LINEAR DICHROISM IN X-RAY-ABSORPTION SPECTROSCOPY OF STRONGLY CHEMISORBED PLANAR MOLECULES - ROLE OF ADSORPTION INDUCED REHYBRIDISATIONS

Dipole selection rules require the pi-resonances, which dominate X-ra y absorption spectra for planar aromatic molecules, to have zero inten sity for X-ray photons linearly polarized in the molecular plane. For molecules adsorbed in a flat geometry on metal surfaces, however, very often considerable intensities are observed for s-polarized light. We will present experimental and theoretical evidence that for two molec ules, benzene and pyromellitic dianhydride (PMDA) adsorbed on Pt(111), this anomalously weak linear dichroism is not due to molecular disord er as assumed previously but arises from molecular rehybridisation cau sed by out-of-plane molecular distortions.