H2 Diffraction from a Strained Pseudomorphic Monolayer of Cu

Article pubs.acs.org/JPCC

H2 Diffraction from a Strained Pseudomorphic Monolayer of Cu Deposited on Ru(0001) C. Díaz,*,† F. Martín,†,‡ G. J. Kroes,§ M. Minniti,∥ D. Farías,∥ and R. Miranda‡,∥ †

Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28038 Madrid, Spain Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), Cantoblanco, 28049 Madrid, Spain § Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands ∥ Departamento de Física de la Materia Condensada, Módulo 3, Universidad Autónoma de Madrid, 28038 Madrid, Spain ‡

ABSTRACT: Diffraction of H2 from surfaces is considered to be a useful tool to characterize molecule/surface interactions and surface topology. In this work, we have studied diffraction of H2 from a strained pseudomorphic monolayer of Cu deposited on Ru(0001), both experimentally and theoretically. Our experimental measurements show a remarkable diffraction probability, both in-plane and out-of-plane. In particular, we observe for the first time third-order diffraction peaks. These striking experimental results have been analyzed by performing theoretical simulations, using both quantum and quasi-classical dynamics methods. Taking into account the relationship between diffraction (quantum phenomenon) and reflection (classical observable), we have performed a classical analysis of a meaningful set of classical trajectories. This analysis reveals that for H2/Cu/Ru(0001) diffracted molecules practically explore the entire surface unit cell and are able to get close to the surface, thus favoring high-order diffraction.

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INTRODUCTION Diffraction of atoms (H, He) and light molecules (H2, D2) from surfaces has been proposed to be a useful tool to determine surface corrugation and to characterize the potential energy surface (PES) of these systems.1,2 Molecules colliding with metal surfaces at low energy can either be scattered, when they hit a repulsive part of the PES, or dissociate on the surface, when they find a nonactivated reaction path or they have enough energy to overcome the reaction barrier. Thus, dissociative adsorption is also sensitive to the corrugation of the PES. Taking into account these considerations, it has been suggested (see ref 2 and refs therein) that H2(D2) diffraction by surfaces could be used to obtain information about dissociative adsorption. Diffraction of H2 (and its isotopes) from a number of metal surfaces has been previously reported.3−18 These experiments have been able to resolve first- and second-order elastic diffraction peaks as well as rotationally inelastic diffraction peaks. One of the most striking features found experimentally was the presence of intense out-of-plane diffraction peaks in nonactivated or weak activated systems,15,17 for which dissociative adsorption is possible in (almost) the whole range of incidence energies. The latter finding suggests that the presence of a pronounced out-of-plane diffraction probability is the signature of a significant dissociative adsorption probability. In this respect, the measured out-of-plane diffraction spectra of H2/NiAl(110), H2/Pt(111), and H2/Pd(111) reported in ref 19 show a clear trend: H2/Pd(111), the most reactive system, exhibits the most pronounced out-of-plane diffraction © 2012 American Chemical Society

probability, whereas H2/NiAl(110), the least reactive system, exhibits the lowest out-of-plane diffraction. Apart from results shown in ref 19, there is not yet an unambiguous proof of the direct relationship between dissociative adsorption and out-of-plane diffraction. One of the aims of the present work is to investigate this question further by considering a completely different system: a bimetallic surface formed by a monolayer of Cu deposited on Ru(0001). Bimetallic surfaces are of great practical interest because very often they present electronic and, as a consequence, chemical properties significantly different from those of their parent metals.20−28 Therefore, bimetallic surfaces can be used, for example, to enhance selectivity of catalytic processes. Among bimetallic surfaces, Cu/Ru(0001) has been the subject of numerous investigations concerning its geometry, electronic properties, and chemical activity toward different molecules.25,29−33 In particular, it has been recently shown that the dissociative adsorption probability of H2 on Cu/Ru(0001)34 is higher than on the parent Cu(111)35 but lower than on the parent Ru(0001) one.36 (See Figure 1.) Therefore, comparison of diffraction probabilities, available in the literature, for H2/Cu(111)35 and H2/Ru(0001),18 with those obtained for H2/Cu/Ru(0001) will allow us to check if the proposed relationship between reactivity and out-of-plane diffraction still holds in this case. In this work, we show that diffraction spectra for H2/Cu/Ru(0001) even exhibit third-order Received: April 9, 2012 Revised: June 5, 2012 Published: June 5, 2012 13671

