Superlattice structure of an Ar monolayer on Ag(111) observed by low-temperature scanning tunneling microscopy

Scanning tunneling microscope (STM) images of a carefully prepared monolayer of Ar on Ag(111) reveal patterns of bright spots, which exhibit perfect ordering over large areas. The dimensions of the slightly oblique unit cell are 1137x2507 pm(2), where the short side is aligned along the [-110] high-symmetry direction of the substrate, and the long side shows a regular division into two parts. In total six domains are observed. Analysis of this pattern, taking into account the tendency of noble gas overlayers to form hexagonal structures, which might be slightly stretched or compressed, leads to a structure for the Ar overlayer, which reproduces all the observed features. Surprisingly, it turns out that the bright STM spots correspond to Ag atoms which are not directly covered by Ar but surrounded by three Ar atoms from the adsorbate layer. A possible electronic origin for this unusual appearance is discussed.