The nitridation mechanism of silicon at room temperature under exposure to 100-1000 eV N-2(+) ion beams has been studied in situ in an ultrahigh vacuum apparatus using x-ray photoelectron spectroscopy. The increase of the nitrogen content in a surface layer as a function of the ion dose was described by a simple formula which was derived by assuming random occupation of the reaction sites in the penetration zone of the nitrogen atoms. A change of the binding energy and the width of the N1s x-ray photoelectron spectrum during the reaction was observed and discussed with the component ratio N/Si-reacted. The Si2p x-ray photoelectron spectra were deconvoluted into five components of Si(0), Si(1), Si(2), Si(3), and Si(4) by curve fitting, where Si(n) represents the component of Si bonded to n nitrogen atoms. Their populations were dependent on the ion dose and the ion energy. The nitride layers formed in the Si surface with low energy beams of 100-200 eV had near-stoichiometric composition of Si3N4. With beams of energy higher than 300 eV, however, they were nonstoichiometric compounds SiNy, (y<1.3) which were mixtures of those components. The influence of the beam energy was observed by the chemical shifts of the N1s and Si2p peaks at the saturation of the N content.