dx.doi.org/10.1021/jp303390x | J. Phys. Chem. C 2012, 116, 13671−13678

The Journal of Physical Chemistry C

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175 meV. The velocity spread of the H2 beam is ∼8%, as estimated from the widths of the diffraction peaks observed in the experiment. According to previous theoretical and experimental work, the rotational population of the incident H2 beam (for stagnation condition P0d = 38 Torr cm) in states with angular momentum quantum number ji = 0, 1, 2, and 3 is 0.21, 0.74, 0.04, and 0.01 at Ei = 75 meV (corresponding to T0 = 300 K) and 0.11, 0.61, 0.14, and 0.13 at Ei = 150 meV (T0 = 600 K), respectively.2 For the energy range used in the experiments, more than 99% of the molecules are in the vibrational ground state. To perform the experimental measurements shown in the above, the molecular beam has been aligned along the [112̅] crystallographic direction of the surface. (See Figure 3.) This Figure 1. Dissociative adsorption probability of H2 on Ru(0001)36 (black curve), CuRu(0001)37 (red curve), and Cu(111)38 (green curve) as a function of the incidence energy.

out-of-plane diffraction peaks. Such pronounced out-of-plane diffraction is not observed either for H2/Cu(111) or for H2/Ru(0001). However, in the latter case, up to second-order out-of-plane diffraction peaks have been resolved experimentally.18 These results suggest that there is not an exact proportionality between the amount of out-of-plane diffraction and the dissociative adsorption probability.

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METHODOLOGY Experimental Setup. The apparatus used for this study has been described in detail elsewhere.39 In brief, it belongs to the so-called rotary detector systems; that is, the detector can be rotated around two axes for any incidence condition. Specifically, in our setup, for a fixed incidence angle, the detector can be rotated 200° in the scattering plane (θ) and ±15° away from it (φ). Here the scattering plane is defined by the direction of the incidence beam and the normal vector to the surface. (See Figure 2.) The two main advantages of this

Figure 3. Reciprocal lattice for an FCC(111) surface. Numbers within parentheses are Miller indices (diffraction peaks). Dashed hexagons show the 2D Wigner−Seitz cell for a few lattice points.

means that measuring with the detector in the in-plane configuration the expected diffraction peaks have the form (n,n), with n being any integer number. All other (n,m) peaks with n ≠ m will be recorded out-of-plane. Atomically clean, crystalline Ru(0001) surfaces were prepared by standard sputtering/annealing cycles, followed by oxygen exposure at 1150 K and a final flash to 1500 K. The Cu(111) sample was cleaned by cycles of Ar+ sputtering at 300 K, followed by annealing to 750 K. Surface cleanliness and order were checked using low-energy electron diffraction (LEED) and helium atom scattering. Cu was deposited from a commercial evaporator equipped with a W basket filled with Cu pellets. The quality of evaporations has been checked by monitoring the changes in the intensity of the specular He beam. While evaporating, the surface roughness varies periodically, reaching a minimum for layer half-filling and a maximum at the completion of each layer. Employing the HAS technique, it is then possible to grow overlayers with a daily precision of 1%. The evaporated Cu atoms reached the sample at nearly normal incidence. The evaporation rate was ∼0.1 ML/min. Theoretical Methods. To study diffraction, a quantum phenomenon, we have carried out quantum dynamics calculations, using a time-dependent wave packet (TDWP) method.40,41 This method has been described in detail in refs 42 and 43 In brief, the calculations are divided in two main steps: (1) As we work within the surface-frozen Born−Oppenheimer approximation, the first step is to compute the PES, that is, the ground-state electronic energy as a function of the nuclear coordinates of the molecule; the surface nuclei are kept fixed in their equilibrium position. The three 6-D

Figure 2. Schematic representation of the experimental setup.

apparatus are: (i) For each set of incidence conditions, energy and incidence direction, all diffraction peaks can be measured. (ii) The incidence molecular beam intensity can be measured, allowing an accurate determination of the absolute diffraction probabilities. On the other hand, the main disadvantages on this kind of setup are that the angular resolution is usually limited to 1.5° and the dynamical range is limited to 3 × 10−3 of the incoming beam intensity. The H2 beam was produced by a free jet expansion of the high-pressure gas (∼45 bar) through a nozzle of 10 μm. The nozzle temperature T0 can be varied between 100 and 700 K, allowing a variation of the H2 incident energy between 25 and 13672



