THE JOURNAL OF CHEMICAL PHYSICS 141, 214708 (2014)

The structure of carbon monoxide adsorbed on the NaCl(100) surface—A combined LEED and DFT-D/vdW-DF study Jochen Vogt and Birgit Vogt Chemisches Institut der Universität Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany

(Received 18 August 2014; accepted 19 November 2014; published online 5 December 2014) The structure of the first layer CO adsorbed on NaCl(100) is investigated experimentally by means of quantitative low-energy electron diffraction at 25 K, and theoretically by means of density functional theory. Consistent with earlier helium atom diffraction results, the monolayer structure has p(2×1) symmetry with a glide-plane along the longer axis of the unit cell. The structure analysis confirms the binding of CO via the carbon end to the NaCl(100) surface. The vertical distance of carbon above Na+ is 2.58 ± 0.08 Å, in good agreement with geometry optimizations based on dispersion-corrected density functional theory, and 0.15 Å lower than predicted in calculations based on the nonlocal van der Waals density functional. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4903192] I. INTRODUCTION

For more than 20 years the first layer CO adsorbed on the NaCl(100) surface has been used as a model system for structural and dynamical phenomena involved with weak molecule surface interaction. Ewing and Chang demonstrated the effect of vibrational pooling1 in this system, which was later analyzed in detail by Corcelli and Tully,2 and, only recently, by Boney and Marcus.3 Moreover, the system CO/NaCl(100) is a prime example for orientational order-disorder phenomena in two dimensions.4–6 Infrared spectroscopy in combination with the helium atom diffraction technique7, 8 revealed the ordering of a monolayer CO in a (2×1) lattice below 35 K. Above this temperature, the translational symmetry of the monolayer was characterized being (1×1).8 In order to derive experimental structural information, polarization infrared spectroscopy (PIRS) has proven to be most fruitful. By means of PIRS, the average tilt angle of the CO molecules in the low temperature phase was determined to be 25◦ relative to the surface normal.9 Moreover, concerning the azimuthal orientation, an antiparallel configuration of the two translationally inequivalent molecules was proposed.9 However, despite vivid experimental research, the adsorption site close to the Na+ cation and the distance of the molecules to the surface could only be predicted by theory.5, 6, 10–12 Several nearly isoenergetic stationary points with different translational symmetry and azimuthal orientations were reported. According to the studies by Meredith and Stone,11 Vu, Jakalian, and Jack,6 as well as Picaud et al.,10 the optimum geometry of the monolayer is characterized by an antiparallel azimuthal orientation. In contrast, the molecular dynamics simulations by Hoang et al.5 support a p(2×1) geometry, in which the two inequivalent molecules are oriented in a herringbone structure with azimuthal orientations of ±45◦ relative to the glide-plane. Towards an experimental approach to confirm one of these structure models, Carré et al. discussed the possibility of helium atom scattering experiments to experimentally de0021-9606/2014/141(21)/214708/8/$30.00

termine the exact adsorption geometry of CO/NaCl(100).13 However, up to now, a complete experimental surface structure analysis, based on helium atom scattering or related techniques, is still missing. An alternative experimental technique for surface structure analysis is low-energy electron diffraction (LEED). Numerous studies demonstrate14–20 that LEED experiments with bulk insulators are possible even at low temperature, if primary electron energies above about 50 eV are used, and, moreover, if the primary current is in the nanoampere range or below. Surface charging effects as well as defect generation are held at a sufficiently low level to give meaningful results, even in the case of weakly bound adsorbates.20 The present study continues the program of LEED based surface structure analysis of molecular species on insulator surfaces, focussing on the system CO p(2×1)/NaCl(100). To the best of our knowledge, we give the first full experimental structure analysis for this system. Theoretical schemes to deal with van der Waals (vdW) interactions at surfaces beyond classical potential calculations are computationally challenging. The most accurate concepts such as the coupled cluster approach are still restricted to small systems. Other methods like dispersion corrected density functional theory introduced by Grimme21 (DFT-D), or vdW-DF22 are tractable, and the development and improvement of these methods within density functional theory are presently a field of high interest.23, 24 In the case of the system N2 /NaCl(100), DFT-D has been shown to reproduce the experimentally observed surface rumpling of the clean and the N2 covered surface.20 In addition, the calculated molecule surface distance overestimated the experimental result by only 0.1 Å at an experimental uncertainty of 0.07 Å. In the present study we directly compare results of DFT-D and vdW-DF for the monolayer CO p(2×1)/NaCl(100). The paper is organized as follows: In Sec. II the experimental setup and the LEED experiments are described. In Sec. III the results of the LEED structure analysis are presented. After the presentation of the results of the DFT calculations in Sec. IV, the paper closes with a discussion in Sec. V.

