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Halogenation at a semiconductor surface follows simple dynamics characterized by "localized reaction" along the direction of the halide bond being broken. Here we extend the study of halide reaction dynamics to the important environment of a smooth metal surface, where greater product mobility would be expected. Extensive examination of the physisorbed reagent and chemisorbed products from two successive electron-induced reactions showed, surprisingly, that for this system product localization and directionality described the dynamics at a metal. The reagent was p-diiodobenzene on Cu(110) at 4.6 K. The first C-I bond-breaking yielded chemisorbed iodophenyl and I-atom(#1), and the second yielded phenylene and I-atom(#2). The observed collinear reaction resulted in secondary encounters among products, which revealed the existence of a surface-aligned reaction. The molecular dynamics were well explained by a model embodying a transition between an a priori ground state and a semiempirical ionic state, which can be generally applied to electron-induced chemical reactions at surfaces.

Injecting an electron by scanning tunneling microscope into a molecule physisorbed at a surface can induce dissociative reaction of one adsorbate bond. Here we show experimentally that a single low-energy electron incident on ortho-diiodobenzene physisorbed on Cu(110) preferentially induces reaction of both of the C-I bonds in the adsorbate, with an order-of-magnitude greater efficiency than for comparable cases of single bond breaking. A two-electronic-state model was used to follow the dynamics, first on an anionic potential-energy surface (pes*) and subsequently on the ground state pes. The model led to the conclusion that the two-bond reaction was due to the delocalization of added charge between adjacent halogen-atoms of ortho-diiodobenzene through overlapping antibonding orbitals, in contrast to the cases of para-dihalobenzenes, studied earlier, for which electron-induced reaction severed exclusively a single carbon-halogen bond. The finding that charge delocalization within a single molecule can readily cause concerted two-bond breaking suggests the more general possibility of intra- and also intermolecular charge delocalization resulting in multisite reaction. Intermolecular charge delocalization has recently been proposed by this laboratory to account for reaction in physisorbed molecular chains (Ning, Z.; Polanyi, J. C. Angew. Chem., Int. Ed. 2013, 52, 320-324).

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Scanning Tunnelling Microscopy (STM) is opening up a new field of reaction dynamics, followed one-molecule-at-a-time, only recently applied to reaction at a metal surface. Here we combine experiment with theory in studying the motions involved in the successive breaking by electron-induced reaction of the two carbon-halogen bonds, C-Cl or C-I, in physisorbed p-dihalobenzene, to form chemisorbed halogen-atoms and organic residue on Cu(110) at 4.6 K. We characterize the geometry of the physisorbed initial state, p-dichlorobenzene (pDCB) and p-diiodobenzene (pDIB), at the copper surface, as well as the successive final states of both chemisorbed reaction products: electron #1 giving rise to the first halogen-atom and a chemisorbed halophenyl and electron #2 giving a second halogen-atom and a chemisorbed phenylene. The major findings reported are (a) the distance and angular distributions of the chemisorbed reaction products relative to the physisorbed reagent molecule, (b) an approximate ab initio calculation, coupled with classical molecular dynamics (MD), of the repulsion between the products on the excited potential-energy surfaces, pes*, following excitation by electrons #1 or #2, and subsequently MD on the ground-state pes with inclusion of inelastic surface-interaction as a means to understanding the above, (c) observation of the changing dynamics with the chemistry of the halogen-atom, and (d) characterization of the effects of secondary encounters among the reaction products in the constrained space of the more highly localized reaction of pDIB. Item (d) shows clear evidence of high reactivity in surface-aligned collisions with restricted impact parameter, termed Surface Aligned Reaction, SAR, characterized by STM.