Dynamics of Ethene Adsorption on Clean and C ... - Unige

Nov 11, 2009 - the presence of a nanosized staircase with open step edges, since it forms neither when dosing ethene on low Miller index surfaces. Fig...
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J. Phys. Chem. C 2009, 113, 20875–20880

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Dynamics of Ethene Adsorption on Clean and C-Contaminated Cu(410) V. Venugopal,†,‡ L. Vattuone,*,†,‡ T. Kravchuk,†,‡,¶ M. Smerieri,†,‡ L. Savio,§ J. Jupille,| and M. Rocca†,§ Dipartimento di Fisica dell’UniVersita` di GenoVa, Via Dodecaneso 33, 16146 GenoVa. Italy, CNISM Unita` di GenoVa, Via Dodecaneso 33, 16146 GenoVa, Italy, and IMEM-CNR, Sede di GenoVa, Via Dodecaneso 33, 16146 GenoVa, Italy ReceiVed: May 22, 2009; ReVised Manuscript ReceiVed: October 6, 2009

As recently established (J. Am. Chem. Soc. 2008, 130, 12552), ethene adsorption on Cu(410) occurs both molecularly and dissociatively, the latter resulting in carbon contamination of the surface. Here we report on the coverage-dependent dynamics of C2H4 adsorption on clean and carbon-contaminated Cu(410). For the bare surface, the initial sticking probability has a very weak dependence on kinetic energy and is almost independent of angle of incidence. Molecular adsorption is in both cases precursor-mediated and nonactivated. Ethene dissociation takes place during adsorption as well as upon annealing. Both paths proceed via a molecular precursor. The former is translationally activated, while the latter depends strongly on the heating rate. The presence of preadsorbed carbon, resulting from previous uptakes, affects both the sticking probability and the attained saturation coverage. The latter quantity is shown to be a sensitive probe of carbon precoverage. A scheme of the complicated potential energy surface of this system is derived and discussed. 1. Introduction A detailed understanding of surface dynamics is of crucial interest to properly model the properties of real catalysts and to obtain reliable estimates of reaction rates. This has prompted molecular beam reactivity studies over wide ranges of translational energies and angles of impingement for a number of gas/ surface systems. To date, efforts were nevertheless mostly concentrated on low Miller index single crystal surfaces. Much less has been done about surfaces involving defects such as steps and kinks, although they are present on catalysts and were often shown to play a key role in the reaction dynamics because of the significant corrugation of the surface potential they produce and of the under-coordination of some of the atoms.1 The reactive adsorption of ethene on copper is such a case in which reactivity was demonstrated to be profoundly modified by steps. On all low-index surfaces,2-6 vibrational spectra of adsorbed ethene are indicative of π-bonded species. In the case of Cu(100), desorption was shown to occur at 140 K for a heating rate of 2.5 K/s.4 Ethene is more strongly bound on Cu(410), a vicinal surface of Cu(100), on which a selective chemistry occurs.7,8 A combined analysis by high-resolution electron energy loss spectroscopy, thermal desorption, and photoemission spectroscopy showed that di-σ-bonded species are formed on the step edges and also dehydrogenation occurs. The importance of the change in surface chemistry is illustrated (a) by the presence of a shoulder in the desorption peak of π-bonded ethene, which extends up to 200 K at a heating rate of 3.6 K/s, (b) by the occurrence of an additional desorption peak around 240 K for C2H4/Cu(410), and (c) by the gradual change in the desorption spectra of subsequent adsorption * Corresponding author. E-mail: [email protected]. † Dipartimento di Fisica dell’Universita` di Genova. ‡ CNISM Unita` di Genova. § IMEM-CNR. | Permanent address: Institut des Nanosciences de Paris, CNRS UMR 7588-Universite´ Paris 6, Campus de Boucicaut, 75015 Paris, France. ¶ Current address: Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa 32000, Israel.