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FIG. 1. LEED patterns of the CO covered surface, recorded at an electron energy of 142 eV. (a) After 8 min of exposure to CO, the substrate temperature is 40 K. (b) Recorded at 25 K at stopped exposure to CO.

II. EXPERIMENTAL

The ultrahigh vacuum (uhv) apparatus, in which the LEED experiments were performed, was described in several previous publications.15–20 It is equipped with a closed-cycle helium refrigerator for sample cooling to temperatures of 25 K. Higher temperatures, measured with a silicon diode with an absolute accuracy of 2 K, can be set by means of a tungsten filament located at the sample holder. The LEED apparatus is a double microchannelplate optics (Omicron) with primary electron currents in the range of 1 nA. Surface charging and defect generation, as well as extensive distortions of the molecular adlayer, are avoided at these low electron exposures, as outlined and discussed in a previous paper.20 LEED diffraction patterns were recorded with a 12 bit digital camera at normal incidence conditions with an estimated alignment error below 0.5◦ . Background corrected diffraction peak intensities were extracted from the images using an integration scheme described in Ref. 25. A clean NaCl(100) surface was prepared by cleavage of a NaCl single crystal in dry nitrogen atmosphere. Subsequently, the NaCl sample was brought into vacuum within 30 min. After bakeout of the uhv chamber, the residual gas pressure was better than 5 × 10−10 mbar. Under measurement conditions at low temperature, the residual gas pressure was below 1 × 10−10 mbar. The CO adsorbate was prepared at sample temperatures below 40 K by exposing the clean NaCl sample to 1 × 10−8 mbar CO (purity 99.97%). In Fig. 1(a) a diffraction pattern is shown, which was recorded after 8 min of exposure at a crystal temperature of 40 K. Clear signs of superstructure, assigned to beam order {1 12 }, are visible, which indicate the formation of the CO adsorbate under these conditions. The superstructure peaks are of elliptical shape and appear broadened in comparison to the integral order peaks, indicating a certain degree of disorder in the adsorbate at this temperature. After the spot intensities were stable, the temperature was reduced to 25 K at stopped exposure. LEED pattern (b) in Fig. 1 was recorded

under these conditions. It shows sharper superstructure peaks, consistent with a higher degree of order of the adsorbate after temperature reduction. The apparent increase of diffraction peak intensities of all beam orders reflects the well-known Debye Waller effect.26 Subsequently, diffraction patterns of the CO covered surface were recorded between 80 and 310 eV in steps of 2 eV. The set of I(V) data of five inequivalent beam orders, including one superstructure spot, is depicted in Fig. 2. The beam orders {0 12 } and {0 32 } were not observed over the entire investigated energy range, consistent with the glide-plane symmetry deduced from earlier helium atom diffraction experiments.8 The peaks of beam order {1 32 } were only faintly visible near 120 eV. Similarly, spots of beam order {2 12 } were too weak to be observed at energies above 142 eV. It is worth to mention that the availability of fractional order beam data increases the sensitivity of a LEED structure analysis towards adsorbate coordinates. However, the extinction of fractional order beams due to glide-plane symmetry has also an important payback: As explained in Sec. III, glide-plane symmetry reduces the number of independent molecular structure parameters from 10 to 5 in the monolayer CO-p(2×1)/NaCl(100). Moreover, it is well known that

FIG. 2. LEED I(V) curves of CO-p(2×1)/NaCl(100).