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measurements. As described in refs 49 and 50, classical diffraction probabilities can be obtained through a binning method. Quantically speaking, diffraction occurs whenever the variation of the parallel momentum ℏΔK∥ = ℏ(Kf − Ki) (Ki and Kf being the initial and final parallel wave vectors, respectively) coincides with one of the vectors of the reciprocal lattice; see Figure 3. In contrast, changes in parallel momentum are continuous in classical dynamics; that is, ℏΔK∥ can be any vector and will not necessarily be equal to one of the reciprocal lattice vectors. Therefore, to obtain classical diffraction peaks, we have to discretize ΔK∥. To do so, we divide the reciprocal space using as a pattern the Wigner−Seitz cell associated with each lattice point. (See Figure 3.) Therefore, the classical diffraction probability for a given diffraction peak (n,m) is given by the number of trajectories leading to a change in parallel momentum contained in the (n,m) Wigner−Seitz cell divided by the total number of trajectories. To compute the final rotational state (Jf) from the continuous classical angular momentum, we evaluate the closest integer that satisfies the formula of a quantum rigid rotor and obeys the selection rule ΔJ = ± 2nJ (n being an integer).52,53 To compare with the experimental results, we have taken into account the rotational populations of the incident molecular beam. Although the experimental setup used in this work does not allow us to measure these populations, they have been estimated from previous work.54−56 (See Table 2.)

PESs used in this study were built by applying the CRP (corrugation reducing procedure) method to a set of DFT (density functional theory) data points. A detailed description of these PESs can found in ref 34 for H2/Cu/Ru(0001), ref 35 for H2/Cu(111), and ref 36 for H2/Ru(0001). In all cases, the DFT calculations were performed within the generalized-gradient approximation (GGA) method using the PW91 functional.44 Although it has been recently shown that a SRP (specific reaction parameter) functional developed specifically for H2+Cu(111)45 yields a better description of reaction and rotationally inelastic scattering of H2 from Cu(111) and of dissociative chemisorption of H2 on Ru(0001),18 we have used the PW91 PES because: (i) results for diffraction of H2 from Ru(0001) obtained with the PW91 functional are in better agreement with the experimental measurements than those obtained18 with the SRP functional developed for H2+Cu(111)38,45 (a similar comparison for H2/Cu(111) is not available in the literature); (ii) the SRP-PES for H2/Cu/ Ru(0001) is not yet available. Furthermore, to rule out that differences between the three systems are due to the use of different functionals, it seems more appropriate to use the PW91 functional in all cases. (2) In the second step, we solve the nuclear time-dependent Schrödinger equation in three steps: (a) The initial wave packet is written as a Gaussian wave packet corresponding to the initial rovibrational state of the molecule, which describes molecules moving toward the surface within a range of translational energies. (b) The wave packet is propagated according to the time-dependent Schrödinger equation using the split operator method.46 A discrete variable/finite basis presentation is used for all degrees of freedom. (c) A scattering amplitude formalism47 is used to analyze the reflected wave packet. Relevant parameters used in the quantum calculations are summarized in Table 1.

Table 2. Estimated Population in % of the Rotational Levels J for the Experimental Molecular Beams

a

J=0

J=1

J=2

J=3

50 175

20 8

73 52

6 17

1 23

Diffraction Patterns. In Figure 4, we show diffraction spectra for H2/Cu/Ru(0001), corresponding to three different choices of the incidence energy, Ei, and incidence angle, θi: (75 meV, 36°), (75 meV, 53°), and (175 meV, 30°). (See Figure 2.) In all cases, out-of-plane diffraction appears to be a prominent feature in the spectra. In fact, we can see from Figure 4 that the first-order out-of-plane diffraction peaks (1̅,0) and (0,1) are more intense than the first-order in-plane peaks (1̅,1̅) and (1,1). Furthermore, the spectrum recorded at θi = 53° and Ei = 75 meV shows an out-of-plane third-order diffraction peak, (3̅,2̅). The observation of pronounced out-ofplane diffraction has been previously reported (see, e.g., previous work on H2/Pt(111),17 H2/Pd(111),15 and H2/Ru(0001)18), but, to our knowledge, a third-order out-of-plane diffraction peak has never been observed. The H2/Pt(111), H2/Ru(0001), and H2/Pd(111) systems are either slightly activated or nonactivated;36,57,58that is, out-of-plane diffraction is supposed to be a very prominent feature in the diffraction spectrum. However, H2/Cu/Ru(0001) is an activated system with a computed minimum reaction barrier of 210 meV.34 Thus, unexpectedly, we have found an activated system with very pronounced out-of-plane diffraction, even more than that observed in the nonactivated systems mentioned above. To understand these findings, we have carried out time-dependent wave-packed propagation calculations on H2/Cu/Ru(0001) by using the same incidence conditions as in the experiment. As shown in Figure 4, our theoretical results reproduce fairly well the experimental spectra. Although the theoretical results overestimate the intensity of the out-of-plane peaks, they give

