signal as a function of time for different pumping intensities with the probe at 7800 angstrom of the DTS peak are shown and discussed. An interpretation based on Frenkel exciton terminology successfully accounts for the observed results.