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structure information of the adsorbate layer is also contained in integral order beam spots.26 The structure model presented in Sec. III does not have more free parameters than a (1×1) structure with one molecule per surface unit cell. A structure analysis based on the integral order beam data and one fractional order beam is thus realistic, and we will pick up this point again in the discussion in Sec. V. III. LEED STRUCTURE ANALYSIS

The LEED structure analysis of the monolayer CO/NaCl(100) was based on full dynamic calculations of diffraction peak intensities using the Barbieri/van Hove SATLEED package,27 in combination with the Barbieri/van Hove phase shift package.28 Multiscattering theory as the underlying basis of these codes is described extensively elsewhere.26, 29 LEED scattering phase shifts were considered up to maximum angular momentum of Lmax = 8, and were corrected for isotropic mean square vibrational displacements of the scatterers. The latter were treated as nonstructural parameters and were optimized during the structure search. For an illustration of the structure model of the CO covered surface see Fig. 3. The NaCl(100) substrate was represented by a stacking of composite layers29 consisting of one sodium and one chlorine ion with (1×1) translational symmetry. The topmost substrate layer was treated as a composite layer with (2×1) symmetry containing two sodium and two chlorine ions, respectively. The vertical positions of ions in this layer, related by glide-plane symmetry, were varied in order to consider the possibility of a top-layer rumpling  = 12 xNa1 − xCl1 , where xNa1 and xCl1 are the vertical positions of toplayer Na and Cl ions, respectively. Note that the positive x-axis points towards the substrate. Hence, a positive rumpling indicates a shift of sodium in the first layer towards the bulk, while chlorine shifts towards the vacuum. A further structure parameter characterizing the substrate surface is the  layer distance between the first and second layer, d = 12 xNa1 + xCl1 − xNa2 − xCl2 . Consistent with the observed diffraction patterns, the surface model considered two energetically equivalent CO molecules denoted M1 and M2 in a p(2×1) unit cell, related by glide-plane symmetry along the longer axis. This surface model reduces the number of independent molecular structure parameters to five: the three cartesian coordinates of the carbon atom of molecule M1, its tilt angle θ , and its azimuthal angle φ (see Fig. 3). The C–O bond length was initially fixed at the gas-phase value of 1.128 Å. This is reasonable, because the only weak electrostatic and van der Waals interaction with the substrate is unlikely to change the internal structure of the CO molecule substantially. Mahmud and Davidson12 have calculated the change of CO bond length upon adsorption to be 0.002 Å. Nevertheless, in the late stages of the structure analysis, the C–O bond length was also varied in order to check the plausibility of our results. Due to the multiscattering nature of the interaction of low-energy electrons with the surface, a direct determination of the surface structure is hampered. Instead, the structure search requires a comparison of calculated and experimental I(V) curves in a trial and error procedure, in which the struc-

FIG. 3. Structure of CO p(2×1)/NaCl(100) from LEED I(V) analysis in side view (upper diagram) and top view (lower diagram). The red square indicates the (2×1) unit cell, the blue dashed line the glide-plane. The unit cell contains two molecules denoted as M1 and M2.

ture parameters are systematically adjusted. For the quantitative measure of agreement between calculated and measured I(V) data, Pendry’s definition30 of the reliability factor RP was used. Poor agreement between experimental and calculated I(V) data corresponds to higher values of RP , while values near or below 0.2 indicate good agreement in general. The calculation of error bars for structure parameters followed the rules described in Ref. 30. Moreover, the comparison of experimental and calculated sets of I(V) data involved the consideration of domain averaging.29 This is necessary, because the adsorbate has in general a lower point symmetry than the C4v symmetry of the NaCl(100) substrate surface. Hence, the

