We considered an "end-on" site over an adatom and over a rest adatom, and a "side-on" site between a rest and an adatom (Figure ). These three adsorption sites have been previously identified as the most favorable adsorption sites for N2 on the SiC(111) surface. In contrast to the SiC(111) surface, our DFT results indicate that N2 is weakly bonded to Si(111)-7x7 (Eads ~ 0.4 eV) with relatively long Si-N equilibrium distances (dSi-N ~ 0.3 nm), characteristics of a physisorbed state. For the "end-on" site over a silicon adatom, the equilibrium distance of N2 over the Si(111)-7x7 surface is 0.3 nm (Figure ). For this equilibrium distance, the projected density of states (PDOS) of N2 indicates that the lowest p*-antibonding level of adsorbed N2 is centered at ~ 3 eV above Fermi level of the system (Figure ). However, if we consider that an increase in the pressure may increase the absolute number (but not the fraction) of N2 molecules with sufficient energies to bring N2 closer to the Si(111)-7x7 surface (Figure ), the interaction of N2 molecule with the Si adatom significantly changes. In this case, the lowest p*-antibonding level of N2 is shifted toward Fermi level, and becomes partially occupied (Figure ). Respectively, the two other cases (end-on over rest-atom or side-on) exhibit a similar shifting of the p*antibonding level of N2 molecule toward Fermi level for Si-N distance shorter than the equilibrium one (Figure ). Therefore, as the PDOS of the p*-antibonding level moves toward Fermi level for smaller Si-N distance, the partial electron transfer from the Si(111)-7x7 surface to the lowest p*- antibonding orbital of N2 molecule improves. By populating the lowest p*-antibonding orbital of N2, such electron transfer initiates the subsequent spontaneous dissociation process by N≡N partial bond cleavage at the interface, as observed in XPS spectra (Figure ). Then, a thermal annealing at 800 K leads to the complete dissociation of any remaining N≡N bonds to give atomic N triply coordinated with silicon adatoms. This electron transfer, occurring even at room temperature, is not the driving force of the process. Indeed, the occupation of the lowest p*antibonding orbital of N2 is possible when the distance of N2 from the Si(111)-7x7 surface is smaller than in the physisorbed phase. In this case, the p*-orbital of N2 pinned at Fermi level, and electron transfer occurs. This charge transfer is achieved when N2 molecules collide the surface, with a sufficient energy to be brought closer to the Si(111)-7x7 surface. Finally, as we observed a decreasing of the extinct silicon adatoms when we increase the temperature (Figure ), we can conclude that the energy barrier for the complete dissociation of N2 molecules is weaker than those for the desorption from the surface. 5g-h In this case, the process of dissociation is called a "facile system". This is also consistent with our DFT results indicating that N2 is physisorbed on Si(111)-7x7; low temperature conditions should improve the "sticking" of N2 on the Si(111)-7x7 surface.