The diffusive motion of O atoms adsorbed on Ru(0001) was observed on the at
omic scale by scanning tunneling microscopy with temporal resolution of app
roximate to 10 ms at room temperature. From statistical analysis of the cha
nges of the atom configurations in sequences of more than 1000 images, mean
residence times of oxygen atoms in the neighborhood of other oxygen atoms
were extracted. The residence times vary by more than 1 order of magnitude
over distances of three lattice constants, indicating the necessity of incl
uding the lateral interaction in modeling collective phenomena such as surf
ace diffusion or reactions.