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beam intensities of four different energetically equivalent domain orientations need to be averaged prior to comparison with experimental I(V) data. Starting with an upright geometry of the CO molecules on top of Na+ and a distance between the lower carbon and the sodium ion of 2.6 Å (RP = 0.56), the structure search commenced with an adjustment of the vertical positions of the sodium and chlorine ions in the topmost substrate layer. As in the related system N2 /NaCl(100),20 the rumpling of the first NaCl layer turned out to be the most significant structural feature with a very pronounced influence on the R-factor in the vicinity of the best-fit minimum. Then the vertical positions of the admolecules were adjusted, followed by a variation of the lateral positions of carbon and oxygen in compliance with the glide-plane symmetry of the system. This sequential fitting of vertical and then lateral positions was iterated several times accompanied by an adjustment of the rms vibrational amplitudes of the scatterers. In the final step, the R-factor landscapes around the minimum were generated in order to get information about error bars. The set of best-fit I(V) curves are shown in Fig. 2 in comparison to the measured beam intensities. The overall Rfactor is 0.216, the R-factors of individual beams are 0.260 for the {10} beam order, 0.233 for {11}, 0.142 for {20}, 0.179 for {21}, and 0.271 for {1 12 }, respectively. Apart from the quite satisfactory overall R-factor, good agreement of relative intensities in all beam orders is recognized. It is worth to mention that the Pendry R-factor is designed to be rather sensitive to the presence of features in I(V) curves, however to less

extent to their intensities. The good agreement of intensities is thus an additional hint for the validity of the presented best-fit results. The obtained best-fit geometry is shown in Fig. 3 and is further characterized in Table I. The surface rumpling of the CO covered surface of  = 0.06 ± 0.02 Å is weak but significant, and has, within the experimental uncertainty, the same value as the bare NaCl(100) surface,17 and, moreover, the N2 covered surface.20 The vertical position of the carbon atoms is 2.59 ± 0.08 Å above the sodium ions in the first layer. In addition, a significant lateral displacement of 0.4 Å away from the glide plane is recognized. The molecules are tilted 28◦ ± 5◦ with respect to the surface normal. This value is in satisfactory agreement with results from polarization infrared spectroscopy.9 However, due to the large error bar of 50◦ , the azimuthal orientation is not significant. This is consistent with the very shallow shape of the R-factor surface over a large range of azimuthal angles shown in Fig. 4. These structural results are further discussed in Sec. V. In order to further test the validity of the located minimum, the C–O bond length was varied in the best-fit geometry, while all other structure parameters were locked. The corresponding change of the Pendry R-factor with bond length is depicted in Fig. 5, and shows a distinct increase if the bond length is either stretched or shortened from the gas-phase value. A bond length of (1.1 ± 0.1) Å is consistent with our LEED data, close to the gas-phase value of the C–O bond length. In order to investigate the sensitivity of LEED to the position of carbon and oxygen, the class of structure

TABLE I. Parameters characterizing the experimental best-fit structure of the monolayer CO p(2×1)/NaCl(100), and the minimum energy structures of CO p(2×1)/NaCl(100) from DFT-D and vdW-DF. The parameters  and d are defined in the text. For the definition of the tilt and azimuthal angle see Fig. 3. The parameters x(Na1–C), y(Na1–C), and z(Na1–C) denote the distance between the carbon atom of molecule M1 and the topmost Na ion underneath (see Fig. 3). Theory (structure 1)a

Experiment LEED Adsorbate structure C–O bondlength (Å) x(Na1–C) (Å) y(Na1–C) (Å) z(Na1–C) (Å) θ (tilt angle) (deg) φ (azimuthal angle) (deg)

1.1 ± 0.1 2.59 ± 0.08 0.4 ± 0.2 0.1 ± 0.3 28 ± 5 30 ± 50

Substrate structure Rumpling,  (Å) Average layer distance, d (Å)

0.06 ± 0.02 2.76 ± 0.05

Thermal displacements (Å) Carbon Oxygen Sodium Chlorine

0.27 0.23 0.13 0.11

Inner potential (eV) Real part V0r Imaginary part V0i

−8.5 −3.0

a

Herringbone structure, see Fig. 6(c). Antiparallel structure, see Fig. 6(d). c Reference 9. d Lattice constant 5.66 Å. e Lattice constant 5.75 Å. b

