Optical absorption spectra of ultrathin PTCDA films on gold single crystals: Charge transfer beyond the first monolayer

We report on the optical absorbance behavior of ultrathin 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films epitaxially grown on Au(1 1 1) and Au(1 0 0). Although PTCDA on Au(1 1 1) has been classified before as weakly interacting without noticeable metal–organic charge transfer (CT), we present clear optical evidence for ionized PTCDA at the interface. Unexpectedly, the charging of PTCDA is not restricted to the layer of contact, but extends into the second monolayer at least. A new feature in the optical spectra is identified as PTCDA cation, and we reason that the second monolayer is (partly) positively charged. We propose the CT to be related to the interface dipole formation between the first and second PTCDA monolayers on Au.