Repulsion-Induced Surface-Migration by Ballistics and Bounce.

The motion of adsorbate molecules across surfaces is fundamental to self-assembly, material growth, and heterogeneous catalysis. Recent Scanning Tunneling Microscopy studies have demonstrated the electron-induced long-range surface-migration of ethylene, benzene, and related molecules, moving tens of Angstroms across Si(100). We present a model of the previously unexplained long-range recoil of chemisorbed ethylene across the surface of silicon. The molecular dynamics reveal two key elements for directed long-range migration: first 'ballistic' motion that causes the molecule to leave the ab initio slab of the surface traveling 3-8 Å above it out of range of its roughness, and thereafter skipping-stone 'bounces' that transport it further to the observed long distances. Using a previously tested Impulsive Two-State model, we predict comparable long-range recoil of atomic chlorine following electron-induced dissociation of chlorophenyl chemisorbed at Cu(110).

Vibrational excitation induces double reaction.

Electron-induced reaction at metal surfaces is currently the subject of extensive study. Here, we broaden the range of experimentation to a comparison of vibrational excitation with electronic excitation, for reaction of the same molecule at the same clean metal surface. In a previous study of electron-induced reaction by scanning tunneling microscopy (STM), we examined the dynamics of the concurrent breaking of the two C-I bonds of ortho-diiodobenzene physisorbed on Cu(110). The energy of the incident electron was near the electronic excitation threshold of E0=1.0 eV required to induce this single-electron process. STM has been employed in the present work to study the reaction dynamics at the substantially lower incident electron energies of 0.3 eV, well below the electronic excitation threshold. The observed increase in reaction rate with current was found to be fourth-order, indicative of multistep reagent vibrational excitation, in contrast to the first-order rate dependence found earlier for electronic excitation. The change in mode of excitation was accompanied by altered reaction dynamics, evidenced by a different pattern of binding of the chemisorbed products to the copper surface. We have modeled these altered reaction dynamics by exciting normal modes of vibration that distort the C-I bonds of the physisorbed reagent. Using the same ab initio ground potential-energy surface as in the prior work on electronic excitation, but with only vibrational excitation of the physisorbed reagent in the asymmetric stretch mode of C-I bonds, we obtained the observed alteration in reaction dynamics.

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 C-I 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···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.

Single-electron induces double-reaction by charge delocalization.

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).

Charge delocalization induces reaction in molecular chains at a surface.

The molecular dynamics of an electron-induced reaction in a self-assembled molecular chain of four dimethyldisulfide molecules on Au(111) are studied. Charge delocalization weakens all the S-S bonds causing a concurrent reaction along the entire chain. All the original S-S bonds are broken and new S-S bonds form giving three altered S-S bonds and two chemisorbed thiyl radicals.

Surface aligned reaction.

This paper reflects on three decades during which the study of surface aligned reaction (SAR) has advanced. The objective in SAR, which in considerable part still lies ahead, is the simultaneous control of atomic and molecular "collision energies, collision angles, and impact parameter." Following a discussion of the benefits of such an approach we review the progress made, and, as a stimulus to experiment, present new calculations of SAR dynamics for bimolecular reaction at a metal surface. It seems reasonable to suppose that we are now entering a decade in which a combination of scanning tunneling microscopy and femtosecond laser spectroscopy will bring the full realisation of SAR.

Localized reaction at a smooth metal surface: p-diiodobenzene at Cu(110).

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.

Quantitative analysis of nonequilibrium spin injection into molecular tunnel junctions.

We report quantitative analysis of nonequilibrium spin injection from Ni contacts to the octanethiol molecular spintronic system. Our calculation is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. The first principles results allow us to establish a clear physical picture on how spins are injected from the Ni contacts through the Ni-molecule linkage to the molecule, why tunnel magnetoresistance is rapidly reduced by the applied bias in an asymmetric manner, and to what extent ab initio transport theory can make quantitative comparisons to the corresponding experimental data. We found that extremely careful sampling of the two-dimensional Brillouin zone of the Ni surface is crucial for accurate results.