Abortive reaction leads to selective adsorbate rotation.

Electron-induced dissociation of a fluorocarbon adsorbate CF (ad) at 4.6 K is shown by Scanning Tunnelling Microscopy (STM) to form directed energetic F-atom 'projectiles' on Cu(110). The outcome of a collision between these directed projectiles and stationary co-adsorbed allyl 'target' molecules was found through STM to give rotational excitation of the target allyl, clockwise or anti-clockwise, depending on the chosen collision geometry.

Reversible 1D chain-reaction gives rise to an atomic-scale Newton's cradle.

An F-atom with ∼1 eV translational energy was aimed at a line of fluorocarbon adsorbates on Cu(110). Sequential 'knock-on' of F-atom products was observed by STM to propagate along the 1D fluorocarbon line. Hot F-atoms travelling along the line in six successive '' cycles paralleled the rocking of a macroscopic Newton's cradle.

Long-range migration of H-atoms from electron-induced dissociation of HS on Si(111).

The electron-induced dissociation of chemisorbed HS to give recoiling H-atoms was investigated on a Si(111)-7 × 7 surface at 270 K by scanning tunnelling microscopy and modelled by density functional theory. Two different H-atom migratory pathways were identified: 'short-range' (S-R; 37%) and 'long-range' (L-R; 42%). In S-R reaction the H-atom recoiled by only 4 Å whereas in L-R the average H-recoil distance was 17 Å extending up to 72 Å.

Direct Observation of Knock-on in Surface Reactions at Zero Impact Parameter.

Reaction dynamics examines molecular motions in reactive collisions. The aiming of reagents at one another has been achieved at selected miss distances (impact parameters, ) by using the corrugations on crystalline surfaces as collimator. Prior experimental work and calculation showed single atoms aimed at chemisorbed molecules with = 0 gave knock-on of atomic reaction products through a linear transition state.

Direct observation of knock-on reaction with umbrella inversion arising from zero-impact-parameter collision at a surface.

In Surface-Aligned-Reactions (SAR), the degrees of freedom of chemical reactions are restricted and therefore the reaction outcome is selected. Using the inherent corrugation of a Cu(110) substrate the adsorbate molecules can be positioned and aligned and the impact parameter, the collision miss-distance, can be chosen. Here, substitution reaction for a zero impact parameter collision gives an outcome which resembles the classic Newton's cradle in which an incident mass 'knocks-on' the same mass in the collision partner, here F + CF → (CF)' + (F)' at a copper surface.

Contrasting Efficiency of Electron-Induced Reaction at Cu(110) in Aliphatic and Aromatic Bromides.

We report a comparative study of the electron-induced reaction of pentyl bromide (PeBr) and phenyl bromide (PhBr) on Cu(110) at 4.6 K, observed by scanning tunneling microscopy (STM). The induced dissociation of the intact adsorbed molecule for both reagents occurred at an energy of 2.

Electron-induced molecular dissociation at a surface leads to reactive collisions at selected impact parameters.

Crossed molecular beams of gases have provided definitive information concerning the dynamics of chemical reactions. The results have, however, of necessity been averaged over collisions with impact parameters ranging from zero to infinity, thus obscuring the effect of this important variable. Here we employ a method through which impact parameter averaging is suppressed in a surface reaction.

Approaching the forbidden fruit of reaction dynamics: Aiming reagent at selected impact parameters.

Collision geometry is central to reaction dynamics. An important variable in collision geometry is the miss-distance between molecules, known as the "impact parameter." This is averaged in gas-phase molecular beam studies.

Direct and Delayed Dynamics in Electron-Induced Surface Reaction.

The electron-induced reaction of physisorbed vinyl bromide (ViBr) and allyl bromide (AllBr) on Cu(110) at 4.6 K was studied experimentally by scanning tunneling microscopy and theoretically by molecular dynamics. ViBr and AllBr were found to react by two pathways: "Direct", in which the molecule reacted under the tip, and "Delayed", in which reaction occurred spontaneously after the molecule had diffused across the surface away from the tip.

Bond selectivity in electron-induced reaction due to directed recoil on an anisotropic substrate.

Bond-selective reaction is central to heterogeneous catalysis. In heterogeneous catalysis, selectivity is found to depend on the chemical nature and morphology of the substrate. Here, however, we show a high degree of bond selectivity dependent only on adsorbate bond alignment.

Clocking Surface Reaction by In-Plane Product Rotation.

Electron-induced reaction of physisorbed meta-diiodobenzene (mDIB) on Cu(110) at 4.6 K was studied by Scanning Tunneling Microscopy and molecular dynamics theory. Single-electron dissociation of the first C-I bond led to in-plane rotation of an iodophenyl (IPh) intermediate, whose motion could be treated as a "clock" of the reaction dynamics.

Retention of chirality in electron-induced reactions.

Two enantiomers were observed by Scanning Tunneling Microscopy (STM) when meta-iodopyridine was physisorbed on a 4.6 K Cu(110) surface. The chirality of the reagent was retained in the products of the electron-induced reaction.

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.

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).

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".

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.

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.

Adsorbate alignment in surface halogenation: standing up is better than lying down.

Bromine atom transfer to a silicon surface as a function of physisorbed adsorbate alignment (see picture: left, vertical 1-bromopentane; right, horizontal 1-bromopentane) of 1-bromopropane and 1-bromopentane on Si(111)-7×7 has been studied by STM. In both thermal and electron-induced bromination reactions, the vertical alignment is more reactive.

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.

Stereo-isomerism controls surface reactivity: 1-chloropentane-pairs on Si(100)-2×1.

Chloropentane forms asymmetric ('A') and symmetric ('S') pairs on Si(100)-2×1, differing in the direction of curvature of one pentane tail. Surprisingly this renders the rate of thermal reaction of 'A' fifteen times greater than 'S' in chlorinating room-temperature silicon. Correspondingly, for electron-induced reaction the energy threshold for A is 1 eV less than for S.

Molecular calipers control atomic separation at a metal surface.

If a molecule controls the length of some other moiety, it can be termed a "molecular caliper". Here we image individual molecular calipers of this type by scanning tunneling microscopy. These consist of linear polymers of p-diiodobenzene, (pDIB)n, of varying length, 0.

Facile charge-displacement at silicon gives spaced-out reaction.

Adsorbates on metals, but not previously on semiconductors, have been observed to display long-range repulsive interactions. On metals, due to efficient dissipation, the repulsions are weak, typically on the order of 5 meV at 10 Å. On the 7×7 reconstruction of the Si(111) surface, charge transport through the surface has been demonstrated by others using charge injection by STM tips.

Multiple pathways of dissociative attachment: CH3Br on Si(100)-2×1.

We describe the dissociative attachment (DA) of methyl bromide to form chemisorbed CH(3) and Br on a Si(100)-2×1 surface at 270 K. The patterns of DA were studied experimentally by ultra-high vacuum scanning tunneling microscopy (STM) and interpreted by ab initio theory. The parent molecules were found to dissociate thermally by breaking the C-Br bond, attaching the resulting fragments CH(3) and Br at adjacent Si-atom sites.

Directed long-range molecular migration energized by surface reaction.

The recoil of adsorbates away (desorption) and towards (reaction) surfaces is well known. Here, we describe the long-range recoil of adsorbates in the plane of a surface, and accordingly the novel phenomenon of reactions occurring at a substantial distance from the originating event. Three thermal and three electron-induced surface reactions are shown by scanning tunnelling microscopy to propel their physisorbed ethylenic products across the rough surface of Si(100) over a distance of up to 200 Å before an attachment reaction.