Cu(111) System: Real-Time

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Adsorption Properties of Xe on Ag/Cu(111) System: Real-Time Photoemission Investigation Azzedine Bendounan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08721 • Publication Date (Web): 28 Sep 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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The Journal of Physical Chemistry

Adsorption Properties of Xe on Ag/Cu(111) System: Real-Time Photoemission Investigation Azzedine Bendounan∗ Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France E-mail: [email protected] Phone: +33 (0)169 359799

1

ACS Paragon Plus Environment


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Abstract We discuss here the adsorption process of rare gas atoms on heterogeneous surface, namely Xe on Ag sub-monolayer covering partially Cu(111) surface. As a function of the Xe dosing, we have monitored in real-time the evolution of the Xe 4d core levels and of the LEED patterns, as well as the modification of the Shockley state band. We confirmed that the reduction in the density of states near the Fermi level strongly promotes the adsorption of Xe atoms on the surface. More interesting, it seems that the Xe overlayer on the Ag monolayer exhibits a disordered structure, which differs substantially from those observed for Xe on most of the single crystal metal surfaces, where the Xe atoms form generally an epitaxial layer with hexagonally closed-packed structure. Additionally, the commensurate (9×9) reconstruction characterizing the 1 ML Ag/Cu(111) is preserved after adsorption of Xe, and consequently leads to a band back-folding of shifted interface state on the 1ML Ag islands covered by Xe. This finding is essential to understanding effect of the surface characteristics on the adsorption properties of rare gases on metal systems.

Introduction Adsorption of rare gases on metal surfaces has been studied extensively over many years and still remains the subject of recent and ongoing theoretic and experimental works. 1–4 It has been tremendously used to determine the surface area of powered catalytic substances 5,6 and represented as well a means of access to fundamental physical properties, such as to measure a local variation of the work function over the surface originated from an image charge screening model inducing a peak shift in the photoemission spectra (see below). 7 It has enabled the measure of the decay length of ionized atoms inducing local field on the surface, as in the case of K on Rh(111). 8 In the past, much interest has been devoted to photoemission of adsorbed xenon (PAX), 9,10 which was often applied as a sensitive probe of the local work function and has proved to be a powerful tool for investigation of heterogeneous 2


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surfaces, in particular for monitoring changes in the composition and tomography of the outermost surface layer, 11 and thus predicting presence of chemical or structural defects 12 , as well as a possible formation of surface alloy 13,14 . Moreover, this technique can provide valuable information on the growth mechanism of immiscible deposits on metallic surfaces 15 . Being characterized by high lateral resolution, of the order of few ˚ Angstroms, PAX has the capability of providing surface potential differences on an atomic scale. 8 In fact, the presence of unlike chemical or structural sites on the same surface leads generally to a variation of local work function over the surface and since the binding energy of the Xe photoemission peaks depends directly on the magnitude of the local work function, one gets in the spectrum a superposition of peaks originated from Xe atoms residing on those different sites. The adsorption mechanism of Xe on metal surface is described as a physisorption process characterized by a potential containing two terms of interactions at the interface. 16,17 On one hand, the long-range and dispersive van der Waals attractive interaction – known also as London force – which can be seen asymptotically as the interaction between the induced dipole in the approaching adsorbate and the electron sea of the metal, and on the other hand the Pauli repulsion acting at a short-range distance and resulting from overlapping between valence electronic states of the metal and the closed shell of the inert atoms of rare gases. Although this interaction is considered rather week (adsorption energy ∼ few hundreds of meV), 18 it affects significantly the properties of the so-called Shockley surface state developing on the (111) face of noble metals. Namely, the binding energy of the surface state, depending on the parallel momentum, is significantly reduced and the spin-orbit splitting is increased after adsorption of rare gases atoms, as for example on Au(111). 19–21 For instance, the underlying mechanism has been invoked on the basis of the Pauli exclusion inducing a local force pushing the wave function of the surface state towards the nuclei of the metal atoms. Note however that the modification of the Shockley state has been widely studied during the past decade for various adsorbates such as noble metals over-layer, 22,23 alkali atoms 24 and more recently organic molecules. 25–30 Thereby, it has been pointed out that the surface states are extremely

