Letter pubs.acs.org/JPCL
Repulsion-Induced Surface-Migration by Ballistics and Bounce Si Yue Guo,† Stephen J. Jenkins,‡ Wei Ji,§ Zhanyu Ning,† John C. Polanyi,*,† Marco Sacchi,‡,∥ and Chen-Guang Wang†,§ †
Lash Miller Chemical Laboratories, Department of Chemistry and Institute of Optical Sciences, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada ‡ Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom § Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-nano Devices, Renmin University of China, Beijing 100872, China S Supporting Information *
ABSTRACT: The motion of adsorbate molecules across surfaces is fundamental to selfassembly, 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 skippingstone ‘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).
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translational energy at surface corrugations, large recoil distances for recoiling atoms were not to be expected. As against this, Nørskov and co-workers12 and also Wahnström, Lee, and Strömquist11 noted that an exception could be the case of high-flying ballistic atomic trajectories. Shortly thereafter, Diebold et al.13 proposed that the 26 Å pairseparation of chlorine-atoms from Cl2 dissociation at TiO2(110) might be due to such ‘cannon-ball’ motion of negatively charged chlorine ions. Long-range recoil of atomic oxygen by 20−40 Å (7−14 lattice constants) across Ag(001) was reported by the Schneider laboratory.14 In subsequent work involving both O2 thermal and electron-induced reaction, the Morgenstern laboratory proposed that the recoil of atomic oxygen included ‘cannon-ball’ motion.15,16 This proposal gained theoretical support from a detailed nonadiabatic scattering study performed by Katz, Kosloff, and Zeiri17 for gas-surface collisions between O2 and aluminum that showed the concurrent involvement of surface-hopping between electronic states on semiempirical potentials leading to multiple outcomes, including ballistic motion to give atoms recoiling by ∼25 Å. Surprisingly long-range migration of physisorbed sexiphenyl has been observed when this molecule was dropped from an STM tip on to Ag(111).18 Here we apply a simple adiabatic classical model to the previously unexplained long-range recoil of a polyatomic molecule, ethylene, across the rough surface of silicon
ively interest is attached to the possibility that exothermic reaction at surfaces may result in energetic products that exhibit high transient mobility, thereby influencing the rates of subsequent reactive steps such as heterogeneous catalysis.1,2 Scanning Tunneling Microscopy (STM) is the favored tool for investigating such recoil. Brune et al. found evidence by STM for translationally ‘hot’ recoiling O-atoms in the dissociative attachment of oxygen at room temperature Al(111), leading to a surprisingly large separation of the chemisorbed O-atoms of >80 Å.3,4 Subsequent experiments in the Kummel laboratory5 placed this in question as did a further STM study by Varga and co-workers of the O2/Al(111) room temperature reaction, which gave pairs of O-atoms separated by only one surface lattice constant of 2.5 Å despite the exothermicity of approximately 5 eV per O-atom.6 Similarly for O2/Pt(111) STM experiments by Wintterlin et al. showed again that each O-atom recoiled only ∼5.5 Å (2 lattice constants).7 An STM study by Rust, Bradshaw, and co-workers of the O2/Cu(110) system evidenced hot-atom recoil to yield an intrapair separation of only ∼2 lattice constants, i.e., 5 Å.8 Transient mobility of recoiling O-atoms was also reported by Du, Dohnálek, and Lyubinetsky in an STM study of the 3.6 eV exothermic O2 dissociation on rutile TiO2(110), but the recoil distance was again short, being 1−2 lattice constants, or 6 Å.9 Theoretical molecular dynamics (MD) studies from two groups have shed light on these findings of short-range recoil of reaction products.10,11 In these MD studies, the recoil of highly energetic, 3.5 eV, O-atoms was computed across Al(111) leading to O-atom recoil distances of only 5−10 Å or 2−4 lattice constants. It was concluded that due to randomization of © 2015 American Chemical Society
Received: August 20, 2015 Accepted: September 25, 2015 Published: September 25, 2015 4093
DOI: 10.1021/acs.jpclett.5b01829 J. Phys. Chem. Lett. 2015, 6, 4093−4098
Letter
