Adsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Since the properties of the resulting materials are largely dependent on the organic/semiconductor interface, fundamental research on the covalent bonding of molecules with the surface can provide useful information. Problems that have been addressed include the structure of the resulting systems and spectroscopic measurements often in concert with theoretical calculations can assess the orientation and geometry of the molecular adsorbate. In this respect NEXAFS spectroscopy represents a powerful technique to investigate the interaction of molecules with a surface. The theoretical simulation of NEXAFS spectra of molecules adsorbed on a surface represents a significant challenge both for a proper modelling of the adsorbate system as well as for the size of system which needs theoretical methods capable to fulfill requirements of accuracy and computational economy. Here we present DFT simulations of NEXAFS spectra of pyridine adsorbed on a regular Si (100) surface, considering several adsorption models.1 The surface and the adsorbate models have been previously optimized through periodic calculations, then suitable finite clusters have been be cut out from the optimized periodic structures and used for the simulation of the angle resolved NEXAFS spectra of the adsorbed molecule. The spectra have been calculated employing a molecular DFT methodology based on the transition potential scheme in order to include the core hole relaxation effect.2 Both the N1s and C1s adsorption edges of the adsorbed pyridine have been investigated. The results show that a careful analysis of the calculated polarized spectra can provide important information on specific details of the adsorbtion geometries; the reliability of the computational strategy is proved by the comparison of the theoretical results with the experimental data.1,3 The periodic optimization of the molecules adsorbed on the surface plays a fundamental role in the design of clusters which correctly model the adsorbed system.
