Direct molecular imaging of NO monomers and dimers and a surface reaction on Ag{111}

Molecular and atomic resolution STM images are presented of NO monomers and dimers and the reaction products of the dimer thermal decomposition, N2O and O, on a Ag(111) surface. The weakly chemisorbed NO monomer is formed at low coverages and temperatures between 4 and 65 K and is imaged as a 0.25 Angstrom deep depression. Continued adsorption leads to multilayer dimer cluster formation at 4 K and smoother multilayer dimer packing at 65 K, with imaging of each component of some of the dimers in the top layer. Removal of the multilayer by evacuation or heating leaves the dimer chemisorbed layer, when each dimer is imaged as a protrusion. Large domains of four different ordered phases are present, which we designate as alpha, beta, chi, and delta. This layer is quite stable at 65 K. Images including clean patches of Ag(111) showing atomic resolution of the top Ag layer enable us to present structures for each of the dimer phases, with full site assignments. The dimers are adsorbed across bridge sites at low coverage and across bridge and an atop to 3-fold site at the highest coverage, 0.25 ML. On heating to 105 K, the dimer phases are largely converted to two new ordered phases, each of which is a mixed N2O + O phase. Some residue of the beta (NO)(2) phase remains, however, indicating that it is the least reactive phase. A mechanism for the dissociation involving thermal excitation of the hindered NO rotation about the N-N axis is consistent with the reactant-to-product image sequences. Further heating to 125 K invokes N2O desorption, leaving a residual silver oxide ring structure, which is a precursor to the stable Ag(lll)-p(4x4)-oxide structure.