Cleaved (111) surfaces on CaF2 were imaged with scanning force microscopy o
perated in the dynamic mode in ultrahigh vacuum. Imaging the pristine surfa
ce reveals an atomic scale contrast with the structure expected For the flu
orine terminated surface. We always reproduced the perfect surface periodic
ity never observing stable defects. However, after exposing the surface to
280 L of oxygen while constantly scanning, we detected,stable atomically re
solved defects that are assigned to OH- groups incorporated into the surfac
e. We could identify a jump of one of the groups from one atomic cell to th
e next. The observed contrast at regular lattice sites as well as at defect
s is discussed and qualitatively explained.