The Journal of Physical Chemistry Letters (Si(100)) observed in a recent STM study.19 In these experiments, three thermal and three electron-induced reactions (−3 V electron energy) were found to expel individual chemisorbed ethylenic molecules distances of up to 200 Å across the direction of the Si dimer-rows, before the ethylene reattached on top of a distant Si dimer-pair. This ‘directed long-range migration’ was attributed to adsorbate− substrate repulsion, as was also done in the comparable case of electron-induced long-range recoil of benzene by an average distance of 48 Å and up to 180 Å across the same surface, Si(100).20 This observation of long-range migration across a silicon surface, known for its roughness, has until now posed an unsolved dynamical problem. The mechanism has not previously been the subject of an MD study. Since there was experimental evidence that the adsorbate tumbled end-to-end, the authors19,20 suggested that their molecules, ethylene, propylene, trans-2-butene, and benzene, cartwheeled across the surface. The electronic excitation of chemisorbed ethylene was in fact shown to give rise to asymmetric repulsion between the ethylene and a Si dimer-pair at the surface, providing a source of rotation. However, in our dynamical model, presented here, we find that this rolling occurs out of contact with the surface, and consequently is not the cause of longrange migration. We further show that rotation interacting randomly with the rows of Si in Si(100) (which have a different periodicity from the rotation) gives rise to friction that prevents long-range migration, rather than inducing it. The MD model explains the observed migration as due, instead, to the recoil of the ethylene away from a tilted Si dimer in a ballistic trajectory that takes the ethylene several Angstroms above the surface, out of range of its roughness, as will be described in detail here. The MD-based model, applied here to both silicon and metal, shows (i) for achievable recoil energies and tilt angles away from the surface, an atom or molecule can be lofted into the physisorption layer (3−8 Å height) in a ballistic arc, returning to the surface under the influence of van der Waals attraction for Si or chemisorptive attraction for metal, with concurrent directed migration across the surface by tens of Angstroms, and (ii) that the recoiling species can travel additional tens of Angstroms across the surface in the same direction, propelled, in skipping-stone fashion, in successive arcs by bounces. We characterize the two elements of this model of long-range migration as ‘ballistics’ and ‘bounce’. It is significant that the migrating species can travel to substantial heights above silicon or metal and return. Passage across the surface at these heights ensures that the surface will not impede migration. The dynamics discriminate, on the basis of the required alignment for surface reaction, between intermediate surface-collision-geometries leading to a ‘bounce’ as against those that lead to chemisorption, which terminates the trajectory. In ref 19 for ethylene and ref 20 for benzene, both on Si(100), it was made evident that electron-induced repulsion between the adsorbate and the surface was the source of the observed recoil momentum. The molecular migration in each case starts and ends with an intact chemisorbed adsorbate at Si(100), hence it is thermoneutral. The additional energy available to induce the observed long-range migration across the surface constitutes the ∼3 eV electronic excitation. The conversion of the electronic energy to recoil was linked in earlier work to anionic or cationic excited states of the adsorbate located in the region of 2−3 eV relative to the ground state.19,20 In the present dynamical study, we achieved
this repulsion by compressing the chemisorbed ethylene against the underlying silicon dimer. The compression was chosen such that the repulsive energy matched the energy of the electron from the STM tip giving rise to electron-induced migration, approximately 3 eV. This value also agrees with the energy release previously reported for the three cases of exothermic reaction that were found to lead to long-range thermal recoil of ethylene reaction product, namely 2.5−3.6 eV.19 In the present work, the system was tracked by molecular dynamics during the release of the repulsive energy. This gave insight into the manner in which in a recoiling molecule (rather than the recoiling atoms studied heretofore, described in the introduction) the vertical momentum can be channeled through internal excitation into horizontal momentum across the surface, giving rise to long-range recoil. Density functional theory (DFT) and molecular dynamics (MD) were performed on a seven-layer slab incorporating 112 silicon atoms, employing the Vienna Ab initio Simulation Package (VASP)21,22 (see Supporting Information for details). For ethylene on Si(100), three categories of initial configurations were examined: (1) equal compression of the two Catoms of the ethylene against the underlying Si dimer away from