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The adsorption of NO on Ru(001) and on O(2 × 1)Ru(001) revisited
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surface s c i e n c e ELSEVIER
Surface Science 370 (1997) L185-L192
Surface Science Letters
The adsorption of NO on Ru(001) and on 0 ( 2 x 1)/Ru(001) revisited P. Jakob
*, M . S t i c h l e r , D . M e n z e l
Physik-Department E 20, Technische Universitat Mfinchen, D-85747 Garching, Germany
Received 21 May 1996; accepted for publication 27 August 1996
Abstract
The infrared absorption spectra of NO adsorbed on Ru(001) and on 0(2 x 1)/Ru(001) have been re-examined to clarify several open questions in these systems, including an old puzzle regarding the insensitivity of IRAS to detect NO/Ru(001) on threefold bridge sites despite its clear presence in (specular) HREELS spectra. We find no indication for such an inconsistency in vibrational spectra as we observe both, the multiply coordinated vl-NO and the linear v2-NO species; we are able to detect their internal stretching modes with signal-to-noise ratios up to 1000: 1. We attribute this to better instrumentation as well as to improved crystal perfection and layer homogeneity. These improvements reflect themselves in unparalleled 3 cm- 1 linewidths of the vibrational bands and very narrow peaks in thermal desorption spectroscopy. As in the case of CO/O(2 x 1)/Ru(001), a (2 x 2) honeycomb structure must form for NO/O(2 x 1)/Ru(001) after annealing to 400 K. A consistent new picture of the processes occurring upon coadsorption and annealing can be obtained. Keywords: Infrared absorption spectroscopy; Low index single crystal surface; Nitrogen oxides; Oxygen; Ruthenium; Surface structure, morphology, roughness, and topography; Thermal desorption spectroscopy; Vibrations of adsorbed molecules
T h e ability to sensitively a n d reliably detect i n d i v i d u a l surface species is a crucial p o i n t for the a p p l i c a b i l i t y of the v a r i o u s surface spectroscopies as general analytic tools. W h e r e a s each of the m a n y techniques has its strengths a n d weaknesses it is p a r t i c u l a r l y d i s t u r b i n g w h e n e v e r two qualitatively equivalent m e t h o d s such as specular H R E E L S a n d infrared a b s o r p t i o n s p e c t r o s c o p y ( I R A S ) exhibit strikingly different results. Two articles in the early 1980s are of p a r t i c u l a r relevance to the t o p i c of this Letter: an I R A S a n d T P D s t u d y of v a r i o u s N O a n d N O + O layers on * Corresponding author. Fax: +49 89 289 123 38; e-mail: [email protected]
Ru(001) b y H a y d e n et al. [ 1 ] a n d a H R E E L S w o r k on the same subject b y C o n r a d et al. [ 2 ] . B o t h articles are b a s e d on earlier w o r k on this a d s o r p t i o n system b y W e i n b e r g ' s g r o u p [ 3 - 6 ] a n d at this institute [7,8]. T h e m o t i v a t i o n for r e - e x a m i n i n g the infrared a b s o r p t i o n spectra of a d s o r b e d N O / R u ( 0 0 1 ) was a r e m a r k m a d e in Ref. [ 1 ] t h a t despite strenous a t t e m p t s to observe the triple-bridge b o n d e d v l - N O - s p e c i e s (expected at 1 3 8 0 - 1 6 0 0 c m -1) n o t h i n g was f o u n d in this region. H R E E L S s p e c t r a at i m p r o v e d r e s o l u t i o n a n d sensitivity [ 2 ] , on the o t h e r hand, c o u l d resolve the related v i b r a t i o n a l feature w i t h o u t p r o b l e m s , in a g r e e m e n t with the earlier ( H R E E L S ) w o r k in Refs. [ 3 - 6 ] . I n a d d i t i o n , C o n r a d et al.