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sensitive to e.g., growth mode 31 , atomic structure 32 , surface morphology 33,34 , doping process 24 ..etc. Usually close relationship between the nature of the adsorption process and the observed change has been established. Recently, increasing attention is being paid to the large π-electron type organic molecules grown on metal surface. For example, over-layer of aromatic molecules like PTCDA (perylene tetracarboxylic dianhydride) or NTCDA (1,4,5,8naphthalene tetracarboxylic dianhydride) is strongly chemisorbed on the Ag(111) surface and induces a significant displacement of the Shockley state 28 and the appearance of the so-called interface molecular orbital. 35 The displacement of the surface state is rather weak when the molecules are physisorbed on Au(111), which induces also an enlargement of the spin-orbit splitting. 27 Such a situation is very similar to the case of rare gases on Au(111) 21 . This statement is consistent with recent work by Kilian et al. 36 who observed a rather big distortion of the PTCDA molecule on Ag(111) in such a way that its functional groups are located closer to the surface than the perylene core, while on Au(111) the molecule stays rather flat with larger distance to the substrate. Differently, pentacene on Cu(110) leads to a shift of the surface state to higher binding energy, suggesting a charge transfer from the substrate to the molecules 25 . Hence the strength of the interaction at the interface depends on both, the chemical composition of the used adsorbed molecule and chemical properties of the substrates. On the other hand, when the adsorbates form a well ordered monolayer and exhibit like a surface reconstruction generally a band back-folding of the surface and/or the bulk states are observed, as it has been seen in several systems 37–40 . An opening of a band gap accompanied by of saddle points in the density of states can be observed, particularly for the low-dimensional electronic states. In turn, it is well known that electronic states near the Fermi level are not only crucial for the transport and the thermodynamic properties but are also extremely important for the understanding of the bonding mechanism between adsorbate and surface. It was proven that the surface state, which is immediately involved in the adsorption and chemical processes on surfaces, can have a significant effect on the physical properties 41 . In this context, it has been shown that depopulation of surface states

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by dopants acting as surfactants promotes layer by layer growths 42 . Dissociation of small molecules like H2 is also believed to be influenced by populated surface state. 43 Furthermore, we have recently demonstrated the predominant role of the surface state in the adsorption mechanism of rare gases, e.g. Xe 44,45 . It is highly important to study the behavior of rare gas films on different surface terminations other than of the single metal surfaces in order to understand the effect of the surface geometry, corrugation, dipole moment induced on the physisorbed atom as well as the electronic states near the Fermi level and the surface reconstruction. In the present paper, I have studied how the Xe atoms arrange on sub-monolayer of Ag on Cu(111) by LEED (low energy electron diffraction) and ARPES (angle resolved photoelectron spectroscopy) using the synchrotron radiation. Before presenting the surface superstructures observed by LEED, we will discuss the adsorption process of Xe and how it can be used to determine la variation of the work function at the surface. We will then emphasize a direct connection with the results obtained by ARPES. Combining structural and spectroscopic data, relevant evidence in the adsorption properties of Xe on metal surface is given. The contribution of the surface reconstruction and the surface state on the adsorption properties of the Xe atoms are discussed as well.

Experimental Section The experiments have been carried out using synchrotron radiation at TEMPO beamline of Synchrotron SOLEIL in France and at the Surface/Interface Spectroscopy (SIS) beamline of Swiss Light Source (SLS) in Switzerland. The XPS measurements of the Xe core levels were performed at hν = 84 eV, taking advantage of the real time chemical reactivity set-up available on the experimental station of the TEMPO beamline. The ARPES data were measured at two different photon energies 40 eV and 65 eV corresponding to energies providing high spectral weight (cross section) of Ag(111) and Cu(111) surface states, respectively (see ref. 46 ). The data have been recorded with a Scienta SES 2002 electron energy analyzer char-