equilibrium, (2) uneven compression of the ethylene such that one carbon was closer to the surface, and (3) altered tilt of the underlying Si dimer. For the case of chlorophenyl on Cu(110) given later, the repulsion between the recoiling chlorine atom and its parent phenyl radical was obtained from an ‘Impulsive Two-State’ model that had been tested in various STM studies of electron-induced reactions referenced below. From these selected starting points for ethylene at Si(100), six trajectories (see Supporting Information) were computed in which the ethylene recoiled by as much as 50 Å ‘perpendicular’ to the dimer rows. This gives the first evidence that the observed directed long-range migration can indeed be described by the classical equations of motion. Figure 1a,b shows the center-of-mass height, z, plotted against the horizontal distance, x, for the six migratory trajectories; Figure 1c is a top view showing directionality along x. Five of the six cases showed long-range migration; in the sixth case (trajectory 2), the ethylene moving at a low elevation was deflected back to its starting point by a lateral collision with row 1. The trajectories were lengthy (up to 5 ps duration) and consequently sensitive to small changes in the initial-state forces, giving rise to large variations in the number of rows traversed. Figure 1a designates three approximate height-regimes: ‘chemisorption’ up to 3 Å, ‘physisorption’ from 3 to 8 Å, and ‘desorption’ above 8 Å. Significantly, in every case, the migrating molecule traversed the physisorption region. The roughness, RO, of the surface is shown in Figure 2. The roughness is defined as the height of the energy-barrier to migration, computed in the plane parallel to the surface. Values of RO are given for five heights: 1.9 Å for chemisorbed ethylene and 3, 3.8, 5, and 6 Å for physisorbed ethylene. Significantly, RO falls by several orders of magnitude as the height above the surface increases from chemisorption to the mean value of 5 Å computed for the ballistic migration described here. Figure 1b shows the height of trajectories 1, 3, and 4 as solid lines, with the best ballistic fit shown as dashed curves. The fits assumed a constant van der Waals attraction between the ethylene and the surface for the average height reached by the molecule, whereas the trajectory used the downward 4094
DOI: 10.1021/acs.jpclett.5b01829 J. Phys. Chem. Lett. 2015, 6, 4093−4098
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The Journal of Physical Chemistry Letters
its ballistic trajectory was 0.26 eV and the angle of release was 28° ± 2°. In the following, we examine the manner in which the repulsive energy-release along the vertical coordinate, z, is converted to migration across the surface, along x. For this we examine trajectory 1, in which ethylene travels 54 Å along x. Figure 3 shows how, in 300 fs, the repulsion along z is
Figure 1. Trajectories obtained from ethylene−Si(100) surface repulsion, showing the center-of-mass coordinates for the six cases examined (the computed rotational periodicity for trajectory 1 is indicated over the time-scale at the top of panel a): (a) height vs distance, (b) the ballistic fit as dashed lines for trajectories 1, 3, and 4, and (c) a top view evidencing the directed motion along x.
Figure 3. Generation of initial velocity and angle in trajectory 1. For the first 300 fs, (a) vertical (z) and horizontal (x) accelerations showing the role of bending, with ethylene bending motion pictured above; (b) vertical (z) translational energy becomes horizontal (x) translational energy.
converted into motion along x. Frames from an MD movie at ∼50 fs intervals show successive geometries. The ethylene first recoils vertically with a resultant sharp peak in Etrans,z. Formation of gaseous ethylene from chemisorbed is endothermic by 1.65 eV23,24 accounting for a major part of the loss of energy, Etrans,z, in the first 100 fs. In going from a chemisorbed, disigma-bound ethylene to a physisorbed molecule, the CH2 flattens from a sp3 to sp2 configuration, transferring energy into the CH2 bending mode. The bending has a characteristic period of ∼35 fs, evident in the up−down positions of the H-atoms, labeled ‘U’ for up and ‘D’ for down in Figure 3a. Concurrently the underlying Si dimer flips from a flat to normal tilted configuration. The ‘U’−‘D’ wagging motion of the ethylene encounters the tilted dimer ∼5 times, resulting each time in acceleration of the molecule along x (see Figure 3a). Cessation of the acceleration along x is followed by approximately constant Etrans,x at 220 fs (see the vertical dashed line in Figure 3b); this marks the onset of inertial ballistic motion.
