Adsorption of organic molecules on semiconductor surfaces has been attracting a growing attention for its importance in emerging technologies. Fundamental research on the covalent bonding of molecules with the surface can provide useful information on the organic/semiconductor interface. The NEXAFS spectroscopy represents a powerful technique to investigate the orientation and geometry of molecules adsorbed on surfaces and is widely used to characterize adsorbate structures, often in concert with theoretical calculations. The computational simulation of NEXAFS spectra of such systems 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 a DFT-TS simulation of the NEXAFS spectra of pyridine adsorbed on a regular Si (100) surface, considering several adsorption models. The models have been previously optimized through periodic calculations performed with the Quantum Espresso code, 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, employing traditional molecular DFT techniques (through the ADF code). The results compare well with the experimental spectra and show that the calculated polarized spectra can provide important information on specific details of the adsorbtion geometries and that the methodology employed is reliable to describe the K-shell spectra of this kind of systems.