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acterized by high angle and energy resolutions (∆θ ∼ 0.3◦ and ∆E ∼ 10 meV at low photon energy). To prepare the substrate, standard Ar+ sputtering and annealing cycles have been made. The evaporation of Ag has been done using a Knudsen cell with a deposition rate of about 0.5 ML/min. The xenon coverage could be accurately controlled until reaching the second monolayer by adjusting the temperature of the sample to around 58 K and the pressure of the Xe in the gas phase to 7 × 10−9 mbar. These parameters were chosen in order to ensure sufficient mobility of the Xe atoms over the surface and prevent the formation of separated 3D islands, which can occur at lower substrate temperatures and/or higher gas pressures. Moreover, these conditions are comfortably within the low-coverage region of the phase diagram for Xe adsorption which was reported by Unguris and co-workers. 47 The adsorption of Xe atoms was realized directly in the ARPES chamber. Also, the LEED experiments have been made at the same conditions and as a function of time.

Results and Discussion Variation of the Work Function: It is highly important to measure exactly the value of the work function at the surface, which is among the fundamental parameters for designing the electronic structure of the materials. Many techniques give access to the work function, such as photoemission by measuring the cut-off in the valence band 48 , Kelvin probe using as principle the dipole interaction with the surface, 49 Scanning tunneling microscopy by using the image potential type state 50–52 . Here, we have performed time-dependent PAX experiment in order to follow the evolution of the work function magnitude in heterogeneous surface, namely Ag/Cu(111). We have simply monitored in real time the progress of the Xe adsorption on Cu(111) surface covered by Ag submonolayer by measuring the Xe-4d core level peaks in the photoelectron spectroscopy experiment. In Figure 1(a), we show the time dependent evolution of the 6


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25

Xe/Ag/Cu(111) Xe4d3/2

high

Xe4d5/2 Exposure time to Xe

20

Photoemission intensity (arb. units)

23 min

Adsorption kinetic (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

10

5

15 min

10 min

7 min

Xe4d3/2

0 65

64

Xe4d5/2 63

62

61

5 min

low

60

65

Binding energy (eV)

64

63

62

61

60

Binding energy (eV)

Figure 1: Real-time photoemission of adsorbed Xe (PAX) data measured on Cu(111) surface covered partially by 0.5 ML of Ag. It is shown the evolution of the Xe 4d core levels as a function of the exposure time to Xe pressure. One can see the Xe 4d3/2 and Xe 4d5/2 levels separated by the spin-orbit splitting energy. Both levels are sub-splitted in two contributions due to different values of work function on the Cu-terraces and on Ag-islands, respectively. In the right panel, spectra at different exposure times are presented with fits using lorenzian peaks associated with the first and second monolayer of Xe on Ag and Cu surface terminations, respectively. Xe-4d photoemission peaks as a function of the Xe dosing. The measurements have been performed at photon energy 84 eV with Xe partial pressure 7 × 10−9 mbar. The substrate temperature during adsorption is estimated about (58 ± 2) K. At the beginning, we observe that as soon as the Ag/Cu(111) surface is exposed to Xe, two spectroscopic features occur at binding energies of 63.37 eV and 61.39 eV, which corresponds to the 4d3/2 and 4d5/2 core level peaks of Xe, respectively. These peaks are associated with Xe atoms adsorbed on the Ag islands present on the Cu(111) surface. Only when these features are saturated, new pair

