Scanning Force Imaging of Atomic Size Defects on the ${\mathrm{CaF}}_2\left(111\right)$ Surface

Cleaved (111) surfaces on ${\mathrm{CaF}}_2$ were imaged with scanning force microscopy operated in the dynamic mode in ultrahigh vacuum. Imaging the pristine surface reveals an atomic scale contrast with the structure expected for the fluorine terminated surface. We always reproduced the perfect surface periodicity never observing stable defects. However, after exposing the surface to 280 L of oxygen while constantly scanning, we detected stable atomically resolved defects that are assigned to ${\mathrm{OH}}^{-}$ groups incorporated into the surface. We could identify a jump of one of the groups from one atomic cell to the next. The observed contrast at regular lattice sites as well as at defects is discussed and qualitatively explained.