Adsorption of benzene on Mo(100) and MgO(100)/Mo(100) studied by ultraviolet photoelectron and metastable impact electron spectroscopies

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1 Surface Science 415 (1998) Adsorption of benzene on Mo(100) and MgO(100)/Mo(100) studied by ultraviolet photoelectron and metastable impact electron spectroscopies J. Günster a, G. Liu a, V. Kempter b, D.W. Goodman a,* a Department of Chemistry, Texas A&M University, PO Box 3001, College Station, TX , USA b Physikalisches Institut, Technischen Universität Clausthal, Leibnizstrasse 4, D Clausthal-Zellerfeld, Germany Received 5 February 1998; accepted for publication 3 June 1998 Abstract By employing metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy ( UPS) together with work-function measurements, three adsorption states of benzene have been identified on the clean Mo(100) surface at 100 K: a chemisorbed layer, a second physisorbed layer with the aromatic rings plane parallel to the surface, and at higher coverages, an adsorbed layer with the molecular planes essentially perpendicular to the surface. The rapid appearance of the upright (edge-on) phase for benzene adsorbed on the MgO-covered Mo(100) surface suggests that the adsorption dynamics on MgO are similar to those for the Mo(100) surface, but no chemisorbed layer forms for the MgO surface. In addition, it was found that benzene adsorption on the MgO-covered Mo(100) surface does not lead to perfectly formed, closely packed layers Elsevier Science B.V. All rights reserved. Keywords: Adsorption kinetics; Aromatics; Electron emission experiments; Magnesium oxides; Molybdenum; Photoemission (total yield) 1. Introduction cate that this low-coverage adsorption geometry is always found on transition metal surfaces (see The adsorption geometry of benzene (C H )on Ref. [7] for a review). A physisorbed layer, with 6 6 metal [1 7] and oxide surfaces [8,9] has received the same orientation and approximately the same considerable attention. From investigations of benpacked molecular density, condenses on top of the close- zene adsorbed on transition metal surfaces, the chemisorbed layer. Further adsorption following picture emerges [3, 6 ]: at coverages up leads to a layer with a more dense saturation to ~1.0 monolayer (ML), the molecules adsorb coverage, with the benzene molecules lying edge-on with their ring planes essentially parallel to the (ring planes approximately perpendicular to the substrate surface. This first monolayer is assumed surface). Finally, additional adsorption leads to to be a chemisorption layer. Previous studies indibenzene. However, presently, it is unclear whether the formation of bulk-like, randomly orientated the growth of the bulk-like phase on top of the * Corresponding author. Fax: ; intact layer of tilted benzene molecules or an goodman@chemvx.tamu.edu island-type growth leads to the coexistence of /98/$ see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S ( 98 )

2 304 J. Günster et al. / Surface Science 415 (1998) islands of a tilted-molecular phase and a bulk-like adsorption on Mo(100) and MgO-covered phase in a single layer. For the adsorption of benzene on MgO and Al O thin films in the lowcoverage regime up to 1 ML [9], it was found that 3 Mo(100). adsorption leads to a significantly weaker interaction. Experimental between the molecular p-system and the oxide substrate compared to typical transitionhigh-vacuum The experiments were carried out in an ultra- metal surfaces. However, those results are mainly ( UHV ) system (base pressure below derived from TPD experiments, which provide Torr) consisting of two interconnected information about the desorption kinetics at eleature-programmed chambers, one for sample treatment and temper- vated substrate temperatures only. Thus, only with desorption ( TPD) ( UTI mass the condition that the adsorbed benzene layers spectrometer), and the other for electron spectro- undergo no phase transition during heating, can scopy. In the latter are capabilities for X-ray the behavior during adsorption be derived from photoelectron spectroscopy ( XPS) (PHI Model those data. For that reason, Jacob and Menzel [6] ), Auger electron spectroscopy (AES) (PHI recently utilized in-situ infra-red reflection adsorp- Model 15-55G), UPS and MIES (home made). tion spectroscopy ( IRAS) during the adsorption MIES and UPS spectra were measured simulta- of benzene on Ru(001) and were able to confirm neously using a cold-cathode discharge source, their earlier [3] adsorption model. which has been described previously [14, 15]. In the present