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Ag Cu Cu

EV

Ag

EV

Xe

Xe

Figure 2: Drawing describing the principle of using the PAX method as a sensitive probe to measure the variation of the local work-function on Ag submonolayer grown on Cu(111) surface. This schematic indicates that the energy separation between the photoemission peaks on Ag and Cu surface terminations corresponds to the work function difference between these two locations. For this model, the 4d3/2 level was taken as an example to illustrate the observed effect. In principle, the same behavior should be also obtained in the other Xe core levels. of structures appears at low binding energies 63.05 eV and 61.07 eV, which are respectively related to the 4d3/2 and 4d5/2 core levels of Xe absorbed on the free Cu terraces. Note that the binding energies of the different features are obtained from a fit using Lorentzian lines, as it is shown in the right hand panel of Figure 1. At this stage, the surface of the sample is entirely covered by one Xe monolayer and the photoemission spectrum displays that both 4d3/2 and 4d5/2 core levels are split each into two sub-levels originating from the presence of different chemical terminations on the surface; one sub-level is associated with the Xe on the Ag islands and the second with Xe on the Cu terraces (spectrum at 10 min exposure in Figure 1). As illustrated in the schematic of Figure 2, the energy separation between the two sublevels can be approximated by the work function difference between the Ag and Cu surface terminations, as follows: ∆EB (4d3/2 ) = ∆EB (4d5/2 ) = ∆φAg

islands−Cu terraces

= 0.32 ± 0.01

eV. Indeed, an electron screening develops due to the presence of Xe on the surface and 8


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Table 1: Binding energy values of the 4d core levels inferred from fits of the photoemission spectra measured upon adsorption of Xe on Ag/Cu(111), on Cu(111) and on Ag(111) surfaces, respectively. Material

ML number

E4d3/2 ±0.01(eV)

E4d5/2 ±0.01(eV)

1ML Xe on Ag/Cu(111) 1st ML Xe/Ag islands 1st ML Xe/Cu terraces

63.37 63.05

61.39 61.07

2ML Xe on Ag/Cu(111) 1st ML Xe/Ag islands 1st ML Xe/Cu terraces 2nd ML Xe/Ag islands 2nd ML Xe/Cu terraces

63.35 63.01 63.91 63.56

61.38 61.04 61.94 61.59

1ML Xe on Cu(111)

1st ML Xe/Cu terraces

63.04

61.07

2ML Xe on Cu(111)

1st ML Xe/Cu terraces 2nd ML Xe/Cu terraces

63.02 63.61

61.05 61.63

1ML Xe on Ag(111)

1st ML Xe/Ag terraces

63.3

61.33

is directly proportional to the local work function. Therefore, since the Ag and Cu sites have different work function values, the energy position of the core level peak in the free Cu terraces is different from its position in the Ag islands. After the first monolayer is saturated, one observes in the photoemission spectrum the appearing of new couple of features at 63.91 eV (4d3/2 ) and 61.94 eV (4d5/2 ), which are related to the built-up of the second Xe monolayer on the Ag islands. The binding energy of such feature is rather different, due to the fact that the Xe atoms in the second monolayer are surrounded only by others Xe atoms whereas those of the first ML are in direct contact with the Ag atoms. In this case the adsorption of the second Xe monolayer provides information of how much the work function is changed upon the adsorption of the first Xe monolayer 9



Xe4d3/2

high

Xe4d5/2 Exposure time to Xe


40

20

Xe4d3/2

0 65

Xe4d5/2

64 63 62 61 Binding energy (eV)

Xe4d3/2

40 min

30 min

Xe4d5/2


Xe/Cu(111) 60


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Xe/Cu(111)

Xe/Ag/Cu(111)

Xe/Ag(111)

7 min low

60

65


60

65


60

Figure 3: Real-time PAX plot obtained on Cu(111) surface (left panel) and the corresponding spectra at different exposure times to Xe (middle panel). For comparison, it is shown in the right panel PAX spectra measured on bare Cu(111) and Ag(111) surfaces, together with that from Cu(111) surface covered by 0.5 ML of Ag. compared to its value on clean Ag islands. Similarly, only when the second monolayer on the Ag islands is completed, spectroscopic structures from the second Xe monolayer on Cu terraces start occurring. This effect can also be explained by the influence of the density of states in the adsorption properties of Xe. In Table 1, we summarize the binding energy values derived from the best fit of the PAX spectra for Xe on Ag/Cu(111) and Xe adsorbed separately on Ag(111) and Cu(111) surfaces. We present in Figure 3 photoemission data taken in real-time upon adsorption of Xe atoms on pristine Cu(111). In this case, the surface has only Cu termination type, and consequently there is the appearance solely of Xe core levels associated with Xe atoms on Cu. Depending on the exposure time, it occurs progressive growth of the first monolayer and then the second monolayer giving rise to peaks at different binding energies. Similarly, the energy difference between the 4d3/2 peaks (or 4d5/2 peaks) is related to the variation in the Cu(111) work function after adsorption of the first Xe monolayer. The right panel of Figure 3 displays photoemission spectra that illustrate a comparison between the Xe 4d core level peaks measured on Xe monolayer formed 10