experiments performed without sputtering the surface. The first point indicates that some π-bonded molecules are stabilized at the undercoordinated sites at the step edges; the second that a di-σ-bonded moiety forms; and the third that carbon accumulation, resulting from ethene dissociation, occurs.7,8 Beyond the overall step-induced changes in reactivity of the copper surfaces, questions arise about the dynamics of ethene/ Cu(410) from a series of experiments which some of us performed for O2/Cu(410). Dissociative oxygen chemisorption9 as well as initial oxidation10 came out thereby to be anisotropic, being different for molecules impinging step up or step down. The present paper is aimed at exploring such anisotropy in the dynamics of ethene adsorption on Cu(410) by dosing the surface with a supersonic molecular beam impinging at different angles, from step up to step down, and investigating the effects of kinetic energy and of the presence of preadsorbed carbon resulting from previous uptakes. 2. Experimental Details Experiments are performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10-10 mbar, equipped with a supersonic molecular beam (SMB), a quadrupole mass spectrometer (QMS) not in line of sight with it (used for detection of beam molecules reflected off the surface), lowenergy electron diffraction optics, a cylindrical mirror analyzer for Auger spectroscopy, and all other typical UHV facilities.11 The sample is a 10 mm diameter disk cut within 0.1° from the (410) plane. The Cu(410) surface consists of three lattice spacing wide (100) terraces separated by (110)-like monatomic step rises. Ethene adsorption leads to the formation of several coexisting moieties, namely π-bonded, di-σ-bonded, and carbon resulting from total dehydrogenation.7 Removal of the molecular species can be obtained by annealing above the respective desorption temperature. Elimination of carbon requires sputtering (performed at 25° off normal with 1.5 keV Ne+ ions) followed by flashing to 900 K. Annealing the ethene covered surface to 900

10.1021/jp9047924 CCC: $40.75  2009 American Chemical Society Published on Web 11/11/2009

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Venugopal et al. uptake does not necessarily imply a difference in the real saturation coverage, since uptakes are evaluated by integration and the integral can be extended only as long as the sticking probability is large enough to be measurable by KW (with the present signal-to-noise ratio, until S ∼ 0.02). Above this point, S is no longer measurable but the uptake may continue slowly up to the real saturation coverage, provided the exposure lasts long enough. In the following we shall then address the coverage at which S is no longer measurable with KW as apparent saturation coverage. In the experimental setup, the angle θ at which the beam impinges on the surface can be varied in the plane perpendicular to the steps and is measured with respect to the normal to the surface. The positive (negative) values correspond to step up (down). Herein, θ is explored from +55° to -60° (see Figure 1). Since the surface makes an angle of 14° with the (100) terraces, the angular directions θ ) +31° and -14° correspond to normal incidence on the step rises and on the (100) nanofacets, respectively (see inset of Figure 1B).

Figure 1. Uptake curves corresponding to different angles of incidence for the ethene beam impinging on Cu(410) with Ei ) 0.10 eV (A) and Ei ) 0.36 eV (B) and at T ) 145 K. The inset in panel B shows a schematic view of the surface structure composed of terraces and step rises.

K leads to a carbon-free surface, causing, however, penetration of the latter into the bulk. Ethene is dosed by SMB and monitored by detecting the QMS signal at m/e ) 26, i.e., the peak of the C2H4 cracking pattern with the lowest background. The gas energy is varied by seeding the carried molecules either in He or in Ne (4% C2H4 in both cases), with an achieved translational energy of Ei ) 0.36 and 0.10 eV, respectively. The beam flux is calibrated by comparison with the one estimated for O2/Pd(100) with the same supersonic beam and employing a mixture (4% C2H4, 96% He) with the same carrier concentration.10 The value of the C2H4 flux is then corrected for the density of the Cu(410) surface and for the relative pressure (determined by the carrier) obtained on the forefront of the rotary pump operating on the first stadium of the SMB. The corrected flux is (0.17 ( 0.01) ML/s for Ei ) 0.36 eV and (0.073 ( 0.007) ML/s for Ei ) 0.10 eV [ML ) monolayer; in ML of Cu(410), i.e. 1 ML ) 1.53 × 1015 molecules cm-2]. The sticking probability is measured by the retarded reflector method of King and Wells (KW).12 The partial pressure of the gas of interest is recorded by QMS during the overall exposure. The beam trajectory is initially intercepted by a first shutter, located in the second stadium of the SMB. As shown by the traces of Figure 1, the initial increase of the QMS signal corresponds to the entrance of the supersonic molecular beam into the vacuum chamber. A second inert flag, located in the experimental chamber, prevents the beam from striking the sample. When the latter is turned down, the beam hits the sample, causing an abrupt decrease in the partial pressure because of the surface gettering action. The relative drop is a measure of the sticking probability, S. The coverage, Θ, is then determined by integrating the missing signal vs time and multiplying it by the beam flux. We stress that a difference in