paper, metastable impact electron Briefly, a helium cold-cathode gas discharge prospectroscopy (MIES) and ultraviolet photoble He* 3S/1S (E*=19.8/0.6 ev ) atoms with vides both ultraviolet photons ( HeI) and metasta- electron spectroscopy [ UPS ( HeI )] have been used thermal kinetic energy. The triplet-to-singlet ratio to characterize the adsorption of benzene on the has been measured by He* Ar impact as 7:1, but Mo(100) and MgO-covered Mo(100) surfaces at a very efficient conversion into He* 3S has been 100 K. Since MIES selectively probes the electronic observed on metallic and semiconducting surfaces structure of the outermost surface and UPS, the [16 18]. Thus only de-excitation from He* 3S average character of the several top layers, the has been observed. Metastable and photon contricombination of these techniques is very useful in butions within the beam were separated by means order to gain information about the geometrical of a time-of-flight method using a mechanical orientation of molecules on surfaces [10 13]. chopper. The collection of a MIES/UPS spectrum However, for a clear identification of molecular requires approximately 180 s. This methodology orbitals on the surface and in the bulk phase, it is yields MIES spectra essentially free from UPS crucial that the character of the observed molecular contributions, however, due to the broad energy orbitals remains unchanged during the mole- distribution of the metastable helium atoms, cule molecule interaction. For benzene molecules, approximately 1.6% of the MIES intensity does which interact via van der Waals forces, the appear in the UPS spectra. In order to obtain a electronic structure of a single molecule remains MIES-free UPS measurement, optional quenching essentially unchanged during condensation. Thus, of the metastable helium via helium atom atom the analysis of the relative peak intensities in MIES collision in a buffer chamber were carried out. can provide information about the geometrical The AES, MIES and UPS spectra were acquired orientation of molecules in the outermost surface with the incident electron/metastable/photon layer in situ during adsorption. An approach sim- beams 45 with respect to the surface normal in a ilar to our experiment was adopted by Kubota constant pass energy mode using a double-pass et al. [1], who investigated benzene adsorption cylindrical mirror analyzer (CMA, PHI Model on graphite by means of MIES and UPS. However, 15-55G). It is noteworthy that the MIES spectra the superior quality of our data allows a more acquired in the present study with an CMA, which detailed discussion and comparison of the benzene integrates over a relatively wide angular accep-

3 J. Günster et al. / Surface Science 415 (1998) tance, are comparable to those measured previously on MgO surfaces using a hemispherical analyzer, which has a relatively small acceptance angle. The energies, denoted by E, in the spectra f correspond to electrons emitted from the Fermi level of the Mo(100) substrate. In the following spectra, all binding energies are referenced to E. f Since the metallic Mo substrate and the analyzer are in electrical contact, the Fermi energy appears at a constant position and permits the workfunction change of the surface to be measured directly from the high energy cut-off of the spectra. MgO films were grown on the Mo(100) surface by depositing Mg in Torr O ambient at 550 K substrate temperature. The Mg source was made from a high-purity Mg ribbon wrapped around a tantalum filament. In order to insure a stoichiometric MgO layer, the as-prepared surface was further annealed to 700 K in a Torr O background for 0 min. As shown in previous investigations, MgO films prepared under these conditions grow epitaxially on the Mo(100) substrate [19, 0]. After further purification in the vacuum manifold via several freeze pump thaw cycles (vacuum Fig. 1. Comparison of a MIES and UPS spectrum (raw data) measured on multilayer benzene condensed on Mo(100) at 100 K. were assigned as 1a /3e and 1b /3e, respectively. The ionization potentials [ IP=(excitation u g u 1u distillation), benzene (C D ) (Spectro Grade, energy) (electron kinetic energy)] and a comparison to the respective gas-phase data are presented 6 6 Caledon Laboratories) was dosed by backfilling the vacuum chamber. in Table 1. The measured ionization potentials are approximately 0.5 ev higher compared to the corresponding data from Kubota et al. [], but the relative peak positions and intensities are in excel- 3. Results lent agreement. Compared to the gas-phase data, Fig. 1 compares a UPS and MIES spectrum