Phenylacetylene chemisorbed on Pt (111), reactivity and molecular orientation as probed by NEXAFS. Comparison with condensed multilayer and polyphenylacetylene

Near-edge X-ray absorption spectroscopy (NEXAFS) has been used to investiga te the reactivity of phenylacetylene (PA) towards a Pt(lll) surface and its molecular orientation. Spectra have been taken in the photon energy region 280-320 eV at both grazing and normal X-ray beam incidence angles with res pect to the sample surface. NEXAFS K-edge spectra have been recorded for PA adsorbed in the monolayer as well as in the multilayer regime and, for com parison, the spectrum of the PA related polymer, polyphenylacetylene (PPA), has also been measured. The assignment of the pi* and sigma* resonances ha s been made on the basis of the chemical structure and interpreted with the help of ab initio calculations. Both the ionisation potentials for the C I s electrons and the theoretical C K-edge absorption spectra have been calcu lated for all the six inequivalent carbon atoms of PA and of phenylethylene (PE) which is a model molecule for PPA. From the analysis of the data we h ave indication for an interaction involving mainly the C=C triple bond of t he acetylenic functional group of PA that undergoes opening and leads to th e formation of a di-sigma bond with Pt, while the benzene C=C bonds seem no t affected. A polarization dependence has been evidenced for the pi* resona nces of PA adsorbed in monolayer condition indicating a molecular orientati on of the phenyl ring pi* orbitals between 37 degrees and 34 degrees relati ve to the normal to the crystal surface. (C) 1999 Elsevier Science B.V. All rights reserved.