Figure 2. Surface roughness, RO, for ethylene on Si(100), namely the barrier to movement along x, as a function of height above the surface, z, in the three designated height regimes: chemisorption, physisorption, and desorption. RO decreases by orders of magnitude with increasing height from 2 to 7 Å.
acceleration due to van der Waals attraction, appropriate to each value of z. From the ballistic fit to the five long-range trajectories in Figure 1a, the average initial velocity to launch the ethylene on 4095
DOI: 10.1021/acs.jpclett.5b01829 J. Phys. Chem. Lett. 2015, 6, 4093−4098
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The MD trajectories exemplify three broad categories of behavior: (1) recoil across the surface at chemisorption heights (z = 2−3 Å), short recoil, (see trajectory 2, Figure 1a), (2) recoil through the physisorption layer, analyzed here as ballistic (z = 3−8 Å) with a mean migratory distance of 22 Å, and (3) bounces through the physisorbed layer resulting subsequently in further migration by an average of 16 Å per bounce. The results of the room temperature experiments detailed in Figure S3 of ref 19 (Supporting Information) can now be interpreted in terms of these three regimes. Of the 158 cases of electroninduced ethylene migration on Si(100), roughly one-third (30%) correspond to regime (1), resulting in short recoil to a neighboring dimer. Another third (40%) correspond to regime (2), suggestive of ballistic recoil followed by reattachment to the surface. The final third (30%) belong to regime (3) with initial ballistic flight followed by an average of three bounces to achieve the observed long-range migration. The experimental range of migration (0−130 Å) suggests as many as 6 bounces, rather than the 2−3 given by the model, implying a surface elasticity greater than that exhibited by our theoretical slab. In the experimental study (ref 19) there was reported a decrease in diffuse scattering and increase in directed scattering across the dimer rows, with increased surface temperature from room temperature to 150 °C, and a consequent increase in migration distance. The effect of changing surface temperature is beyond the scope of this investigation; however, a qualitative explanation of the experimental observation may be the expected decrease in collision duration at elevated surface temperature, leading to decreased inelastic energy loss to the surface. In a related study of long-range migration, a comparison of trajectories computed at 75 and 110 K showed negligible difference in the migratory dynamics, suggestive of insensitivity to surface temperature. In what follows, we predict long-range ballistic surfacemigration at a metal rather than semiconductor. The system, electron-induced reaction of halophenyl, is one whose dynamics have been investigated by experiment and theory in recent published work.26−28 In the case of chlorophenyl (ClPh) studied experimentally on Cu(110), the authors observed and also computed by an ‘Impulsive Two-State’ model the dynamics following electron attachment.26−28 Here we use their theoretical approach to calculate the recoil of atomic Cl from ClPh on Cu(110) in four different adsorption geometries of comparable stability, varying in their in-plane alignment angle, ϕ, between the C−Cl bond and the [001] direction. We selected these four states with ϕ angles, ϕA = 60°, ϕB = 70°, ϕC = 78°, and ϕD = 86° since, due to the anisotropic underlying copper, they are calculated to have significantly different ClPh out-of-plane tilt angles (θ) with respect to the surface. Increasing tilts, which correlate with increasing ϕ, will result in greater ranges of ballistic recoil of the light Cl-atom away from the chemisorbed phenylene, Ph, as illustrated in Figure 5a (side view) and 5b (top view, showing directionality). The four ϕ angles yielded the θ tilts: θA = 16°, θB = 25°, θC = 35°, and θD = 44°, with configuration A most closely matching that obtained in ref 27. The experiments of ref 27 showed the presence of ClPh with a range of angles ϕ, indicating that the correlation of tilt with ballistic migration range should in the future be observable. The present calculation showed configuration A to be marginally the most stable with Eads,A = 3.51 eV, followed by D with Eads,D = 3.48 eV, then B and C with Eads,B = Eads,C = 3.47 eV. Electron-induced dissociation (see