of the ionization potentials of the condensed phase multilayer (approximately 4 ML) benzene con- are between 0.4 and 0.8 ev lower, which is consistent densed on Mo(100) at 100 K. Since benzene has with an estimated energy shift in the condensed no unoccupied states in resonance with the imping- densed phase of 0.9 ev [5] due to polarization of ing He s electron, the MIES spectrum is dominated the neighboring molecules. However, the ioniza- by an Auger de-excitation process. Thus, a tion potentials determined by MIES are closer to direct comparison between MIES and UPS data the gas-phase data compared to the IP determined is possible [1]. by UPS, which is probably due to a lower polarization The spectra shown in Fig. 1 agree well with of molecules in the outermost surface layer. previously reported MIES and UPS results for The low intensity of the 3a band in UPS, compared to MIES, can be explained by the relative 1g benzene condensed on copper at 150 K []. Peak assignments, based on gas-phase UPS measure- low sensitivity of UPS for s-derived levels: the ments (see Refs. [3,4] for a review), are noted 3a structure belongs to molecular orbitals that 1g in the figure. Since the features corresponding to are composed of C(p) and H( 1s) orbitals [3]. the ionization of the 1a, 3e and 1b, 3e However, UPS provides a superior sensitivity for u g u 1u molecular orbitals were not resolvable, the peaks the detection of p-derived levels, which results in

4 306 J. Günster et al. / Surface Science 415 (1998) Table 1 Comparison of our MIES and UPS ionization potentials with previous data from Kubota et al., measured on benzene condensed on Cu, and gas phase data from Turner et al. MO MIESa UPS (HeI)a MIESb UPS (HeI)b Gas UPS (HeI)c 1e 1g e g /1a u /1. 3e 1u /1b u /14.7 b 1u a 1g (15.9) a Data from the present study. b Kubota et al. [19]. c Turner et al. [31]. a high intensity of 1e and 1a. Typically, MIES benzene-induced features up to the 1th spectrum 1g u does not discriminate among the various molecular suggests a chemisorbed interaction between the levels, but is strongly influenced by the various benzene molecules and the substrate at initial contributions of molecular orbitals to the local exposures. Thus, the appearance of the benzene- electron density on the outermost surface. Since induced features in the 1th MIES and UPS the metastable de-excitation process is governed spectrum (dotted spectra in Fig. ) together with by an exchange type interaction, the determining the saturation of the work function at about the factor for the de-excitation process of the metastable same exposure time (see Fig. 3) are attributed to helium is the overlap between the projectile the completion of the first chemisorbed benzene and the target wave functions. layer, i.e. the onset of second layer physisorption The MIES and UPS spectra of a Mo( 100) [3, 6, 7]. For higher coverages, the benzeneinduced surface acquired during a benzene exposure are features become more apparent. In the presented in Fig.. For convenience, in the rest sequence of MIES spectra of Fig., the intensity of the text, the benzene bands are simply denoted of the benzene bands B and B increases monoton- 3 4 by B, B, B and B, respectively (see Fig. 1). ically versus exposure time, whereas for the features assigned as B and B, a sudden decrease in Benzene was dosed at a background pressure of Torr (measured with a nitrogen calibrated intensity is clearly visible (marked in bold) between vacuum gauge) and a substrate temperature of the 8th and 33rd spectrum. This modulation of 100 K. MIES and UPS spectra were recorded the spectral intensity with benzene coverage in continuously during exposure. Since the metallic MIES is not apparent in the sequence of UPS Mo(100) surface (bottom spectrum) has unoccu- spectra. For this modulation of the MIES spectral pied states in resonance with the impinging He s intensity to be meaningful, it is critical that the electron, the MIES spectra of Fig. are dominated observed change in peak intensities is not simply by an Auger capture (AN ) mechanism in the lowcoverage caused by changes in the background. Fig. 3 com- regime. At increasing benzene coverages, pares the absolute MIES peak intensities for the four distinct features ( B1 B4), originating from five highest occupied molecular orbitals and the the ionization of the six highest occupied molecular intensities in the minima between these three orbitals of benzene, are apparent in MIES and spectral features as a function of exposure time. UPS. The presence of these features demonstrates The intensities of these minima are a measure of that the metastable He atoms are quenched predominantly the overall background. The smooth intensity via Auger de-excitation involving the increase in the minima as a function of exposure molecular orbitals of benzene. The intense feature time demonstrates that the intensity decrease of at c. 15 ev (high energy cut-off of the spectra) is B and B in MIES is not induced by a rapid 1 assigned to scattered electrons. The absence of change in the spectral background. As indicated