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0039-6028(96)01176-4
P. Jakob et al. / Surface Science 370 ( 1997) L185-L192
also studied NO postadsorbed on a O ( 2 x l ) / R u ( 0 0 1 ) layer for which a modified bridge-bonded NO-state (v~ (O)-NO) was identified which Hayden et al. likewise had failed to detect. No further IRAS studies have been devoted to this system despite the quoted contradiction. In view of the interesting question of whether dipolecoupled HREELS and IRAS possess different sensitivities for certain modes, we reinvestigated this adsorbate system with an IRAS system we recently constructed, for which it constitutes an excellent test system. Our new data also give access to an improved understanding and a reinterpretation of the rather complex N O and N O + O layers. Our U H V system consists of a highly stable setup for infrared absorption measurements in reflection geometry (incidence angle 84 ° from the surface normal) which was designed to allow high pressure measurements as well. A MCT-detector is used in general to detect the reflected p-polarized light. The standard settings of data collection are coaddition of 1000 scans at 2 cm -~ resolution which takes around 5 min, resulting in a noise level (transmission spectra) of 6 x 10 .5 at 2000cm -~. Besides IRAS, the apparatus contains LEED, XPS and TDS facilities. Surface cleanliness from carbon is monitored by TDS of desorbing CO during heat-up of an oxygen covered layer: any traces of desorbing CO above 5 0 0 K indicate residual carbon contaminants. Clean Ru(001) is prepared by flashing the sample to 1560 K twice (maintaining it at that temperature for 10-20 s) to remove adsorbed oxygen. Contaminants other than carbon were detected by XPS. Also, the shape of TDS curves and of vibrational bands has been found to be sensitive to surface contaminants, as is well known. The N O layers were prepared at l 1 5 K (NO/Ru(001)) and 150 K ( N O / O ( 2 x 1)/Ru(001)) via a microcapillary dosing array mounted directly above the sample in the IRAS measurement position. For the latter layer, oxygen was first adsorbed at 400 K up to saturation; a subsequent anneal to 700 K ensured proper ordering of the O-layer. Care has been taken to prevent possible contaminants in the N O reservoir (N20, NO2, etc.) to reaching the sample, by passing the N O gas through a liquid nitrogen cold trap.
In Fig. 1 we show infrared absorption spectra of NO/Ru(001) which had been adsorbed at 115 K and then annealed to 200 K while keeping the N O gas flowing. In this manner perfect ordering into the (2 x 2) superlattice is achieved. We note that exposure at 200 K would lead to partial N O dissociation at the beginning of the dosage when the N O layer is non-saturated [8]. Once saturated the NO/Ru(001) layer can be heated up to 350 K without inducing any irreversible changes in the spectra; only after the beginning of molecular N O desorption does dissociation of residual N O become possible. In Fig. la we clearly resolve NO bands at 1504.5 and 1829.9cm -1 which can be attributed to the internal stretch modes of the triply-coordinated vl-NO and the linear v2-NO species, respectively. This assignment follows the earlier work in Refs. [ 2 - 6 ] . The v2-NO and vl-NO modes give 5.9 and 1.1% changes in reflectivity at linewidths of 4.3 and 6.5cm -~, respectively, at 89 K. Elevated temperatures cause negative line shifts of the v2-NO vibrational bands, in parallel with a slight broadening. For vl-NO this shift is opposite in sign and the line broadening is much I
I
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/ 1823.8
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Fig. 1. Infrared absorption spectra of the vN~o-bands of a saturated NO/Ru(001) layer. The spectra were obtained at a sample temperature of 89 K (a) and 300 K (b) and they are offsetfor clarity of presentation. Spectralresolutionwas 2 cm- 1 and no smoothing or baseline correction has been applied.
P. Jakob et al./ Surface Science 370 (1997) L185-L192 Table 1 Vibrational frequencies of adsorbed NO (Vr~o) of the NO/Ru(001) and the NO/O(2 x 1)/Ru(001) layers in Figs. 1 and 2; the sample temperature during data acquisition is denoted as Tin; measured linewidths of the individual bands are quoted in parentheses; for details of the layer preparation see text Layer
T~ (K)
Vibrational frequencies (cm-1) vl-NO
NO/Ru(001) NO/Ru(001) N O + O(2 x 1)/Ru(001)
89 300 150
NO+O(2 × 1)/Ru(001)~
150
vl (O)-NO
1504.5 (6.5) 1510.3 (14.1) 1615.8 (3.8)
vz-NO 1829.9 (4.3) 1823.8 (5.4) ~1835 ( ~ 7 ) 1857.5 (3.7)
a Annealed to 400 K.
stronger, in agreement with the expectation from theory of dephasing [9]. This behavior is demonstrated by curve (b) of Fig. 1 which was recorded at 300 K. The observed modifications are completely reversible and they are summarized in Table 1 together with the results shown in Fig. 2 I
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Wavenumber cm "~ Fig. 2. Infrared absorption spectra of the vN~-bands of a N O + O ( 2 × l ) layer on Ru(001). The spectrum in (a) was taken after adsorption of NO at 150K; spectrum (b) was obtained after annealing the layer to 430 K. Both spectra were measured at 150 K and they are offset for clarity of presentation. Spectral resolution was 2 cm- 1 and no smoothing or baseline correction has been applied. The two inserts display the structural arrangement of oxygen atoms (full circles) and NO molecules (open circles) on the Ru(001) substrate for the two layers in (a) and (b).
