Formation of (n x 1)-O/Ag(110) overlayers and the role of step-edge atoms.

Abstract

Oxygen adsorption on Ag(110) was studied by ultrahigh vacuum scanning tunneling microscopy. Strong evidence that the n\ifmmode\times\else\texttimes\fi{}1),n=7,...,2, oxygen overlayers adopt an ``added-row'' structure similar to the (2\ifmmode\times\else\texttimes\fi{}1)-O/Cu(110) system is presented. Since the oxygen ``added row'' on Ag(110) is a -Ag-O-Ag- chainlike structure atop the original substrate, a source of the substrate atoms must exist. Steps, in principle, serve as an efficient source of these substrate atoms, provided that the detachment rates from the step edges are sufficiently large. The nominal detachment rate (\ensuremath{\sim} 3 Ag atoms/s per step site) is sufficient to supply atoms to form the added-row structure when the oxygen partial pressure is low. For high oxygen pressures (\ensuremath{\sim} ${\mathit{P}}_{{\mathrm{O}}_{2}}$\ensuremath{\gtrsim}${10}^{\mathrm{\ensuremath{-}}5}$ mbar), the rate of dissociative oxygen chemisorption is competitive with the Ag supply rate from step detachments, and a second source of Ag atoms is provided by vacancy-island generation on the terraces. The microscopic structure of the oxygen overlayer of O/Ag(110) is quite similar to those of O/Cu(110) and O/Ni(110), but some differences in the formation kinetics are noted. \textcopyright{} 1996 The American Physical Society.