5 J. Günster et al. / Surface Science 415 (1998) (a) Fig. 3. Comparison of the absolute MIES peak intensities of the 1e 1g (B 1 ), 1a u /3e g (B ) and 1b u /3e 1u (B 3 ) benzene bands and, as a measure of the overall background, the intensities of the minima in between these features in Fig.. (b) Fig.. MIES and UPS spectra from a 100 K Mo(100) substrate as a function of benzene exposure. The bottom spectrum shows the clean Mo(100), and the uppermost spectrum shows the benzene-covered surface. sure are shown in Fig. 4. The thickness of the MgO layer was determined via AES to be approximately 7 ML. Benzene was dosed at a background pressure of Torr with the substrate temperature at 100 K. The MIES and UPS spectra of the clean MgO(100) surface agree well with the corresponding data reported previously [8,9]. Due to the insulating character of the clean MgO(100) surface, no intensity between E and f 3.8 ev binding energy is apparent in the lower spectra of Fig. 4. That is, the MIES spectra are dominated by an Auger de-excitation process. The structure in the lower spectra of Fig. 4, denoted as O(p), corresponds to emission from the O p valence band of the MgO(100) substrate [8,30]. During benzene dosage, a continuous change from a spectrum typical for a MgO surface ( lower spectrum) to a spectrum typical for a condensed benzene layer (uppermost spectrum) is observed in the sequence of MIES and UPS spectra. It should be pointed out that the energy scale in Fig. 4 is the reverse of Fig.. Due to the superposition of the prominent O(p) feature with the peak derived from the ionization of the highest occupied mole- cule orbital (HOMO) of benzene, B, it is not 1 possible to clearly resolve the intensity increase in the HOMO with benzene exposure. However, by the dashed line in Fig. 3, the change in spectral intensity plateaus at approximately the 33rd spectrum. The MIES and UPS spectra of a MgO-covered Mo(100) surface acquired during benzene expo-

6 308 J. Günster et al. / Surface Science 415 (1998) (a) Fig. 5. Comparison of the absolute MIES peak intensities of the O(p)+1e 1g (O(p)+B 1 ) structure and the 1a u /3e g (B ) and 1b u /3e 1u (B 3 ) benzene bands and, as a measure of the overall background, the intensities of the minima in between these features in Fig. 4. (b) Fig. 4. MIES and UPS spectra from a 100 K MgO-covered Mo(100) substrate as a function of benzene exposure. The bottom spectrum shows the clean MgO(100), and the upper- most spectrum shows the benzene-covered surface. The energy scale in this figure is inverse as compared to Fig.. corresponding spectra of benzene on the Mo( 100) surface, i.e. band B and B monotonically increase 3 4 in intensity versus exposure time, whereas the B feature shows a sudden decrease in intensity (indicated by the bold lines in Fig. 4) between the 18th and 4th spectra (see also Fig. 5). Unfortunately, in the corresponding sequence of UPS spectra, the B band is not well-developed and thus does not allow a comparison of the relative B peak intensities in MIES and UPS. However, the continuous decrease in background during the entire benzene exposure of Fig. 5 indicates that even for the relatively high benzene coverages, there is some contribution to the MIES spectra from the MgO substrate. This suggests that benzene does not form continuous layers when adsorbed on MgO(100). 4. Discussion shortly after the onset of the exposure to benzene, the benzene bands B,B and B appear in the 3 4 The adsorption of benzene on the clean MIES and are well-separated from the MgO sub- Mo(100) surface ( Fig. ), in the low coverage strate intensity (dotted spectrum in the MIES and regime, yields only weak MIES and UPS features. UPS of Fig. 4). The increase in intensity of these This is consistent with the formation of a strong three features as a function of benzene exposure interaction (chemisorption) between the adsorbed shows the identical modulation compared to the molecules and the highly reactive Mo surface

