N2 gas and procedure of its deposition Scientific grade molecular N2 gas of purity 99.9999%, purchased from Linde Gas, was used in this work. All details about the gas contained impurities is presented in the datasheet of the Figure .3. In addition, the gas line that drive the gas from the compressed bottle to the chamber was pumped and baked until achievement of UHV condition similar to the one in the preparation chamber with a measured base pressure equal to 6.7x10 -11 mbar. After scanning and approving the surface cleanness, the Si(111)-7x7 sample was placed on the manipulator and held at RT in front of the capillary of N2 gas injecting. The distance between the sample surface and the extremity of the capillary is about 15 cm. Gas flow is controlled by using a high precision leak valve and by following the pressure measured inside the preparation chamber using a filament gauge. This sample exposure to the gas is known as "bulb" method where the flux of molecules contains with a thermal distribution of the kinetic energies following the law of Maxwell-Boltzmann distribution. X-ray photoelectron X-ray photoelectron experiments were performed with a hemispherical analyzer Omicron EA 125 equipped with a dual anode X-ray source (Omicron, DAR 400) using Al Kα X-ray source (hν =1486.6 eV) radiation. The photoelectrons were collected at normal electron emission. The angle of acceptance of the photoelectron analyzer was set to ± 1. The spectrometer energy scale was calibrated to the Ag 3d5/2 core level line measured on clean Ag sample, set at a binding energy of 386.25 eV. The Si(111) sample is still conductive after N2 adsorption and this procedure guarantees a good estimation of the N1s lines binding energy. The spectra were recorded using a pass energy of 20 eV for high resolution and 50 eV for wide scan. The peak was fitted to mixed Gaussian-Lorentzian components using XPS-CASA software 1 , after having subtracted a Shirley-type background.