The choice of the surface plays a crucial role when seeking for an efficient approach to dissociate inert molecule like N2. Therefore, we chose the Si(111)-7x7 surface because this surface is known to be very reactive and is able to convert, for instance, NO into N2O by on-surface heterogeneous process 6 or to dissociate NH3 into layers of Si3N4. Surprisingly, there is no report of the dissociation of N2 on the Si(111)-7x7 surface. After the preparation of the Si(111)-7x7 surface in ultra-high vacuum (UHV) by standard procedure, the surface was exposed to N2 molecular gas for 30 minutes at different pressures ranging from 6•10 -9 mbar to 5•10 -5 mbar. For pressures below 1•10 -6 mbar, STM images of the Si(111)-7x7 surface are unchanged compared to the starting clean surface (Figure ). Surprisingly, after an exposition to N2 gas at a pressure of 3•10 -6 mbar during 30 min, unusual dark spots are observed in the STM images (Figures 1b). The density of these dark spots strongly increases after exposition at a pressure of 1•10 -5 mbar (Figure ). A close inspection of the STM image corresponding to an area of 15x15 nm 2 (Figure ) reveals that darker protrusions can be assigned to silicon adatoms in both faulted or unfaulted half-unit cells. In addition, STM images recorded for negative polarities (i.e. filled states) show that some rest-adatoms appear as dark protrusion (Figure ). In order to detect a potential contribution from dissociated nitrogen that may be produced on the filament gauge used to measure the pressure in the UHV chamber, N2 exposure was performed on a clean Si(111)-7x7 surface while the filament was turned off once the deposition pressure is reached (5•10 -6 mbar). Additional test was performed with all chamber windows covered by aluminum foil insuring a complete darkness inside the chamber during gas exposition. In both cases the same amounts of dark spots was observed on the Si(111)-7x7 surface supporting that there is no effect of the gauge filament or the ambient light on the dissociation reaction.