7 J. Günster et al. / Surface Science 415 (1998) [3,7,6]. In this context, dissociative adsorption in the growth of the intensity of B and B, 1 of benzene is not expected since TPD and followed by a decrease in the intensity of these HREELS data have shown that benzene adsorbs features, whereas B and B grow steadily in 3 4 non-dissociatively on Mo( 110) [ 7]. However, intensity. The latter two orbital features relate from the present data, we cannot exclude the primarily to the C H bonds in benzene. possibility of dissociative adsorption on the mor- The adsorption of benzene on a MgO-covered phological different Mo(100) surface. At the 1th Mo(100) substrate leads to well-developed ben- scan in MIES and UPS (dotted spectra in Fig. ), zene features in the fifth MIES scan. That the the appearance of well-developed benzene features decrease in the intensity of the B band occurs at indicates the onset of second layer physisorption. the 18th scan is consistent with a benzene phase From the 8th to the 33rd spectrum in the in which the molecules are standing on edge. This sequence of MIES spectra (indicated in bold), the phase then is formed significantly earlier on MgO spectral features, corresponding to the ionization compared to metal surfaces. In a previous study of the two highest occupied p-levels of benzene, of benzene adsorption on MgO-covered Mo(100) exhibit a sharp decrease in intensity. It is noteinitially in a geometry similar to metal surfaces [9], it was found that benzene molecules adsorb worthy that the B band (1a /3e ) is not only u g derived from the emission of a benzene p-level, with their ring planes parallel to the surface. The but MIES gas-phase data [31] suggest that this interaction, however, of benzene with the substrate peak is primarily related to the ionization of was found to be relatively weak. In contrast to p-type benzene orbitals. Given that the onset of most metal surfaces, the constant work function second-layer adsorption occurs at the 1th scan, during benzene adsorption on MgO (see Fig. 5) suggests, in this respect, that there is little to no saturation of the second layer is anticipated at charge transfer between benzene and the substrate. the 3rd spectrum. This behavior implies, in With these considerations, an adsorption mechaaccordance with Ref. [3], a nearly constant sticknism of benzene on MgO is suggested that is ing coefficient for the first and second adsorption similar to benzene adsorption on transition-metal layers. Thus, the decrease in intensity of B and 1 surfaces except for the absence of the first chemi- B occurs in a coverage regime in which the sorbed layer. In addition, the relatively high MgO physisorbed layer (benzene adsorbed with planes intensity over the entire benzene coverage regime parallel to the surface) saturates. The peak growth suggests that the adsorption of benzene on our behavior in this coverage regime indicates, as in thin MgO films does not result in the formation previous studies [3, 6 ], that following saturation of completely closed-packed layers. The finding of the second layer, benzene adsorption is characthat the edge-on phase is formed earlier on the terized by a phase transition from a phase where MgO surface is consistent with a recent study of the benzene molecules are adsorbed with their benzene adsorbed on Cu(111) [5], which shows a ring planes parallel to the surface to one where significantly weaker molecule surface interaction the molecules are standing on edge with their ring as well. For this surface, the formation of a stable planes approximately perpendicular to the sur- benzene bilayer was observed during first- and face. Peaks B and B correspond to de-excitation 1 second-layer adsorption, i.e. benzene molecules in of the impinging He* involving essentially p-type direct contact with the surface adsorb with their molecular orbitals [3]. For molecules adsorbed ring planes parallel to the surface, while secondwith their ring planes perpendicular to the surface, layer molecules adsorb with their ring planes an interaction of the metastable helium with the approximately perpendicular to the surface. These p-electron system is hindered, whereas molecular observations together suggest, once again [5], that orbitals related to C H bonds in this configura- the formation of the edge-on phase, which appears tion are exposed to a greater degree to the to be metastable on most transition-metal surfaces impinging He* [ 3]. [6 ], is governed by an intermolecular interaction Together, these effects lead to an initial reduction between molecules in the first and second phy-

