Low temperature scanning force microscopy of the Si(111)-(7 x 7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mo de in ultrahigh vacuum was used to study the Si(111)-(7 X 7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cel l are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atom ic sites are presented. The data are in good agreement with first principle s computations and indicate that the nearest atoms in the tip and sample re lax significantly when the tip is within a few Angstrom of the surface.