C. Mainka et al., LINEAR DICHROISM IN X-RAY-ABSORPTION SPECTROSCOPY OF STRONGLY CHEMISORBED PLANAR MOLECULES - ROLE OF ADSORPTION INDUCED REHYBRIDISATIONS, Surface science, 341(3), 1995, pp. 1055-1060
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.