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respectively on bare Cu(111), on pristine Ag(111) and on Ag submonolayer deposited on Cu(111) surface. These data indicate that the work function decreases upon deposition of Ag monolayer on the surface, nonetheless its value does not reach the work function of clean Ag(111) surface. On the other hand, the PAX approach permits to determine accurately the Ag coverage, since the peak areas of the double Xe 4d3/2 features (or 4d5/2 features) depend directly on the size of the Ag islands and on that of the free Cu regions, respectively. In other respects, it has been shown that adsorption of Xe on 2 ML Ag/Pt(111) induces also peak splitting of the Xe core levels, which was attributed to the presence of surface reconstruction in which a significant variation in the local work function (amounts to 0.3 eV) was obtained. 50 This effect has been corroborated by STS measurements, which enabled determination of the surface potential landscape 50 . In contrast, for the triangular reconstruction of 1ML Ag/Cu(111) the variation of the local work function seems to be rather weak since it is not detected by PAX method.

Modification in the LEED Patterns: In Figure 4a, an STM image taken from earlier work 53 and obtained on 0.6 ML of Ag on Cu(111) is shown. Due to large lattice mismatch between Cu and Ag (13%), quasicommensurate (9×9) reconstruction forms on the surface and uniform distribution of triangular patterns develops upon the relaxation of the surface strain and the formation of dislocation loops in the Cu topmost layer beneath the Ag monolayer 32,54 . In Figure 4, we present also the evolution of the corresponding LEED images as a function of the exposition time to the Xe. Before introducing Xe, the LEED pattern shows multiple diffraction spots close together, which are the signature of the (9×9) superstructure. The latter remains almost unchanged after the adsorption Xe, which prefers to be adsorbed on the Ag monolayer regions rather than on the free Cu terraces. As demonstrated in earlier study, the 11



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0.6 ML Xe

0 ML Xe

a

b

c

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1 ML Xe

d

Figure 4: (a) Atomic resolved STM image of Cu(111) surface covered by Ag-submonolayer. One observed Ag islands displaying the characteristic 9×9 reconstruction with triangularlike patterns on the surface. Inset displays the atomic detail of one triangle. (b-d) LEED images obtained on 0.6 ML of Ag on Cu(111) before and after adsorption of Xe. condensation temperature of Xe on different bare surfaces appears very similar and cannot be the origin of such an effect 41 . We have addressed this behavior in previous work 45 and showed that reduction in the density of the Shockley states favors the adsorption of the Xe atoms on the Ag islands rather than on the Cu terraces. From our LEED investigations, we realize that the (9×9) reconstruction persists even after being covered by Xe monolayer and no other structure occurs. Hence, one can think that the Xe layer is uni-axially compressed along the short distance, i.e. the ΓM-direction. However, the in-plane Ag-Ag distance (d ∼ 2.88 ˚ A) appears much smaller than the Xe-Xe distance in over-layer adsorbed on different metallic surfaces. This latter is ranged from 4.2 ˚ A to 5.0 ˚ A (all the values are summarized in ref. 55 ). Note that, there exists a long debate about the preferred adsorption site of Xe on different metal surfaces. 56–60 The most of the given works arrived at the conclusion that low coordination site, e.g. top position is the favor one, 60 except the example of graphite surface where the gas atoms occupy the high-coordination sites and form a compressed layer √ √ with a commensurate ( 3 × 3)R30◦ reconstruction 61 . The effect of this reconstruction has as well been observed as a folding of electronic states in the valence band 61 . In fact, the Xe monolayer is always forming a hexagonally close-packed over-layer, which is often incommensurate with the substrate lattice, except for the cases of Cu(111) and graphite, where √ √ well-ordered and commensurate ( 3 × 3)R30◦ reconstruction is observed. Also, conden12