Table 1. Numerical Values of the Most Relevant Parameters Used in the Quantum Calculationsa Nz (no. of points used in Z) 48 Nsp z (no. of points used in Z for specular grid) ΔZ (grid spacing in Z) Zmin (minimum value of Z) Nr (no. of points used in Z) Δr (grid spacing in r) rmin (minimum value of r) NX (no. of points used in X) NY (no. of points used in Y) Jmax (maximum J value in basis) total propagation time Δt (time step of split operator propagator) Z0 (initial distance to the surface) Z∞ (analysis value of Z) perpendicular energy range/eV

Ei (meV)

140 180 0.12 −1.0 56 0.15 0.4 24 24 20/21 42 000 3.0 12.0 9.08 0.025 to 0.20

Atomic units are used.

Although diffraction is a quantum phenomenon, it has been previously shown15,49,50 that quasi-classical dynamics51 can be a useful tool to analyze qualitatively diffraction experimental 13673



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Figure 5. In-plane (n,n) and out-of-plane (n,m) diffraction spectra for H2/Cu/Ru(0001) (black line) and H2/Cu(111) (red line). Black solid line: H2/Cu/Ru(0001) experiment; red solid line: H2/Cu(111) experiment; red dashed line: H2/Cu(111) 6D quantum dynamical (QD) calculations. Theoretical results have been convoluted with a Gaussian function of width σ = 0.7° (the angular resolution of the measurements). All peaks have been normalized to the experimental specular peak for H2/Cu/Ru(0001). The experimental surface temperature is 100 K.

Figure 4. In-plane (n,n) and out-of-plane (n,m) diffraction spectra for H2/Cu/Ru(0001). Solid lines: Experiment; dashed curves: 6D quantum dynamical calculations. Theoretical results have been convoluted with a Gaussian function of width σ = 0.7° (the angular resolution of the measurements). The experimental peaks have been normalized to the specular peak (0,0) that arises from quantum calculations. The experimental surface temperature is 100 K.

the correct relative intensities. The latter finding ensures that the 6D PES used in this work is accurate enough to allow us to investigate the origin of the pronounced out-of-plane diffraction observed in the experiment. Note that quantitative accuracy of the computed diffraction probabilities (peaks) would only be expected with the use of a PES that also accurately described the dissociative chemisorption reaction for the metal surface considered, as accomplished in ref 17. However, most likely, the PW91 function used here produced barriers for the Cu/Ru surface that are too low.18,45 For the sake of both completeness and comparison, we have also measured H2 diffraction for Cu(111) under the same experimental conditions shown in Figure 4. Results are shown in Figure 5, where we compare diffraction spectra for H2 on Cu/Ru(0001) and Cu(111). From these Figures, we can clearly see that out-of-plane diffraction is much higher for the former

system than for the latter one. In fact, in the case of Cu(111), only out-of-plane first-order diffraction peaks are observed. In this Figure, we also show quantum results for H2/Cu(111) at Ei = 75 meV and θi = 36°. In this case, the agreement between experiment and quantum results is excellent, which validates the PES used here for studying diffraction of H2 scattering from Cu(111).

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ANALYSIS To analyze the relationship between diffraction and dissociative adsorption probabilities, we have further analyzed our quantum dynamics results for H2/Cu(111) and H2/Ru(0001). In Figure 6A, we show the comparison between theoretical diffraction patterns for the three systems considered here for two typical experimental incidence conditions, (75 meV, 36°) and (175 meV, 53°). In this Figure, we show the absolute 13674



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Figure 7. Quantum diffraction probabilities as a function of the diffraction order for: H2/Cu/Ru(0001) (black solid bars), H2/ Ru(0001) (green dashed bars), and H2/Cu(111) (red dotted bars).