In summary, the CH2 wagging interacting with the tilted Sidimer below gave ethylene the translational energy at an angle for ballistic launch (0.28 eV, at 27°) accounting for ballistic trajectory 1. The same two elements, bending vibration and dimer tilt, converted repulsion along z to translation along x in the other ballistic trajectories. We note that even for cases 1, 5, and 6, trajectories for which the ethylene traveled the furthest, a single ballistic arc carried the ethylene only two dimer rows, i.e., 15.4 Å. A further mechanism must therefore be operative to explain the computed outcomes of 38 Å migration for trajectory 6 and 54 Å for 1 and 5. This extension of the range of migration is important since experiment gave migration by in many instances > ∼100 Å. Examination of trajectories 1, 5, and 6 shows that this extended migration is due not to the first ballistic arc, but to subsequent arcs following ‘bounces’ at the surface. Bounces explain the substantial migratory distances observed by experiment, as well as the retention of direction since direction is preserved in elastic scattering, analogous to ‘skipping-stone’ motion across water. The in-plane alignment of the colliding ethylene, ϕ, with respect to the Si dimer-row direction following the initial ballistic trajectory was found to be important in determining whether the migrating ethylene subsequently ‘bounced’ or ‘stuck’ (chemisorbing). These alternatives are illustrated in Figure 4. In the six cases studied, the computed trajectories
Figure 4. In-plane rotational alignment required for chemisorption (‘stick’) as compared with the ‘bounce’ taken from the trajectories. Ethylene aligned along a silicon dimer sticks, whereas that aligned across, bounces.
showed that sticking (i.e., reaction) occurred when the ethylene’s CC axis was within 20° of the Si−Si dimer-bond direction. Assuming a random distribution in ϕ, this implies a yield of approximately 1/4 sticking versus 3/4 bouncing in accord with the computed number of bounces. The higher second arc observed following some bounces (see Figure 1a) was found to be due to bouncing off a tilted dimer.25 Migration is terminated by reactive capture of the ethylene at the silicon surface. As has been noted, it was suggested in earlier work that the observed long-range migration might be due to cartwheeling rotation across the surface. The Supporting Information summarizes a study of the transport along x obtained from various rotational energies combined with a range of translational energies. Migration due to rotation was found to be by only one dimer row, leading thereafter almost invariably to desorption but never to long-range migration. We can therefore discount rotation as being the principal cause of the observed migration. 4096
DOI: 10.1021/acs.jpclett.5b01829 J. Phys. Chem. Lett. 2015, 6, 4093−4098
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For ethylene on Si(100) we showed that the unexpected observation of migration by tens of Angstroms can be understood in terms simple classical mechanics. By Molecular Dynamics (MD) we found that the two elements required to account for directed long-range migration are ‘ballistics’ and ‘bounce’. The ballistics resulted from the ethylene being ejected from the surface with a velocity sufficient to become physisorbed, after which it traveled across the surface at a height of 3−8 Å. Significantly, at this height surface roughness was reduced by orders-of-magnitude, making possible longrange migration. The ethylene followed a ballistic arc, returning to the surface under the influence of van der Waals attraction. Conversion of the initial vertical recoil into horizontal migration resulted from collisions between vibrationally excited ethylene and the tilted Si dimer beneath, giving recoil across the dimer-rows. The longest-range migration was enabled by a skipping-stone effect, in which the initial ballistic arc was converted by ‘bounces’ into a succession of arcs, with retention of direction. Similar behavior can be expected in other cases where an atom or molecule recoils at an angle away from a surface. We illustrate this for chlorophenyl (ClPh) chemisorbed on Cu(110), in which the ClPh is shown to point away from the surface. For such cases, using a tested ‘Impulsive Two-State’ model, we have computed a comparable ballistic long-range migration of atomic chlorine as the outcome of electroninduced reaction.
Figure 5. Trajectories of the chlorine recoil from ClPh adsorbed on Cu(110) calculated using the I2S model: (a) shows the side view with the ballistic fit in dashed lines, (b) shows the top view indicating the directed nature of the Cl-atom motion along the extension of the original C−Cl bond.
