To determine the evolution of extinct silicon adatoms as a function of the pressure of N2, we analyzed more than 50 STM images recorded at positive sample voltage (empty states) with different pressures but by keeping the duration of exposition at 30 minutes. The counting of extinct adatoms in each STM image was determined by using the particle detection module included in the SPIP software. The increasing number of dark protrusions as a function of the N2 pressure for an exposition of 30 minutes at RT clearly highlights a threshold at P = 1•10 -6 mbar and a plateau when P > 1•10 -5 mbar (Figure ). After exposition to N2, the Si(111) surface has been then heated at 800 K during 30 min. No modifications were observed on the resulting STM images, recorded in the two polarities (Figure ).