The N(1s) line is broad and asymmetric, as expected for a multicomponent line containing at least two species. The full width at half maximum (FWHM) of this line is about 2.2 eV. This peak has been fitted onto three components with binding energies (BE) of 400.1 ± 0.1 eV (A1, blue line), 398.6 ± 0.1 eV (A2, green line), and 397.6 ± 0.1 eV (A3, red line), respectively (Figure ). Each component (A1, A2 and A3) was fitted with a FWHM value of 1.6 eV. Furthermore, the Si(111)-7x7 surface exposed to a pressure of N2 (P = 2•10 -5 mbar, 30 minutes) was annealed at 800 K for 30 minutes. After this thermal annealing, N(1s) line is sharper and composed of a single line with a BE of 397.6 eV. Its FWHM is found to be 1.5 eV. This value is consistent with that obtained for the three components of the N(1s) line before annealing (Figure ). Note that the area of the N(1s) lines before and after annealing is almost the same (within a few percent). Therefore, this result clearly indicates that the nitrogen species, adsorbed on Si(111)-7x7 after exposition at room temperature, are not desorbed after annealing at 800 K. The A1 and A2 lines (Figure ) observed before annealing correspond to intermediate species partially dissociated N2 molecules grafted onto Si(111)-7x7 surface. The main line before annealing (A3, Figure ) and the single line measured after annealing (Figure ), both located at a BE of 397.6 eV correspond to atomic N triply coordinated with silicon adatoms, e. g. Si3-N, in which each N2 molecule is fully dissociated into two nitrogen atoms bounded to the surface. 9 Thereby, the extinct silicon adatoms observed in the STM images are attributed to the silicon adatoms covalently bounded to nitrogen atoms. This result is also supported by the partial second order relative to surface-adsorption sites discussed above (Figure ). Therefore, all experimental data, including STM images with atomic resolution and XPS spectra, indicate that adsorbed N2 molecules are dissociated on the Si(111)-7x7 surface at room temperature after an exposition to a pressure of N2 greater than 1•10 -6 mbar for 30 minutes. In order to support our experimental results, we have evaluated the adsorption energy and the electronic structure of N2 molecules on different adsorption sites of a Si(111)-7x7 surface with DFT calculations.