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below) was computed to yield atomic chlorine recoiling to distances of several lattice constants, increasing in recoil distance as the out-of-plane tilt angle of the parent ClPh at the copper surface was increased. Figure 5 shows the four ab initio ClPh initial state geometries and the computed Cl recoil following electron-induced dissociation. We applied the Impulsive Two-State model (I2S)26−29 using VASP, adding an electron to the Cl 3p orbital, and selecting a residence time on the ionic surface of 70 fsthe minimum value that gave reaction. After 70 fs, the system was returned to the ground-state with all its momenta from the excited state, and evolved according to MD. The details of the calculation are given in the Supporting Information. In the ionic state, an electron from a bonding orbital has been excited to an antibonding orbital localized on C−Cl, giving repulsion between the Cl and the phenylene radical at the locations in red in Figure 5a. The extension of the C−Cl bond continued on the ground state as the chlorine was ejected with a kinetic energy of 1.1 eV along C−Cl. Of the four trajectories, three gave clear ballistic trajectories where the Cl traveled up to 21 Å away, indicative of directed long-range migration on copper. In the remaining case, the chlorine desorbed. Figure 5a also shows, as three dashed curves, the calculated ballistic fits to the trajectories in which the Cl returns to the surface. In its final state, the Cl forms a chemisorption bond at the copper. The strong chemisorption attraction explains the small excursions in the Cl bounces following the initial ballistic arc, as compared to the case of ethylene above. In summary, we have applied a model based on surface repulsion to illuminate the experimentally observed long-range migration of ethylene across Si(100), and have also predicted comparable long-range recoil of atomic chlorine from chlorophenyl across Cu(110).
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01829. Details of the theoretical methods, the migratory trajectories, and a study of rolling ethylene on Si(100) (PDF) Additional data related to this publication are available at the DSpace@Cambridge repository (https://www.repository.cam.ac.uk).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address ∥
(M.S.) Department of Chemistry, The University of Reading, Whiteknights, Reading, UK. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), by the University of Toronto NSERC General Research Fund, and by the Xerox Research Centre Canada (XRCC). Computations were performed on the SciNet supercomputer, funded by the Canada Foundation for Innovation under Compute Canada, the Government of Ontario, Ontario Research Fund, and the University of Toronto. We thank Drs. Lydie Leung and Kai Huang for helpful discussions. W.J. was supported by the Ministry of Science and Technology (MOST) of China, Grant 2012CB932704, and the National Natural Science Foundation of China (NSFC), Grants 11274380 and 91433103. C.-G.W. thanks the Basic Research Funds of Renmin University of 4097
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(22) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (23) Clemen, L.; Wallace, R. M.; Taylor, P. A.; Dresser, M. J.; Choyke, W. J.; Weinberg, W. H.; Yates, J. T., Jr. Adsorption and Thermal Behavior of Ethylene on Si(100)-(2×1). Surf. Sci. 1992, 268, 205−216. (24) Nagao, M.; Umeyama, H.; Mukai, K.; Yamashita, Y.; Yoshinobu, J.; Akagi, K.; Tsuneyuki, S. Precursor Mediated Cycloaddition Reaction of Ethylene to the Si(100)c(4×2) Surface. J. Am. Chem. Soc. 2004, 126, 9922−9923. (25) Backward rotation could, in principle, cause a high bounce, but this effect was negligible since the rotational period (∼1 ps for a typical rotational energy ∼ 0.05 eV) was ∼10× the duration of an encounter with the surface. (26) Leung, L.; Lim, T.; Ning, Z.; Polanyi, J. C. Localized Reaction at a Smooth Metal Surface: p-Diiodobenzene at Cu(110). J. Am. Chem. Soc. 2012, 134, 9320−9326. (27) Eisenstein, A.; Leung, L.; Lim, T.; Ning, Z.; Polanyi, J. C. Reaction Dynamics at a Metal Surface; Halogenation of Cu(110). Faraday Discuss. 2012, 157, 337−353. (28) Huang, K.; Leung, L.; Lim, T.; Ning, Z.; Polanyi, J. C. SingleElectron Induces Double-Reaction by Charge Delocalization. J. Am. Chem. Soc. 2013, 135, 6220−6225. (29) Ning, Z.; Polanyi, J. C. Surface Aligned Reaction. J. Chem. Phys. 2012, 137, 091706.
China and the Central Government for support under Grant 15XNH068.
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