3. Results 3.1. Ethene Adsorption on the Bare Cu(410) Surface. Figure 1 shows the result of KW measurements recorded during the exposure of ethene on Cu(410) at a temperature T ) 145 K, with Ei ) 0.10 eV (Figure 1A) and Ei ) 0.36 eV (Figure 1B). The following points should be noted: (1) The initial sticking probability, S0, depends little on Ei and θ. (2) At Ei ) 0.10 eV, S remains constant for several seconds, while at Ei ) 0.36 eV it increases initially. It eventually decreases abruptly in both cases. (3) When the surface is close to saturation and the exposure is stopped by intercepting the beam with the inert flag in the experimental chamber, an increase of the C2H4 partial pressure above the steady state level is observed. (4) At Ei ) 0.10 eV, the uptake is significantly larger for θ ) -60° than for θ ) +45°, while at Ei ) 0.36 eV this difference is definitively smaller. Note that the effect is due to the stable fraction since the difference persists after subtracting the contribution of the metastable species. This result is highlighted in Figure 2, which reports the attained surface coverage vs time. Outcomes (2) and (3) are expected and have been reported already, e.g., for C2H4/Ag(410)13 and C2H4/Ag(210).14 In particular, outcome (2) indicates that an extrinsic precursor is active both at thermal and hyperthermal energy. The increase of S with coverage at hyperthermal energy has been traditionally explained with a mechanical effect known as adsorbate-assisted adsorption,15 according to which ethene molecules hitting on preadsorbed companions are able to more efficiently dissipate their excess energy due to the better mass matching. The phenomenon was not, however, observed for some systems showing a nearly perfect mass matching.16 This observation led to the alternative hypothesis that the increase of S is connected to an adsorbate-induced enhancement of the corrugation of the gas-surface potential.17 The latter enables an easier conversion of the normal momentum of the impinging molecule into parallel momentum. As we will show later on, this is most probably the relevant physics for the present system. Result (3) implies that, at T ) 145 K, some of the adsorbed molecules are in a metastable state in equilibrium with the beam pressure.1 We identify such molecules with ethene at the terraces, which is known to be unstable at the temperature of

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Figure 4. Temperature dependence of the initial sticking probability, S0, for ethene impinging at normal incidence on the clean Cu(410) surface.

Figure 2. Ethene uptake curves obtained from the data of Figure 1. The highest coverage reached corresponds to the total uptake, Θ, while the asymptotic value after the flag intercepts the beam again corresponds to the stable coverage.

Figure 3. Sticking vs coverage for 4% C2H4 in Ne (Ei ) 0.10 eV) and in He (Ei ) 0.36 eV) at T ) 145 K. The coverage indicates the total uptake on the surface.

our experiment.4 In the following, we shall denote Θ as the coverage corresponding to the total uptake, which is defined as including both stable and unstable species. Outcomes (1) and (4) are new and unexpected. The former indicates that, as long as only the interaction with the bare surface is concerned, S0 is nearly independent of Ei and θ (see also Figure 3). The ineffectiveness of kinetic energy is surprising for nonactivated adsorption, for which S0 usually decreases markedly with increasing translational energy. The independence of S0 on θ, on the other hand, is expected as long as the normal energy of the molecules is small compared to the depth of the chemisorption well, while in the present case, it is observed also at high kinetic energy. We shall come back to this point later on in the paper. The energy and angle dependence of the saturation coverage (point (4)) is at variance with previous observations on the (otherwise similar) C2H4/Ag(410) system, for which it depends on θ at all energies and is largest when impinging step up. The