intensity, the zeroth order peak being by far the most intense one. Results shown in Figures 6 and 7 reveal that there is not a direct proportionality between dissociative adsorption and outof-plane diffraction because dissociative adsorption is more important for H2/Ru(0001) than for H2/Cu/Ru(0001) (see Figure 1), whereas diffraction behaves the other way around. Dissociative adsorption depends on both the energetic corrugation (difference between the lowest barrier height at the site displaying the lowest overall barrier and the lowest barrier height at the site displaying the highest overall barrier) and the geometric corrugation (difference between the position (Z value) of the lowest barrier at the site displaying the overall earliest barrier and the position of the lowest barrier at the site displaying the overall latest barrier) of the surface and also on the height and location of the barriers. Diffraction, in contrast, depends on the corrugation, but not necessarily on the height of the barriers. Therefore, the results in Figures 6 and 7 seem to indicate that H2/Cu/Ru(0001) is a highly corrugated system. In Table 3, we give the values of the geometric and energetic

Figure 6. In-plane (n,n) and out-of-plane (n,m) H2 quantum theoretical diffraction spectra for H2/Cu/Ru(0001) (back dashed line), H2/Ru(0001) (green dashed line), and H2/Cu(111) (red dashed line). Results have been convoluted with a Gaussian function of width σ = 0.7° (typical angular resolution in experiments). (a) Absolute diffraction peaks. (b) Normalized diffraction peaks. All of the peaks have been normalized to the intensity of the specular diffraction peak for H2/Cu/Ru(0001).

probabilities for in-plane (φf = 0) and out-of-plane (φf ≈ 15°) diffraction peaks. We can see that whereas the specular diffraction peak for H2/Cu(111) is more than five times higher than for H2/Cu/Ru(0001) and more that three times higher than for H2/Ru(0001), the out-of-plane peaks for the three systems have a similar intensity. These results indicate that, in principle, the sole observation of out-of-plane diffraction peaks in experiments cannot be taken as the only signature of a very corrugated system. The key parameter to interpret diffraction experiments is the relative intensity of the different diffraction peaks. In Figure 6B, we show the same diffraction spectra but normalized to the probability of the specular peak (0,0) for H2/Cu/Ru(0001). It can be seen that H2/Cu/Ru(0001) presents the most intense out-of-plane diffraction peaks with respect to the specular one. This phenomenon is more clearly seen in Figure 7, where we show the probability of the different diffraction orders for H2/Cu/Ru(0001), H2/Cu(111) and H2/Ru(0001). The diffraction orders are defined with respect to concentric hexagons built around the (00) point: diffraction peaks lying on the same hexagon belong to the same order. (See Figure 3). Figure 7 shows that diffraction probabilities for H2/Cu/Ru(0001) are similar for all diffraction orders (up to the fourth order), whereas, for H2/Ru(0001), only diffraction probabilities up to the second order are large. In the case of H2/Cu(111), only zeroth and first-order peaks have a relevant

Table 3. Geometric Corrugation (GC), Energetic Corrugation (EC), and Minimum Reaction Barrier (MRB) Height system

GC (Å)

EC (eV)

MRB (eV)

H2/Cu/Ru(0001) H2/Ru(0001) H2/Cu(111)

0.25 0.64 0.21

0.43 0.24 0.39

0.21 0.01 0.49

corrugations for the three systems investigated here. It can be seen that H2/Cu/Ru(0001) presents the highest energetic corrugation, whereas the highest geometric corrugation corresponds to H2/Ru(0001). These results show that the measured diffraction patterns cannot be explained by only using arguments based on the static corrugation features of the PES. We have also analyzed the different regions of the PES where the molecules are scattered. To carry out this analysis, we have performed quasi-classical trajectory (QCT) calculations as described above. To validate the QCT results, we have compared them with the TDWP method. (See Figure 8.) From Figure 8, we can see that although the QCT method overestimates the diffraction peak intensities, the QCT spectra 13675



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Figure 9. Schematic representation of the irreducible cell for C6v symmetry.