Theory (structure 2)b

IRc

DFT-Dd

vdW-DFe

DFT-Dd

vdW-DFe

25 90

1.143 2.65 0.36 0.36 24 44.7

1.142 2.80 0.36 0.36 27 42.9

1.143 2.58 0.77 0.06 34.9 90.0

1.141 2.74 0.57 0.02 30.0 90.0

0.06 2.81

0.05 2.86

0.05 2.81

0.05 2.86

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IV. DFT CALCULATIONS

FIG. 4. Polar diagram illustrating the variation of the Pendry R-factor RP with tilt angle θ and azimuthal angle φ (cf. Fig. 3). The minimum is located at θ = 28◦ and φ = 30◦ .

models with bonding via the oxygen end of the CO molecule was also tested. If in the best-fit structure shown in Fig. 3 carbon and oxygen atoms are simply exchanged, the Pendry R-factor worsens from 0.216 to a value of 0.270. The difference demonstrates the extent of chemical sensitivity of LEED in this special case. However, a potential minimum of the Rfactor for models with oxygen pointing towards the surface is expected to be characterized by a different set of molecular structure parameters. By means of systematic variation of the latter, the R-factor could indeed be further minimized to a value of 0.220. This value is so close to the above bestfit R-factor, that judged from the LEED experiment only, no clear decision can be made, whether CO binds via carbon, or oxygen. However, with bonding via oxygen, the optimum tilt angle increases markedly to θ = 37◦ , which is 12◦ larger than the value from polarization infrared spectroscopy.9 Taking together the results from both experimental techniques, there is strong support for the bonding of the CO molecules via the carbon end.

FIG. 5. Variation of Pendry R-factor RP with C–O bond length.

The CO covered NaCl(100) surface was modeled using DFT as implemented in the Quantum Espresso code, version 4.3.1.31 Ultrasoft pseudopotentials based on Perdew-BurkeErnzerhof (PBE) functionals were taken from the Quantumespresso pseudopotential repository.32 In the case of sodium, a pseudopotential with semicore 2s and 2p states was used. A cut-off energy of 60 Ry in combination with a 6 × 6 × 1 Monkhorst Pack grid33 was used to calculate the electronic structure for a given adsorbate geometry. Two different approaches to treat van der Waals interaction were used, the model of dispersion correction (DFT-D) by Grimme,21 which adds a dispersion term to the DFT energy, and the nonlocal van der Waals density functional vdW-DF42,34 by Dion et al.22 The underlying structure model was an asymmetric slab of 4 NaCl layers with lateral (2×1) symmetry, periodically repeated with a gap of 58 Å, to suppress interactions of neighboring slabs. The ions in the deepest layer were fixed at their bulk positions. Appropriate lattice constants of the NaCl substrate were determined in preceding calculations by a relaxation of bulk NaCl with variable unit cells, resulting in a lattice constant of 5.66 Å for the DFT-D method, and 5.75 Å for the vdW-DF, respectively. The somewhat larger value for vdW-DF reflects the general trend of this method to overestimate lattice constants of solids.35 The 0 K adsorption energy per molecule of a given geometry is defined as Eads =

1 (E − 2 ECO − ENaCl ), 2 CO/NaCl

(1)

where ECO/NaCl , ECO , and ENaCl are the total DFT energies of the above described system CO + NaCl, an isolated CO molecule, and the bare relaxed substrate, respectively. Geometry optimizations based on both methods lead to the following results: a quasi p(1×1) structure with CO molecules on top of Na+ and an exactly upright orientation is a local minimum on the potential energy surface in a (2×1) unit cell. Such a structure is shown in Fig. 6(a). However, a tighter bonding to the surface is found, if the CO molecules are slightly displaced from their exact on-top positions. In this case, the molecular axes become tilted with respect to the surface normal. Both, a quasi (1×1) geometry (Fig. 6(b)), or a (2×1) herringbone structure with glide-plane symmetry as shown in Fig. 6(c) are minima on the PES. Geometric parameters characterizing the latter structure are presented in Table I for both functionals. The energetic differences between these latter structures, however, are very small, in the range of only 0.1 kJ mol−1 . Moreover, an antiparallel (2×1) configuration with alignment of the CO molecules along neighboring Na+ sites has been located as a minimum (see Fig. 6(d), structure parameters in Table I). Both DFT-D and vdW-DF find this geometry energetically most favorable with adsorption energies of −24.9 kJ/mol (vdW-DF) and −23.2 kJ/mol (DFT-D), respectively. The described behavior is qualitatively similar to what has been reported by Meredith and Stone.11 However, the adsorption energies from DFT-D and vdW-DF are about 50% higher than values presented in Ref. 11, and about 30% higher than the value deduced from experimental data.37

