Abortive reaction leads to selective adsorbate rotation.

Electron-induced dissociation of a fluorocarbon adsorbate CF3 (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. Molecular dynamics computation linked the collisional excitation of the allyl target to an 'abortive chemical reaction', in which the approach of the F-projectile stretched an H-C bond lifting the allyl above the surface, facilitating isomerization from 'Across' to 'Along' a Cu row.

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 'to-and-fro' cycles paralleled the rocking of a macroscopic Newton's cradle.

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, b) by using the corrugations on crystalline surfaces as collimator. Prior experimental work and ab initio calculation showed single atoms aimed at chemisorbed molecules with b = 0 gave knock-on of atomic reaction products through a linear transition state. Here we report a study of b = 0 collision between directed CF2 and stationary chemisorbed CF3. Experiments and ab initio calculations again show linear reaction with a linear transition state, despite the additional degrees of freedom for CF2. The directed motion of CF2 is conserved through this linear transition state. Conservation of directionality is evidenced experimentally by the observation of a knock-on chain reaction along a line of chemisorbed CF3.

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 + CF3 → (CF3)' + (F)' at a copper surface. The mechanism of knock-on was shown by Scanning Tunnelling Microscopy to involve reversal of the CF3 umbrella as in Walden inversion, with ejection of (F)' product along the continuation of the F-reagent direction of motion, in collinear reaction.

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 2.0 eV, producing a hydrocarbon radical and a Br atom. Electron-induced C-Br bond dissociation was found to be a single-electron process for both reagents. The impulsive two-state (I2S) model was used to describe the Br atom recoil as due to molecular excitation to a repulsive anti-bonding state, in which recoil of the dissociation products occurred due to C·Br repulsion along the prior C-Br bond direction. The measured reaction yield was 3 orders of magnitude greater for PeBr, 2.0 × 10-7 reactive events per electron, than for PhBr with a yield of 1.7 × 10-10. The low yield of dissociation products from the aromatic PhBr was attributed to the presence of two additional anionic states below the 2.0 eV energy limit, absent for the aliphatic PeBr; these additional anionic states for PhBr could provide a pathway for electron transfer to the surface in the case of the aromatic, but not the aliphatic, anion. The consequent shorter lifetime of the repulsive aromatic anion of PhBr is consistent with the observation of shorter mean recoil distance (3.2 Å) of its Br dissociation product, as compared with the markedly longer recoil (8.7 Å) of Br observed from the anion of PeBr.

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. We aim a highly collimated reactive 'projectile' molecule along a surface at a stationary adsorbed 'target' molecule, with both the projectile and target being observed by Scanning Tunnelling Microscopy (STM). The projectile was CF2 recoiling from electron-induced bond-breaking in chemisorbed CF3 on Cu(110) at 4.6 K. The collimation of the resulting CF2 'surface-molecular-beam' restricted it to a lateral spread of ±1° as a consequence of its interaction with the Cu rows below. This collimation was modelled by molecular dynamics simulations. In the experiments the recoiling CF2 projectile was aimed, successively, at impact parameters of b = 0 and +3.6 Å at a chemisorbed second CF2, b = +1.8 Å at a chemisorbed I-atom, or b = -4.0, b = -0.4 and b = +3.2 Å at a chemisorbed vinyl radical. The pattern of reactive and non-reactive scattering was determined by STM. These collimated surface-molecular-beams have the potential to aim molecular projectiles with selected impact parameters at the many target species identifiable at a surface by STM.

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. By aligning molecules on a surface prior to electron-induced dissociation, we select impact parameters in subsequent inelastic collisions. Surface-collimated "projectile" molecules, difluorocarbene (CF2), were aimed at stationary "target" molecules characterized by scanning tunneling microscopy (STM), with the observed scattering interpreted by computational molecular dynamics. Selection of impact parameters showed that head-on collisions favored bimolecular reaction, whereas glancing collisions led only to momentum transfer. These collimated projectiles could be aimed at the wide variety of adsorbed targets identifiable by STM, with the selected impact parameter assisting in the identification of the collision geometry required for reaction.

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. The novel pathway of Delayed reaction constituted a major route for both vinyl bromide (68%) and allyl bromide (53%). The observed reaction dynamics for ViBr and AllBr gave evidence of a long-lived vibrationally excited intermediate for both Direct and Delayed reactions. Molecular dynamics simulations with reagent excitation by way of selected vibrational normal modes resulted in either Direct or Delayed reaction, depending on the vibrational mode.

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. The system studied is the electron-induced reaction of meta-diiodobenzene physisorbed on Cu(110). Of the adsorbate's C-I bonds, C-I aligned 'Along' the copper row dissociates in 99.3% of the cases giving surface reaction, whereas C-I bond aligned 'Across' the rows dissociates in only 0.7% of the cases. A two-electronic-state molecular dynamics model attributes reaction to an initial transition to a repulsive state of an Along C-I, followed by directed recoil of C towards a Cu atom of the same row, forming C-Cu. A similar impulse on an Across C-I gives directed C that, moving across rows, does not encounter a Cu atom and hence exhibits markedly less reaction.

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. Alternative reaction mechanisms, successive and concerted, were observed giving different product distributions. In the successive mechanism, two electrons successively broke single C-I bonds; the first C-I bond breaking yielded IPh that rotated directionally by three different angles, with the second C-I bond breaking giving chemisorbed I atoms (#2) at three preferred locations corresponding to the C-I bond alignments in the prior rotated IPh configurations. In the concerted mechanism a single electron broke two C-I bonds, giving two chemisorbed I atoms; significantly these were found at angles corresponding to the C-I bond direction for unrotated mDIB. Molecular dynamics accounted for the difference in reaction outcomes between the successive and the concerted mechanisms in terms of the time required for the IPh to rotate in-plane; in successive reaction the time delay between first and second C-I bond-breaking events allowed the IPh to rotate, whereas in concerted reaction the computed delay between excitation and reaction (∼1 ps) was too short for molecular rotation before the second C-I bond broke. The dependence of the extent of motion at a surface on the delay between first and second bond breaking suggested a novel means to "clock" sub-picosecond dynamics by imaging the products arising from varying time delays between impacting pairs of electrons.

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. Dynamical calculations showed this to be a consequence of the reaction occurring on one side of the mirror plane.

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).

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.

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, 0.7-2.9 nm, physisorbed on Cu(110) at 4.6 K. Through electron-induced reaction these chemically imprint their terminal I-atoms on the copper, 0.7 nm further apart than their initial separations. The physisorbed monomer or polymer, therefore, constitutes a molecular-caliper with variable terminal I..I separation. The localized nature of the I-atom reaction at the copper surface relative to the parent molecule, constitutes a novel finding reported here. It ensures that the separation of the I-atoms in the physisorbed molecular caliper correlates with their subsequent separation when chemisorbed at the surface.

Cooperative molecular dynamics in surface reactions.

The controlled imprinting of surfaces with specified patterns is important in the development of nanoscale devices. Previously, such patterns were created using self-assembled physisorbed adsorbate molecules that can be stabilized on the surface by subsequent chemical bonding. Here we show a first step towards use of the bonding within a surface to propagate reactions for patterning, namely the cooperative reaction of adjacent silicon atoms. We exploit the double-bonded silicon dimer pairs present on the surface of Si(100)-2×1 and show that the halogenation of one silicon atom (induced by electrons or heat) results in cooperative halogenation of the neighbouring silicon atom with unit efficiency. The reactants used were two 1-halopentane molecules physisorbed over a pair of silicon atoms. This cooperative pair of halogenation reactions was shown by ab initio calculation to be sequential on a timescale of femtoseconds.