S(Θ) curves for Ei ) 0.10 eV (left) and 0.36 eV (right) and for several angles of incidence are shown in Figure 3. The total uptake at 0.10 eV is larger when impinging step down (θ ) -60°, red) than step up (θ ) +45°, blue), a difference that disappears already below 170 K (data not shown). Since at θ ) -60° the step rises are completely in shadow, the additional coverage must be at the upper side of the step, which can be more easily reached by molecules impinging step down if they keep at least partial memory of the direction of their parallel momentum in the gas phase. No angle dependence of the total saturation coverage is present, within the experimental error, for doses performed at Ei ) 0.36 eV. The initial sticking probability S0 vs T is reported in Figure 4 for the He seeded beam at normal incidence. S0 decreases with T so that no adsorption is observed when dosing at room temperature. This dependence indicates that all pathways, including the dissociative one, are precursor mediated. If a direct path is present, then its contribution is lower than the sensitivity of the King and Wells’ method. 3.2. Ethene Adsorption on C-Covered Surfaces. All the experiments reported so far were performed on the freshly sputtered surface, i.e., in absence of carbon contamination. In the following, we investigate the effects on the ethene adsorption dynamics of residual carbon. We shall refer to “clean” or “carbon-contaminated” to indicate surfaces prepared, respectively, by ion bombardment and annealing to 900 K or surfaces pre-exposed to C2H4 up to saturation and annealed to 273 K. Two heating rates were employed for annealing to this T: the former, denoted as “slow”, takes ∼5 min, while the latter, denoted as “fast”, takes only 1 min and implies the use of electron bombardment on the back of the sample. The average heating rate reads then 0.3 and 1.8 K/s. In Figure 5 we compare the C2H4 uptakes performed with Ei ) 0.10 and 0.36 eV on clean and carbon-contaminated Cu(410). First of all, we note that the initial sticking probability for the latter case is larger than for the clean surface. This agrees with TPD results reported in ref 8 showing an increased total uptake for very low subsequent exposures of 0.25 L ethene by backfilling, when the sample is cleaned only by a rapid annealing to room temperature (without sputtering) after each dose. The presence of preadsorbed carbon increases, therefore, the sticking probability. Next we comment on the coverage dependence of S. If the ethene dissociation probability (and hence carbon contamination) while dosing and annealing to 273 K were small, we would expect a subsequent ethene uptake (on the C-contaminated surface) to nearly superimpose with the one performed on the clean surface. This is approximately the case for Ei ) 0.10 eV and slow heating rate, while it is definitely not true for Ei )

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Figure 5. Sticking vs coverage for an ethene dose performed at normal incidence at 0.10 and 0.36 eV on clean and carbon-contaminated Cu(410) surfaces at T ) 145 K. For the latter case, previous exposures to C2H4 were performed with the same translational energy (0.10 and 0.36 eV, respectively); S(Θ) curves corresponding to slow and a fast annealing to 273 K are reported. During annealing molecularly adsorbed ethene (both π- and σ-bonded) desorbs, while dehydrogenated carbon radicals remain on the surface.

0.36 eV and slow annealing and for fast heating rates at both the Ei values. This indicates that (a) residual carbon at the surface (formed by C2H4 dehydrogenation, so most reasonably in the form of C2 admolecules) poisons part of the adsorption sites, (b) dissociation during the adsorption process is activated (it is negligible when ethene is dosed with Ei ) 0.10 eV), and (c) thermal dissociation depends on heating rate (it is negligible for slow annealing and significant for fast annealing). For slow annealing rate and Ei ) 0.36 eV, the total uptake on the carboncontaminated surface is about one-half of that observed for the clean surface. About every second active site is, therefore, either occupied or poisoned by carbon produced during dissociation of the previous C2H4 layer. If the annealing is rapid, the effect is larger and the (apparent) saturation coverage is then reduced to one-fourth with respect to the clean surface value. In the accompanying paper, we show X-ray photoelectron spectroscopy (XPS) data demonstrating that the C1s signal is reduced by a factor of 2 when the ethene-covered surface is heated to 300 K at a high heating rate,8 thus indicating that about 50% of the admolecules dissociate. From comparison of our data with the XPS results, we can then conclude that the total uptake reduction is due to site occupation by carbon produced by dehydrogenation and not by poisoning of nearby sites. Since the ratio between C2 admolecules and unavailable sites is 1:1, an estimate of the total reduction of the uptake can allow the determination of the amount of carbon present on the surface. Comparison with the experiments reported in Figure 5 indicates, therefore, that for Ei ) 0.36 eV, half of the ethene molecules dissociate directly during the dose while a further half of the remaining admolecules dissociate during the subsequent fast annealing, independently of their translational energy in the gas phase. Two pathways are thus present for ethylene dehydrogenation: a translationally activated one, which takes place during adsorption, and a thermal one proceeding via the preadsorbed admolecules. 4. Discussion Since, as demonstrated in the accompanying paper,8 no dissociation is found when ethene is dosed at room temperature, independently of the kinetic energy in the gas phase, we have to conclude that (a) also the activated dissociative process is mediated by an intrinsic precursor from which the molecules