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(a)

(b)

(c)

(d)

FIG. 6. Geometries of the CO covered NaCl(100) surface from DFT calculations (top view). Geometry optimizations are performed in a (2×1) unit cell. (a) Quasi (1×1) structure with CO in perpendicular orientation. (b) Quasi (1×1) structure with CO in tilted orientation. (c) Herringbone structure with glide-plane symmetry and tilted CO molecules oriented towards chlorine (yellow balls) (cf. Table I, structure 1). (d) Structure with tilted CO molecules in antiparallel azimuthal orientation (cf. Table I, structure 2).

In Fig. 7, Eads as a function of the vertical distance of the CO molecules from the topmost Na+ is shown for both methods and for the two different azimuthal orientations of the molecule (see Fig. 6(c), denoted as structure 1, and 6(d), denoted as structure 2 ). In addition, the potential minimum (−18 kJ/mol) deduced from adsorption experiments37 and the

molecule surface distance supported by our LEED analysis are marked by the green lines (dotted lines indicate the error bars). As can be seen in the diagram, the DFT-D minima are closer to the experimental values than the vdW-DF results. While both methods predict a quite similar behavior in the repulsive part, the vdW-DF curve is more attractive at distances greater than 2.5 Å. By consequence, vdW-DF predicts a less stiff bonding of the molecules, consistent with a lower curvature at the potential minimum. To analyze this further, the well-known Morse potential used in Ref. 7, V (r) = De [1 − exp (−a(r − re ))]2 ,

(2)

with the model parameters De (potential depth), a (range parameter), and the equilibrium distance re was fitted to the DFT data. From the range parameter a, De , and the mass of the CO molecule, the wave number of the frustrated translation perpendicular to the surface,  2De a , (3) ν˜ x = 2π c mCO FIG. 7. Adsorption energy as a function of molecule surface distance from DFT-D and vdW-DF calculations. Structure 1 (solid circles) is depicted in Fig. 6(c), structure 2 (open circles) is the antiparallel geometry shown in Fig. 6(d). The green vertical line marks the experimental best-fit distance from LEED, the dotted lines indicate the corresponding uncertainty. The horizontal line indicates the potential depth derived from previously published adsorption experiments.37

is derived.43 The vdW-DF curve is consistent with a = 1.2 Å−1 and ν˜ x = 60 cm−1 . These values are lower than those discussed by Ewing7 and Gevirzman38 (1.8–2.0 Å−1 for a, and 90 cm−1 for ν˜ x ), but they are closer to the values used in the recent studies on vibrational pooling (˜νx = 69 cm−1 used

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in Ref. 2 and a = 0.816 Å−1 used in Ref. 3). In contrast, the DFT-D potential curve is consistent with a = 1.4 Å−1 , and ν˜ x = 96 cm−1 . Larger curvatures at the potential minimum and thus larger values of the perpendicular vibrational energy predicted by DFT-D in comparison to vdW-DF functionals have also been reported in the case of noble gas physisorption on metal surfaces.36