Figure 10. XcmYcm distribution of the classical turning point for the scattered molecules, projected on the irreducible wedge of the surface unit cell (see Figure 9) as a function of the distance to the surface (Zcm). Figure 8. In-plane (n,n) and out-of-plane (n,m) diffraction spectra for H2/Cu/Ru(0001). Dashed lines: 6D quantum dynamics calculations; dotted-dashed curves: 6D classical dynamics calculations. The results have been convoluted with a Gaussian function of width σ = 0.7° (the angular resolution of the measurements). The classical peaks have been normalized to the specular peak that arises from quantum calculations.

whole unit cell; therefore, they feel the whole corrugation of the surface, which is reflected in the diffraction spectra. If we perform the same analysis for H2 scattered from Cu(111) and Ru(0001) (Figure 11), a reactive system and a nonreactive system, respectively, then we observe a completely different behavior. Both Ru(0001) and Cu(111) reflect the molecules far away from them, Zcm > 2.3 Å. In the case of H2/Cu(111), most molecules are reflected at any site on the surface, but they are so far away that they see a rather flat surface. Ru(0001) reflects most molecules in the region lying between the bridge and the hollow sites. Molecules approaching the Ru surface in a region around the top site will dissociate; that is, this region will not be explored by the reflected molecules. Therefore, as for the Cu surface, scattered molecules see Ru(0001) as a flatter surface than it really is. From these results, we can conclude that the pronounced diffraction measured for H2/Cu/Ru(0001) is a consequence of the ability of the scattered molecules (i) to explore the whole unit cell and (ii) to get close to the surface. This is in contrast with the behavior observed in H2/Cu(111) and for H2/Ru(0001), for which none of the above two effects appears.

resemble the quantum and the experimental ones. Specifically, the QCT calculations approximately reproduce the correct peak-intensity order. In Figure 10, we show the XcmYcm-distribution of the H2 molecules at the classical turning point of trajectories scattered from Cu/Ru(0001), projected on the irreducible cell (see Figure 9), for Ei = 75 meV and θi = 36°. The distributions are shown for three different regions of the classical turning points, associated with three different intervals of Zcm. We observe, first, that a considerable number of molecules gets rather close to the surface (1.5 Å > Zcm > 1.3 Å). Second, far from the surface, the molecules are reflected in the region between the hollow and the bridge sites. (See Figure 9.) Close to the surface, the classical turning point lies in the region between the top site and a site half way between the top and the hollow sites. Therefore, on average, the reflected molecules explore the 13676



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank BSC-RES and CCC-UAM for allocation of computer time. Work supported by the MICINN projects FIS2010-15127, FIS 2010-18847, CTQ2010-17006, and CSD2007-00010 and CAM program NANOBIOMAGNET S2009/MAT1726.