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Figure 6. Comparison of S(Θ) curves recorded for the clean and the C-contaminated surfaces at θ ) -60° (left) and θ ) +45° (right) at T ) 145 K. The initial sticking probability increases in the latter case, while it remains the same for grazing incidence on step rises.

can either desorb or dissociate with a T dependent branching ratio and (b) the barrier for desorption must be higher than the one for dissociation but the prefactor for dissociation must be much smaller than the one for desorption. The potential energy surface for ethene/Cu(410) must therefore present a multiple adsorption well corresponding to the different sites available at the terraces and at the step. A consistent interpretation of the findings reported in this and in the accompanying paper8 implies the existence of five different adsorption states: two π bonded ones (π0 at the terraces and π1 at the step edge), one fully dehydrogenated state (CdC), one σ-bonded, and a metastable state [long bridge (LB) at the step edge], acting as a precursor for dissociation. These potential energy minima are interconnected by several paths. The details are illustrated schematically in Figure 7: (1) π0 and π1 are π-bonded molecules at the terraces and at the step edges, respectively. π1 is most reasonably on top of the undercoordinated step atoms in analogy to the case of ethene/ Ag(n10)18 and, according to DFT,19,20 has a binding energy on the order of 0.52 eV. It is more stable than π0, which again according to DFT has a bond energy of 0.30 eV.20 These numbers imply desorption of π0 at around 100 K and of π1 at 200 K. The latter value is in accord with the HREELS data reported in ref 8 for Cu(410) and in ref 19 for sputtered and Cu adisland-covered Cu(111), which show that ethene at steps is stable up to 190 K. The TDS for Cu(410)8 does not show two well-defined desorption peaks corresponding to π-bonded molecules, but the asymmetry of the 100 K peak may be indicative of desorption around 200 K. π0 is little or not populated in our molecular beam experiments, since they are performed by dosing at 123 K or higher. Both π0- and π1-sites may be reached from the gas phase via nonactivated paths. (2) From π0 the molecules may either desorb or move to the more strongly bound π1- and σ-states. The barrier height to transform from π0 to σ must be slightly larger than the one for desorption out of π0, since (a) in TDS we always observe desorption out of the σ-state which is therefore always populated and (b) as shown by the HREELS experiments8 there is some dependence on the heating rate for the population of the σ-bonded state. (3) The location of the σ-molecules at the surface cannot be determined from our data, but this state must be associated with the presence of a nanosized staircase with open step edges, since it forms neither when dosing ethene on low Miller index surfaces

Dynamics of Ethene Adsorption on Cu(410)

Figure 7. Scheme of the potential energy surface for C2H4/Cu(410) and schematic of the adsorption sites and of the paths connecting them. The left panel shows adsorption states at the step and the right panel at the terraces (see the text for the explanation of the symbols). The adsorption energies for the π-bonded states are taken from refs 19 and 20. The barriers from σ to LB (via π1) and from σ to π0 (desorption route) are taken from the TPD analysis reported in ref 8.

nor on sputtered Cu(111).19 Possibly they may occupy bridge sites, as suggested in a theoretical paper on ethene/Cu(111).21 The latter case corresponds to adsorption at closed packed islands with reasonably close-packed step edges. From the σ-state the molecules can either desorb or dissociate as demonstrated by TDS. The latter allows estimation of the barrier for desorption out of the σ-state which comes out to be 0.466 ( 0.004 eV with a prefactor of 109 s-1. The low value of the prefactor implies that desorption occurs through an unlikely pathway (e.g., if it is mediated by defects). The TDS estimate for the barrier for dissociation is 0.65 ( 0.03 eV with a prefactor of 5 × 1011 s-1. The σ-state is populated efficiently when dosing at low T, as in our TDS experiments,8 implying that it is precursor-mediated by π0, which is populated only at low T. From the values determined from TDS analysis we have to conclude that the di-σ-state is less strongly bound than π1 (though more than π0). (4) Molecules in π1 may either desorb or dehydrogenate. Dissociation dominates at high heating rates, desorption at low rates. The barrier for dissociation must therefore be larger than the one for desorption out of π1. (5) Dehydrogenation must occur through a precursor LB, which can be reached either indirectly through π1 at all kinetic energies of the incoming molecules or directly for translationally swift molecules only. The experiment tells us that at low T all molecules reaching LB dissociate, while at room temperature they all desorb (we observe no dissociation when dosing with the He seeded beam at room temperature). To explain the low T behavior, the barrier for desorption must be larger than the one for dissociation, while to explain the absence of dissociation at 300 K the prefactor for dehydrogenation must be very small so that all molecules desorb once this process becomes possible.

