  We use Ca doping during growth of one and two monolayer thick MgO films on
Ag(100) to identify the adsorption sites of individual adatoms with scanning
tunneling microscopy. For this we combine atomic resolution images of the bare
MgO layer with images of the adsorbates and the substitutional Ca atoms taken
at larger tip-sample distance. For Ho atoms, the adsorption sites depend on MgO
thickness. On the monolayer, they are distributed on the O and bridge sites
according to the abundance of those sites, 1/3 and 2/3 respectively. On the MgO
bilayer, Ho atoms populate almost exclusively the O site. A third species
adsorbed on Mg is predicted by density functional theory and can be created by
atomic manipulation. Au atoms adsorb on the bridge sites for both MgO
thicknesses, while Co and Fe atoms prefer the O sites, again for both
thickness.
