A MULTITECHNIQUE SPECTROSCOPIC INVESTIGATION OF THE ADSORPTION OF PYROMELLITIC DIANHYDRIDE (PMDA) ON PT(111)

The adsorption of pyromellitic dianhydride (PMDA) on the Pt(111) surfa ce was studied by near edge X-ray absorption fine structure spectrosco py (NEXAFS), X-ray photoelectron spectroscopy (XPS), ultraviolet photo electron 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 b roadening of the pi resonances in the carbon K-edge NEXAFS-spectra wh ich is an indication for a strong adsorbate-substrate interaction. An analysis of the polarization dependence of the NEXAFS resonances confi rms the presence of flat-lying molecular PMDA on the surface, but addi tionally indicates a strong (approximate to 20 degrees for the anhydri de units) out of plane distortion of the molecules. UPS on gas phase P MDA reveals that the HOMO is located at 11.8 eV below E(Vac). TPD spec tra recorded at different molecular masses reveal that above 340 K mon olayer PMDA undergoes competing decomposition and desorption; decompos ition to a surface carbon residue is complete by 550 K.