Quantitative measurement of short-range chemical bonding forces

We report direct force measurements of the formation of a chemical bond. Th e experiments were performed using a Low-temperature atomic force microscop e, a silicon tip, and a silicon (111) 7x7 surface. The measured site-depend ent attractive short-range force, which attains a maximum value of 2.1 nano newtons, is in good agreement with first-principles calculations of an inci pient covalent bond in an analogous model system. The resolution was suffic ient to distinguish differences in the interaction potential between inequi valent adatoms, demonstrating the ability of atomic force microscopy to pro vide quantitative, atomic-scale information on surface chemical reactivity.