  The oxidized Ag(111) surface has been studied by a combination of
experimental and theoretical methods, scanning tunneling microscopy (STM),
x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). A
large variety of different surface structures is found, depending on the
detailed preparation conditions. The observed structures fall into four
classes: (a) individually chemisorbed atomic oxygen atoms, (b) three different
oxygen overlayer structures, including the well-known p(4 x 4) phase, formed
from the same Ag$_6$ and Ag$_{10}$ building blocks, (c) a c(4 x 8) structure
not previously observed, and (d) at higher oxygen coverages structures
characterized by stripes along the high-symmetry directions of the Ag(111)
substrate. Our analysis provides a detailed explanation of the atomic-scale
geometry of the Ag$_6$/Ag$_{10}$ building block structures, and the c(4 x 8)
and stripe structures are discussed in detail. The observation of many
different and co-existing structures implies that the O/Ag(111) system is
characterized by a significantly larger degree of complexity than previously
anticipated, and this will impact our understanding of oxidation catalysis
processes on Ag catalysts.
