A multitechnique spectroscopic investigation of the adsorption of pyromellitic dianhydride (PMDA) on Pt(111)

The adsorption of pyromellitic dianhydride (PMDA) on the Pt(111) surface was studied by near edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and temperature programmed desorption (TPD). The XPS data verify a non-dissociative adsorption for the monolayer in agreement with Fourier Transform Infrared Absorption Spectroscopy (FT-IRAS) data reported previously. Adsorption leads to considerable broadening of the pi* resonances in the carbon K-edge NEXAFS-spectra which is an indication for a strong adsorbate-substrate interaction. An analysis of the polarization dependence of the NEXAFS resonances confirms the presence of flat-lying molecular PMDA on the surface, but additionally indicates a strong (approximate to 20 degrees for the anhydride units) out of plane distortion of the molecules. UPS on gas phase PMDA reveals that the HOMO is located at 11.8 eV below E(Vac). TPD spectra recorded at different molecular masses reveal that above 340 K monolayer PMDA undergoes competing decomposition and desorption; decomposition to a surface carbon residue is complete by 550 K.