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REFERENCES

(1) Halstead, D.; Holloway, S. J. Chem. Phys. 1988, 88, 7197−7208. (2) Farías, D.; Miranda, R. Prog. Surf. Sci. 2011, 86, 222−254. (3) Horne, J. M.; Yerkes, S. C.; Miller, D. R. Surf. Sci. 1980, 93, 47− 43. (4) Hayward, D. O.; Taylor, A. O. J. Phys. C 1986, 19, L309−L314. (5) Robota, H. J.; Vielhaber, W.; Lin, M. C.; Segner, A.; Ertl, G. Surf. Sci. 1985, 155, 101−120. (6) Mattera, L.; Musenich, R.; Salvo, C.; Terreni, S. Faraday Discuss. Chem. Soc. 1985, 80, 115−126. (7) Cowin, J. P.; Yu, C. F.; Sibener, S. J.; Wharton, L. J. Phys. Chem. 1983, 79, 3537−3549. (8) Rettner, C. T.; Delouise, L. A.; Cowin, J. P.; Auerbach, D. J. Chem. Phys. Lett. 1985, 118, 355−358. (9) Berndt, R.; Toennies, J. P.; Wöll, C. J. Chem. Phys. 1990, 92, 1468−1477. (10) Shackman, L. C.; Sitz, G. O. J. Chem. Phys. 2005, 122, 1147021−114702-5. (11) Goncharova, L. V.; Braum, J.; Ermakov, A. V.; Bishop, G. G.; Smilgies, D. M.; Hinch, B. J. J. Chem. Phys. 2001, 115, 7713−7724. (12) Boato, G.; Cantini, P.; Tatrek, R. J. Phys. F 1976, 6, L237−L240. (13) Lapujoulade, J.; Cruer, Y. L.; Lefort, M.; Lejay, Y.; Maurel, E. Surf. Sci. 1981, 103, L85−L89. (14) Whaley, K. B.; Yu, C. F.; Hogg, C. S.; Light, J. C.; Sieber, S. J. J. Chem. Phys. 1985, 83, 4235−4255. (15) Farías, D.; Díaz, C.; Rivière, P.; Busnengo, H. F.; Nieto, P.; Somers, M. F.; Kroes, G. J.; Salin, A.; Martín, F. Phys. Rev. Lett. 2004, 93, 246104-1−246104-4. (16) Yu, C. F.; Whaley, K. B.; Hogg, C. S.; Sieber, S. J. J. Chem. Phys. 1985, 83, 4217−4234. (17) Nieto, P.; Pijper, E.; Barredo, D.; Laurent, G.; Olsen, R. A.; Baerends, E. J.; Kroes, G. J.; Farías, D. Science 2006, 312, 86−89. (18) Nieto, P.; Farías, D.; Miranda, R.; Luppi, M.; Baerends, E. J.; Somers, M. F.; van der Niet, M.; Olsen, R. A.; Kroes, G. J. Phys. Chem. Chem. Phys. 2011, 13, 8583−8597. (19) Farías, D.; Busnengo, H. F.; Martín, F. J. Phys: Condens. Matter 2007, 19, 305003-1−305003-18. (20) Mavrikakis, M.; Hammer, B.; Norskov, J. K. Phys. Rev. Lett. 1998, 81, 2819−2822. (21) Rodríguez, J. A. Surf. Sci. Rep. 1996, 24, 223−287. (22) Gsell, H.; Jakob, P.; Menzel, D. Science 1998, 280, 717−720. (23) Schalapka, A.; Lischka, M.; Gross, A.; Käsberger, U.; Jakob, P. Phys. Rev. Lett. 2003, 91, 016101-1−016101-4. (24) Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 4717−4725. (25) Otero, R.; Calleja, F.; García-Suárez, V. M.; Hinarejos, J. J.; de la Figuera, J.; Ferrer, J.; de Parga, A. L. V.; Miranda, R. Surf. Sci. 2004, 550, 65−72. (26) Hoster, H.; Richter, B.; Behm, R. J. J. Phys. Chem. B 2004, 108, 14780−14788. (27) Kibler, L. A.; El-Aziz, A. M.; Hoyer, R.; Kolb, D. M. Angew. Chem., Int. Ed. 2005, 44, 2080−2084. (28) Gross, A. Top. Catal. 2006, 37, 29−39. (29) Günther, C.; Vrijmoeth, J.; Hwang, R. Q.; Behm, R. J. Phys. Rev. Lett. 1995, 74, 754−757.

Figure 11. XcmYcm distribution of the classical turning point for the scattered molecules, projected on the irreducible wedge of the surface unit cell (see Figure 9) as a function of the distance to the surface (Zcm).

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CONCLUSIONS We have studied H2 diffraction from a strained pseudomorphic monolayer of Cu deposited on Ru(0001) (Cu/Ru(0001)), both experimentally and theoretically. Experimental diffraction spectra show pronounced diffraction, both in-plane and out-ofplane, for different incidence conditions (Ei,θi). In particular, diffraction spectra exhibit second and, more strikingly, thirdorder diffractions peaks. These spectra are fairly wellreproduced by TDWP calculations, which ensured the validity of the PESs used in our theoretical analysis. Classical trajectory simulations reveal that H2 molecules scattered from Cu/ Ru(0001) explore the whole surface unit cell, which leads to very pronounced diffraction. We have also compared H2/Cu/Ru(0001) diffraction results with those obtained for H2/Cu(111) and H2/Ru(0001). This comparison has allowed us to investigate the possible relationship between diffraction and dissociative adsorption. H2/ Ru(0001) is a more reactive system than H2/Cu/Ru(0001), which is in turn more reactive than H2/Cu(111). Diffraction, in contrast, does not follow the same trend. Diffraction is more important for H2/Cu/Ru(0001) than for H2/Ru(0001) and H2/Cu(111). In the case of H2/Ru(0001), scattered molecules feel a less corrugated PES because molecules that get closer to the surface dissociate. Therefore, the lower barriers of the PES are not explored by the diffracted molecules. H2 molecules diffracted from Cu(111) do not explore the whole PES either because they are reflected far away from the surface. This is the reason why most diffraction in this case is specular. In contrast with Ru and Cu, H2 molecules scattered from Cu/Ru(0001) explore the whole surface unit cell and are able to get close to the surface. Therefore, our study suggests that the link between reactivity and diffraction is more complex than assumed in previous work. 13677



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H2 Diffraction from a Strained Pseudomorphic Monolayer of Cu

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