J. Phys. Chem. C, Vol. 113, No. 49, 2009 20879 This situation could, for example, occur if LB coincides with the site between two kink atoms of the open step edge and if, for dehydrogenation to occur, the hydrogen atoms of ethene need to be in very close contact with the Cu atoms. This situation may be realized when the molecule sits in the hollow between the kink atoms (the barrier to reach this site could then be associated with the necessity to bring them slightly apart to allow the ethene to fit in) with the molecular axis oriented parallel to the step edge within a narrow cone. Being more quantitative on the barriers involved in these processes goes beyond the possibilities of the present investigation. A theoretical simulation is currently under way.20 (6) Conversion from π1 to σ is very unlikely in accord with the fact that the di-σ-state is less strongly bound. The dissociation of the di-σ-bonded molecules could, however, occur through π1 and LB (otherwise, we would still have another path for dissociation, which is unlikely since no dissociation is observed in absence of open steps). The barrier determined by TDS8 corresponds therefore to the barrier from π1 to LB rather than from σ to CC. This implies that conversion from σ to π1 is easy and does not limit the process. Coming back to the sticking probability data, the increase of S with exposure might be accounted quantitatively by adsorbateassisted adsorption given an increase of 0.2 at 0.2 ML coverage at 0.36 eV (see Figures 5 and 6). The effect is, however, absent for the carbon-contaminated surface, although C2 and C2H4 have very similar masses. The adsorbate-assisted mechanism should therefore be operative in both cases, contrary to experimental evidence. A consistent picture can however be achieved by invoking a pivotal role of the corrugation of the gas-surface potential:17 the increase of S with ethene coverage for the pristine surface is then due to an increase in the corrugation in the presence of adsorbed ethene, while C2 contributes less to corrugation sitting closer to the surface. The initial sticking probability is larger for the carboncontaminated surface than for the bare surface at θ ) 45°, but it is the same at θ ) -60°. This information indicates that, at θ ) 45°, incoming ethene feels the corrugation associated to the presence of C2, while this is not the case at -60°. Carbon must therefore be at sites which cannot be easily reached by the latter molecules. The hollow site between the kink atoms at the step rises would fulfill this requirement, being in the shadow at the latter angle. Careful inspection of the data shows that at the onset of adsorption, the sticking probability is slightly higher for molecules impinging at +45° than at -60°. In a reverse manner, the apparent saturation coverage is higher for -60° than for +45°. The latter difference is close to the experimental error for 0.36 eV [see also the S(Θ) curves in Figure 3] but definitively larger for Ei ) 0.10 eV. These results imply that the incoming molecules keep the memory of the direction they had in the gas phase and have therefore a different probability to hit against the step edge depending on whether they arrive step up or step down. For ethene colliding against the bare surface, it is thus easier to get rid of its energy when in the subsequent hits it collides with the Cu atoms building up the step rise, thus resulting in a higher sticking probability. For ethene colliding on the ethene-precovered surface, the situation is different, since its kinetic energy can be lost more efficiently when probing the larger corrugation produced by the preadsorbed companions, which sit at the upper side of the step edge when π-bonded. At Θ ∼ 0.2 ML, the sticking probability is thus larger for molecules arriving step down, since they move in the right direction. The energy dependence of the process

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implies that, after the collision with the ethene precovered surface, slow impinging molecules find a site where they can chemisorb, while the fast ones have still a significant chance to be backscattered into the gas phase. The above-reported experiments allow us to rationalize the very little angle dependence in the sticking probability. Indeed, at low Ei the incoming molecules are unable to overcome the dissociation barrier and are then sensitive to the chemisorption well for π (or at most for di-σ) bonding. The ratio between Ei and the potential energy well is low enough to remove the memory of the angle of incidence. At high Ei the C2H4/Cu(410) system is characterized by the possibility of dissociative ethene adsorption, which implies a deep chemisorption well. The latter can be accessed directly through LB, so that also under these conditions the well depth is large compared to Ei. Moreover, the gas surface potential has a reasonably large geometric corrugation, which allows the dissociation barrier to be overcome also using parallel momentum.22 5. Conclusions Ethene uptake on Cu(410) has been studied by means of a molecular beam at different angles and energies of impingement. Molecular adsorption is precursor-mediated and nonactivated. The initial sticking probability is comparable for thermal and hyperthermal energies and nearly independent of the angle of incidence. The latter process occurs via an intrinisic precursor state, unstable even at the lowest investigated T, which is accessed directly if the incoming molecules have a hyperthermal energy or indirectly from the nearby sites when rapidly annealing the predosed surface. After reaching this state the molecules can either desorb or dehydrogenate. The presence of carbon strongly affects the adsorption dynamics, decreasing the total uptake by poisoning sites which would otherwise be available. Comparison with XPS indicates that poisoning involves only the site directly occupied by carbon and suggests the use of the uptake measurements as a sensitive probe for the amount of carbon present on the surface. Acknowledgment. We thank Compagnia S. Paolo, CNR, for the short-term mobility programme and the Italo-French University for financial support. We thank M. Okada for providing

