To understand the precise nature of the molecular changes due to the exposition of N2 molecule of the Si(111)-7x7 surface, we have used the XPS technique to provide elemental-selective information. After exposition to N2 (RT, 30 minutes, P = 2•10 -5 mbar), XPS experiments show a N(1s) line in addition to the two Si(2s) and Si(2p) lines originating from the Si(111)-7x7 surface (Figure ).