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sation of Xe atoms on Bi/Ag(111) surface alloy forms a well ordered structure exhibiting commensurate reconstruction. 39 For Ag/Cu(111), the situation is quite different and the fact that the reduction in the density of states near the Fermi level promotes the adsorption of Xe may influence the arrangement of the adsorbed Xe atoms and induces a disorder on the surface. Besides, it is possible that the triangular corrugation characterizing the (9×9) reconstruction affects significantly the arrangement of the Xe atoms on the surface, thus the distance between atoms should be different from the one of Xe bulk. Therefore, we can suggest the formation of amorphous Xe layer on the surface and the reconstruction seen in the LEED patterns is associated with the superstructure of the Ag overlayer beneath. This finding is consistent with the ARPES data (discussed later). On the other hand, our LEED investigations confirm that the Xe atoms favor to be adsorbed on the Ag islands, because the reconstruction of Xe on Cu(111) appears only after a long time of exposition (see LEED pattern in Figure 4d). To summarize, the Xe monolayer film on Ag(111) single crystal forms incommensurate structure with unit vectors of the over-layer aligned with those of the √ √ substrate 41,58 . On Cu(111), the Xe monolayer presents a ( 3 × 3)R30◦ commensurate superstructure 59 . However, on 1 ML Ag/Cu(111), our present investigations suggest that the Xe overlayer is strongly incommensurate, or even disordered, and for that reason no new structure can occur in the LEED pattern. This behavior can be attributed to the change in the density of states near the Fermi level and/or to the presence of a surface reconstruction.

Modification of the Shockley State Band: In principle, the information obtained from LEED can be directly correlated to the results obtained by photoemission. In Figure 5, we show low temperature ARPES data obtained on 0.7 ML of Ag prepared at room temperature on Cu(111). In agreement with previous publications, two surface states are observed; one associated with non-covered Cu terraces at binding energy EB (SCu ) ∼ 440 meV and a second at EB (SAg/Cu ) ∼ 220 meV localized at the Ag islands. In addition, due to the surface reconstruction an opening of a band gap occurs in 13



G

Binding energy (eV)

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0.0

0.0

0.2

0.2

SAg/Cu

0.4

A

SCu B

-0.2

0.0

0.2 -1

Parallel momentum (Å )

-0.4

-0.2

0.0

0.2

0.2

0.4

0.4

0.6

0.6

C

0.8

0.8 -0.4

SR-SBZ

SR-SBZ

SXe/Cu

0.6

0.8

0.0

0.2

SXe/Ag/Cu

0.4

SCu

0.6

0.0

ΓR-SBZ

Γ

D 0.8

-0.4

-1

-0.2

0.0

0.2

0.4 -1



-0.4

-0.2

0.0

0.2

-1

0.4


Figure 5: ARPES data obtained on 0.7 ML of Ag on Cu(111) before (panel A) and after adsorption of Xe monolayer (panel B) with a photon energy hν = 40 eV. In panel (C) the measurements have been done with hν = 65 eV and panel (D) represents the corresponding second derivative plot. The measurements have been done at T = 60 K. the Ag-island Shockley state band at the boundaries of the reduced surface Brillouin zone (RSBZ) and a back-folding of the this band can be clearly seen in the second R-SBZ, as shown in Figure 5A. This effect is not observed on non-reconstructed surface where usually isotopic band dispersion is obtained. It represents a direct experimental evidence of fundamental phenomenon concerning the behavior of electronic state inside the solid. It confirms the 2D character of Shockley states behaving according to nearly free electron model. Now, as soon as the Ag/Cu(111) surface is exposed to Xe pressure, the state linked to Ag islands starts disappearing and at the same time new state at low binding energy develops EB (SXe/Ag/Cu ) ∼ 90 meV. The surface state associated with non-covered Cu(111) regions (indicated by SCu ) remains at this moment unchanged. It begins vanishing only after further Xe exposition leading to new state SXe/Cu at EB ∼ 335 meV. This indicates immediately that the Xe atoms favor the adsorption on the 1 ML Ag film, where they form closed islands themselves, while the clean Cu(111) surface regions primarily remain uncovered. As displayed in Figure 5, by using photon energy hν = 40 eV, one observes that the photoelectron intensities of SCu and SXe/Cu bands are significantly reduced (panel B), while at hν = 65 eV the same bands occur with greatly enhanced signal (panel C). This behavior is attributed to the cross section effect

