Direct observation of mobility and interactions of oxygen molecules chemisorbed on the Ag(110) surface

The energetics of thermal motions and interactions of oxygen molecules chemisorbed on a Ag(110) surface were investigated by scanning tunneling microscopy at 60-100 K. Surface mobility is anisotropic, preferably in the [110] direction with an activation energy of 0.22 q 0.05 eV and a preexponential factor of 1 x 10(13q3) s(-1). Along the [110] direction a repulsive interaction between nearest neighbors of about 0.02 eV and an attraction of 0.04 q 0.01 eV between next nearest neighbors were derived. Along [001] appreciable repulsion exists between nearest neighbors. while a ''diagonal'' arrangement of molecules is associated with an attraction of 0.02 q 0.01 eV. The data are indicative for the operation of indirect, substrate-mediated molecule-molecule interactions.