8 310 J. Günster et al. / Surface Science 415 (1998) sisorbed layer. In the case of the more reactive Acknowledgements transition-metal surfaces, the character of the first chemisorbed benzene layer in direct contact with We acknowledge with pleasure the support of the surface is significantly changed by the surface this work by the National Science Foundation molecule interaction and thus a second, physisorbed molecular layer with the ring planes paral- (Contract No. DMR ). lel to the surface is formed before the edge-on phase appears. References One important question, thus unanswered, is the growth mechanism of benzene on Mo(100), [1] B.M. Blass, S. Akhter, J.M. White, Surf. Sci. 191 (1987) i.e. layer-by-layer versus island formation. We 406. believe that the physisorbed benzene layers are [] A.C. Liu, C.M. Friend, J. Chem. Phys. 89 (1988) either patchy or otherwise not closely packed. This [3] P. Jakob, D. Menzel, Surf. Sci. 0 (1989) 70. [4] D.R. Huntley, S.L. Jordan, F.A. Grimm, J. Phys. Chem. is based on the fact that the benzene intensities of 96 (199) all MIES features reach saturation well after the [5] M. Xi, M.X. Yang, S.K. Jo, B.E. Bent, P. Stevens, J. Chem. formation of the upright (edge-on) phase. In Fig. Phys. 101 (1994) 91. (see also Fig. 3), saturation is not reached. A [6] P. Jakob, D. Menzel, J. Chem. Phys. 105 (1996) [7] J. Eng, Jr., B.E. Bent, B. Frühberger, J.G. Chen, J. Phys. detailed discussion of the relative MIES intensities Chem. B 101 (1997) for higher benzene coverages as well as benzene [8] G.W. Rubloff, H. Luth, W.D. Grobman, Chem. Phys. TPD will be published elsewhere. Lett. 39 (1976) 493. [9] S.C. Street, Q. Guo, C. Xu, D.W. Goodman, J. Phys. Chem. 100 (1996) [10] Y. Harada, H. Hayashi, S. Masuda, T. Fukuda, N. Sato, S. Kato, K. Kobayashi, H. Kuroda, H. Ozaki, Surf. Sci. 4 (1991) Conclusion [11] T. Pasinszki, M. Aoki, S. Masuda, Y. Harada, N. Ueno, H. Hoshi, Y. Maruyama, J. Phys. Chem. 99 (1995) By employing metastable impact electron [1] H. Kubota, T. Munakata, T. Hirooka, T. Kondow, K. Kuchitsu, K. Ohno, Y. Harada, Chem. Phys. 87 (1984) spectroscopy (MIES) and ultraviolet photo electron spectroscopy ( UPS) together with work- [13] S. Dieckhoff, V. Schlett, W. Possart, O.-D. Hennemann, function measurements, three adsorption states of J. Günster, V. Kempter, Appl. Surf. Sci. 103 (3) (1996) benzene have been identified on the clean Mo(100) 1. [14] W. Maus-Friedrichs, M. Wehrhahn, S. Dieckhoff, V. surface at 100 K: a first chemisorbed layer and a Kempter, Surf. Sci. 37 (1990) 57. second physisorbed layer with the plane of the [15] W. Maus-Friedrichs, S. Dieckhoff, V. Kempter, Surf. Sci. aromatic rings parallel to the surface; at higher 49 (1991) 149. coverages, an adsorbed layer with the molecular [16] J. Lee, C. Hanrahan, J. Arias, F. Bozso, R.M. Martin, H. Metiu, Phys. Rev. Lett. 54 (1985) planes essentially perpendicular (on-edge) to the [17] B. Woratschek, W. Sesselmann, J. Küppers, G. Ertl, H. surface forms. From the earlier appearance of Haberland, Phys. Rev. Lett. 55 (1985) 611. benzene-induced features on the MgO-covered [18] A.G. Borisov, D. Teillet-Billy, J.P. Gauyacq, Surf. Sci. 84 Mo(100) compared to Mo(100) and the modula- (1993) 337. tion of the intensities of the benzene features [19] M.-C. Wu, J.S. Corneille, C.A. Estrada, J.W. He, D.W. Goodman, Chem. Phys. Lett. 18 (1991) 47. characteristic of the upright (edge-on) phase, we [0] M.-C. Wu, C.M. Truong, D.W. Goodman, Phys. Rev. B conclude that on MgO, the adsorption dynamics 46 (199) are similar to the metal surface except for a missing [1] G. Ertl, Surf. Sci. 89 (1979) 55. first or chemisorbed layer. In addition, it was [] H. Kubota, T. Hirooka, T. Fukuyama, T. Kondow, K. Kuchitsu, A.J. Yencha, J. Electron Spectrosc. Relat. found that the adsorption of benzene on MgO- Phenom. 3 (1981) 417. covered Mo(100) does not result in the formation [3] S. Hüfner, Photoelectron Spectroscopy, nd ed., Springer, of perfectly closed layers. Berlin, 1995.

9 J. Günster et al. / Surface Science 415 (1998) [4] E. Lindholm, Faraday Disc. Chem. Soc. 54 (197) 00. [9] J. Günster, G. Liu, V. Kempter, D.W. Goodman, J. Vac. [5] K.Y. Yu, J.C. McMenamin, W.E. Spicer, Surf. Sci. 50 Sci. Technol. A 16 (3) (1998) 996. (1975) 149. [30] L.H. Tjeng, A.R. Vos, G.A. Sawatzky, Surf. Sci. 35 [6] J. Günster, D. Ochs, S. Dieckhoff, V. Kempter, Appl. Surf. (1990) 69. Sci. 103 (1996) 351. [31] S. Masuda, M. Aoyama, K. Ohno, Y. Harada, Phys. Rev. [7] P. Jakob, D. Menzel, Surf. Sci. 01 (1988) 503. Lett. 65 (1990) 357. [8] D. Ochs, W. Maus-Friedrichs, M. Brause, J. Günster, V. [3] K. Ohno, H. Mutoh, Y. Harada, J. Am. Chem. Soc. 105 Kempter, V. Puchin, A. Shluger, L. Kantorovich, Surf. Sci. (1983) (1996) 557.