14


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that has been discussed in clean metal surfaces by Lobo et al. 46,62 The latter have studied the variation of the Shockley state intensity as a function of the excitation energy and this on bare (111) surfaces of Cu and of Ag. On the other hand, a close look at the ARPES plot in panel (C) of Figure 5 allows to discern the presence of additional dispersive features located in the left and in the right sides from the SXe/Ag/Cu band. These two outstanding features (labelled SR−SBZ in Figure 5) are well seen in the second derivative plot shown in Panel (D) of Figure 5. SR−SBZ band represents a replica resulting from the backfolding of the SXe/Ag/Cu Shockley state at the Bragg diffraction plans of a reduced surface Brillouin zone. The distance in the k-direction separating the main and replica bands is defined by G = 0.31 ± 0.01 ˚ A−1 . From this value, we were able to deduce the super-periodicity responsible for the backfolding effect to be also (9×9) type. The size of the superlattice is p calculated as the following: x = (4 2/3 × π/aCu )/|G| ≃ 9, where aCu is the lattice constant of copper and x is the ratio between the period length of the superstructure and the atomic distance at the Cu(111) surface. In fact, when the Ag islands surface is covered by Xe, the SAg/Cu Shockley state undergoes an energy shift and becomes localized at the level of the Xe/metal interface, where it is transformed into an interface-like state. Since, according to the LEED analysis, the (9×9) superstructure of the Ag monolayer remains preserved after Xe adsorption, this superstructure affects the dispersion of the raising interface state, by inducing a backfolding behavior accompanied certainly by an opening of a band gap at the reduced-SBZ boundaries. The gap in this case should be located in the empty states and for that reason it cannot be accessible by ARPES.

Conclusions We have investigated the adsorption properties of Xe on Cu(111) surface covered by Ag submonolayer. By monitoring in real-time the evolution in the photoemission core level peaks of adsorbed Xe atoms on two unlike chemical sites (Ag and Cu sites) on the same surface,

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we were able to demonstrate that the Xe atoms are preferentially adsorbed on the 1ML Ag islands rather than on the free Cu terraces. This observation confirms the hypothesis that the reduction in the density of states near the Fermi level strongly affects the adsorption of Xe atoms. Besides, after the completion of the first Xe monolayer, further exposition to Xe leads to the appearance of core level peaks associated with the formation of the second monolayer, firstly on the Ag islands and then on the regions of Cu terraces. On the other hand, LEED investigations reveal that the (9×9) on surface of the Ag islands is preserved upon adsorption of Xe. More interesting, close inspection of the diffraction patterns suggests the absence of any ordering in the Xe overlayer, which appears different from the case of Xe on most of the low index transition metal surfaces. However, even disordered Xe overlayer induces a shift of the Shockley state which occurs back-folded due the 9×9 Ag reconstruction that remains preserved after the Xe adsorption. A significant effect of matrix element is observed on the balance of the Shockley state intensity.

Acknowledgments I gratefully thank the beamline staff of W09LA at Swiss Light Source for their support and acknowledge also the colleagues at TEMPO beamline of Synchrotron SOLEIL, in particular Ch. Chauvet, for his assistance. Many thanks to SOLEIL direction for the financial support of the present project.

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Figure 6: TOC graphic 25

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62

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Cu(111) System: Real-Time

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