Ultra-thin PTCDA layers studied by optical spectroscopies

3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) was deposited using organic molecular beam deposition (OMBD) onto various substrates, i.e. mica(0001), Au(111) layers on mica, and Se-passivated GaAs(100). Layer thicknesses were from 2 to 30 nm. Reflectance and transmittance measurements were performed in order to identify PTCDA absorption features and find suitable laser wavelengths for subsequent Raman investigations. Despite the low thicknesses the Raman spectra reveal strong scattering by the molecular vibrational modes, in particular above 1200 cm(-1). Frequency shifts of various modes in the layers from their values in PTCDA source material may indicate the influence of the substrates. Similar shifts were also observed in infrared spectra of the same materials.