Venugopal et al. the Cu(410) sample, Giovanni Carraro for collaborating to collect the data on the temperature dependence of ethene adsorption, and A. Kokalj for scientific discussion and for providing preliminary DFT results. V.V. undertook this work with the support of the TRIL Programme of ICTP in Trieste, Italy. M.R. acknowledges the hospitality of INMETRO and CNPq, Rio e Janeiro, Brasil, during which the writing of this paper was finished. References and Notes (1) Vattuone, L.; Savio, L.; Rocca, M. Surf. Sci. Rep. 2008, 63, 101. (2) Linke, R.; Becker, C.; Pelster, Th.; Tanemura, M.; Wandelt, K. Surf. Sci. 1997, 377-379, 655. (3) McCash, E. M. Vacuum 1990, 40, 423. (4) Jenks, C. J.; Bent, B. E.; Bernstein, N.; Zaera, F. Surf. Sci. 1992, 277, L89. (5) Raval, R. Surf. Sci. 1995, 331-333, 1. (6) Nyberg, C.; Tengstal, C. G.; Andersson, S.; Holmes, M. W. Chem. Phys. Lett. 1982, 87, 87. (7) Kravchuk, T.; Vattuone, L.; Burkholder, L.; Tysoe, W. T.; Rocca, M. J. Am. Chem. Soc. 2008, 130, 12552. (8) Kravchuk, T.; Venugopal, V.; Vattuone, L.; Burkholder, L.; Tysoe, W. T.; Smerieri M.; Rocca, M. J. Phys. Chem. C (accompanying paper in this issue)(DOI: 10.1021/jp904794n). (9) Vattuone, L.; Savio, L.; Gerbi, A.; Okada, M.; Moritani, K.; Rocca, M. J. Phys. Chem. B 2007, 111, 1679. (10) Okada, M.; Vattuone, L.; Gerbi, A.; Savio, L.; Rocca, M.; Moritani, K.; Teraoka, Y.; Kasai, T. J. Phys. Chem. C 2007, 111, 17340. (11) Rocca, M.; Valbusa, U.; Gussoni, A.; Maloberti, G.; Racca, L. ReV. Sci. Instrum. 1991, 62, 2172. (12) King, D. A.; Wells, M. G. Surf. Sci. 1972, 29, 454. (13) Savio, L.; Vattuone, L. Surf. Sci. 2005, 587, 110. (14) Vattuone, L.; Savio, L.; Rocca, M. J. Phys.: Condens. Matter 2004, 16, S2929. (15) Stephan, J.; Burghaus, U. Surf. Sci. 2002, 507, 736. (16) Stinnett, J. A.; Weaver, J. F.; Madix, R. J. Surf. Sci. 1998, 395, 148. (17) Carlsson, A. F.; Madix, R. J. J. Chem. Phys. 2000, 113, 838. (18) Kokalj, A.; Dal Corso, A.; Gironcoli, S.; Baroni, S. Surf. Sci. 2004, 566-568, 1018. (19) Skibbe, O.; Vogel, D.; Binder, M.; Pucci, A.; Kravchuk, T.; Vattuone, L.; Venugopal, V.; Kokalj, A.; Rocca, M. J. Chem. Phys. 2009, 131, 024701. (20) Kokalj, M. Private communication. (21) Michalak, A.; Witko, M.; Hermann, K. J. Mol. Catal. A 1997, 119, 213. (22) Darling, G. R.; Holloway, S. Surf. Sci. 1994, 304, L461.

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