V. DISCUSSION

In an effort to get experimental structure information on the monolayer CO/NaCl(100), we have performed the first quantitative LEED structure analysis on this model system of physisorption. The structural model, which fits best to the experimental set of I(V) curves shown in Fig. 2, is the one depicted in Fig. 3. As proposed by all preceding experimental and theoretical studies,5–12, 38 CO binds via carbon to the NaCl surface. Although our LEED data give only a slightly worse R-factor for a geometry with bonding via oxygen, this model implies a larger tilt angle of 37◦ , inconsistent with the results from polarization infrared spectroscopy9 (25◦ ). In contrast, the tilt angle of 28◦ ± 5◦ involved with the best-fit geometry in Fig. 3 is in excellent agreement with the spectroscopic result. For the antiparallel structure 2 (cf. Fig. 6(d)), both DFT methods predict higher tilt angles than determined by the experiments. Our experimental value for the vertical distance of carbon to Na+ is 2.59 ± 0.08 Å, consistent with a vertical center of mass distance of only 3.09 ± 0.10 Å. This value is 0.2 Å smaller than assumed by previous theoretical studies.10, 11 Moreover, as can be seen in Table I, the present vdW-DF calculations also support larger molecule-surface distances than determined experimentally. In contrast, the DFT-D value for x(Na1–C) in the antiparallel configuration (structure 2) is very close to the LEED result. Concerning the azimuthal orientation of the admolecules, the best-fit structure depicted in Fig. 3 comes closer to the model structure shown in Fig. 6(c). Both DFT-D and vdW-DF calculations support the antiparallel orientation (Fig. 6(d)), although the adsorption energy per molecule of structure in Fig. 6(c) is only about one or two kJ/mol higher, as can be seen in the potential curves in Fig. 7. The antiparallel orientation of the molecules is strongly supported by the PIRS experiments,9 because – as discussed in Ref. 9 – dipole coupling of the in-phase stretch vibration in this structure leads to a vanishing absorption in s-polarized spectra, in agreement with the experiment. We are thus led to assume that the structure depicted in Fig. 3 does not reflect the mean orientation of the admolecules. We mention two possible explanations for this point. The first explanation is related to the well-known reduced sensitivity of quantitative LEED to lateral structural features,39 such as the azimuthal orientation φ. The reduced sensitivity to the latter is reflected in the large error bar of 50◦ , and also in Fig. 4, where the only weak dependency of the R-factor on the azimuth angle φ is illustrated. In general, the sensitivity to φ could be increased if additional half-order diffraction spots such as (2, 12 ) and (1, 32 ) (cf. Fig. 1) could be included in the analysis. Unfortunately, these higher in-

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dex beams appear on the screen at higher electron energies, where their intensities are strongly damped, presumably due to their higher momentum transfer. Therefore, no usable experimental I(V) curves could be extracted for these beam orders. An alternative explanation is related to adsorbate dynamics, which – as a consequence of weak molecule-surface interaction – is pronounced even at 25 K, where the LEED data were recorded. Hoang et al. provide distribution functions of the azimuthal orientation derived from their molecular dynamics simulations5 at 25 K and 55 K. Frustrated rotations of the CO molecules imply notoriously anisotropic motions of atoms, especially of the oxygen atoms.5 The thermal displacements provided in Table I, however, assume isotropic thermal movement, including the quantum mechanical 0 K motion. This treatment, which is standard in LEED structure analysis, thus implies an additional systematic error. More sophisticated techniques to account for anisotropic thermal displacements have been described in literature.40, 41 An exact treatment is involved because of the multiscattering nature of electron diffraction. For example, the split-positions model by Over et al.40 averages calculated beam intensities of several structures with thermally displaced atoms before they are compared to experimental data. They show that such a treatment is more accurate than just assuming the scatterers in their mean position. In that sense it is quite conceivable that the enhanced anisotropic librational motions of the CO admolecules are partially reflected in the best-fit result shown in Fig. 3. To summarize, we present the results of the first quantitative LEED structure analysis on the low-temperature phase CO-p(2×1)/NaCl(100). The LEED data in combination with results from polarization infrared spectroscopy9 give an experimental verification of the adsorption site of CO on top of Na+ with bonding via the carbon atoms. The tilt angle of the molecules is 28◦ ± 5◦ with respect to the surface normal. The vertical distance of carbon to Na+ is 2.59 ± 0.08 Å, in reasonable agreement with predictions from dispersion corrected density functional theory (PBE/DFT-D). Calculations using the van der Waals inclusive vdW-DF functionals overestimate the molecule surface distance by 0.15 Å. 1 H.-C.

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