How Adsorbate Alignment Leads to Selective Reaction

There has been much interest in the effect of adsorbate alignment in a surface reaction. Here we show its significance for an electron induced reaction occurring along preferred axes of the asymmetric Cu(110) surface, characterized by directional copper rows. By scanning tunneling microscopy (STM), we found that the heterocyclic aromatic reagent physisorbed meta-iodopyridine, lay with its carbon-iodine either along the rows of Cu(110), A, or perpendicular, P. Electron induced dissociative attachment with the bond initially along A gave a chemisorbed I atom and chemisorbed vertical pyridyl, singly surface-bound, whereas that with C-I along P gave a chemisorbed I atom and a horizontal pyridyl, doubly bound. An impulsive two state model, involving a short-lived antibonding state of C-I, accounted for the different product surface binding in terms of closer Cu center dot center dot Cu atomic spacing along A accommodating only one binding site of the pyridyl ring recoiling from I and wider spacing along P accommodating simultaneously both binding sites, N-Cu and C-Cu, in the meta position on the recoiling pyridyl ring. STM studies combined with dynamical modeling can be seen as a way to improve understanding of the role of surface alignment in determining reactive outcomes in induced reaction at asymmetric crystalline surfaces.