  We insert and manipulate a single chlorine atom in chlorine monolayer on a
Si(100)-2x1 surface using a scanning tunneling microscope. Two objects were
created - a Cl atom in a groove between two dimer rows, and bridge-bonded Cl on
a silicon dimer. Changing the voltage polarity leads to conversion of the
objects into each other. Anisotropic movement of the objects at 77 K is
mediated by two different diffusion mechanisms: hopping and crowdion-like
motion. Insertion of a Cl atom in a groove between two dimer rows leads to the
formation of a dangling bond on a third-layer Si atom. At positive sample
voltage bias, the first object is positively charged, while the second object
can be neutral or negatively charged depending on silicon sample doping.