(obtained from a N O / O ( 2 x 1)/Ru(001) layer, see below). The origin of these changes of the vibrational line shape is the anharmonic coupling of the internal N O stretch mode (VN_o) to thermally excited low frequency modes of N O (frustrated translations or rotations) or to Ru-phonons, and will be presented in more detail in a separate article [ 10]. We note that we also were able to detect the low coverage bands at 1130 and 1400 cm -1 which disappear again at higher ¢9NO. The effect of oxygen coadsorption on N O adsorption is displayed in Fig. 2. N O postadsorption onto a well ordered 0 ( 2 x 1)/Ru(001) layer (O =0.5 ML) at 150 K gives rise to a surprisingly narrow feature at 1615.8cm -1 with a 3.8 cm-1 linewidth and a 7.7% change in reflectivity. Deconvolution with the instrumental resolution of 2 c m - ~ yields an intrinsic width of only 2.8 cm -~. In addition, some intensity is found in the region of the v2-NO species at 1835 cm -1 which we attribute to N O located at O ( 2 x 1) domain boundaries. Heating the layer to 350440 K leads to a dramatic change of the spectrum: the 1615.8 cm -~ band is completely gone and an even sharper ( F = 3 . 7 cm-1; 2.6 cm -1 after deconvolution) and more intense mode ( A T = 13.6%) has grown at 1857.5 c m - 1 (v2-NO) (Fig. 2b). The sharp (2 x 2) overstructure is retained after N O postadsorption at low T and upon heating to 320 K when the spots become slightly more diffuse. Above 400 K they sharpen again, especially after recooling to low temperatures. In a very recent LEED-IVanalysis [11] of the layer after annealing to 430 K, equal occupation
P. Jakob et al. /Surface Science 370 (1997) L185-L192
o! lacp and fcc hollow sites by the oxygen atoms in a honeycomb arrangement has been found, while the NO molecules are located at on-top positions in the center of the hexagons, in agreement with the observation of the linear v2-NO species in IRAS. Note that for the very similar coadsorbate system C O + 2 0 / R u ( 0 0 1 ) a ( 2 x 2 ) honeycomb structure is also formed after moderate annealing, to 360 K [12-14]. This structural similarity also shows up in thermal desorption spectroscopy where a very narrow peak of desorbing NO has been found (see below). The masses 2, 14, 28 and 30 have been followed in parallel in a thermal desorption experiment while ramping the sample temperature linearly at 1 K/s. A glass envelope (with an entrance hole of 4 mm in diameter) around the ionization cage is used to enlarge the desorption signal as well as to improve the signal dynamics [15]. In Figs. 3 and
4000
,,,,,,,,,I
....
4 we show the signals of 28 and 30 amu which correspond to desorbing N 2 and NO, respectively. Whereas the pure NO/Ru(001) layer (curve a) exhibits the well-known double peak structure (30 amu) [8] a single state is observed in this temperature range when NO is post-adsorbed onto a 0 ( 2 × 1)/Ru(001) layer (curve b). However, in the latter case a broad desorption peak at 270 K (with relatively steep increasing slope and a long tail on the high temperature side) is observed. The total amount of desorbing NO is slightly less (80%) for the coadsorbate layer compared to the pure NO/Ru(001) layer. Dissociation, as indicated by the 28 amu signal (traced by the 14 amu signal), on the other hand, is considerably reduced (down to 1/5 of the original level) for the N O + O layer. A quantitative analysis (taking into account the relative signal strength for NO (30 amu) and N2 (28 amu) of 1 : 3.5) shows that 2/3 of the saturated
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Temperature [K] Fig. 3. Thermal desorption spectra of the mass 28 amu signal featuring desorbing N 2. Curves (a) and (b) were obtained from pure NO/Ru(001) and the mixed N O + 0 ( 2 x 1)/Ru(001) layers, respectively.
P. Jakob et aL / Surface Science 370 (1997) L185-L192 1.2
I ' ' ' * ' * ' ' ' l ' ' ' ' ' J ' ' ' l ' ' ' ' ' ' ' ' ' l ' ' ' ' ' ' ' ' ' l ' ' ' ' ' ' ' ' ' l ' * ' ' ' ' ' ' ' r ' ' ' ' ' ' ' ' ' l ' ' l l r l ' '
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Temperature [K] Fig. 4. Thermaldesorption spectra of the mass 30 amu signal featuringdesorbing NO. Curves (a) and (b) were obtained from the pure NO/Ru(001) and the mixedNO + 0(2 x 1)/Ru(001) layers,respectively.
NO/Ru(001) layer desorb molecularly while the residual 0.25 ML (assuming a saturation coverage of 0.75 ML) is dissociated at higher temperatures. For NO + 0(2 x 1)/Ru(001) the dissociation of NO therefore amounts to only 0.05 ML. This small remnant may well occur at defects and domain boundaries of the honeycomb structure. The extremely narrow TDS lineshape of desorbing NO at around 470 K with only 12 K (NO) peak width (after annealing the layer to 400 K for some time) indicates a high pre-exponential factor Ko for the desorption kinetics, which can be explained by an immobile adsorbate species, as has also been found for CO+20/Ru(001) [13]. However, even for k o = l x 1018 s -1 the TDS peak would be about twice as broad as observed in our experiment. A reasonable fit of the experimental data can be obtained by the assumption of an increase of the
NO binding energy of 4% (EB(ONo=0)= 168 kJ/mol) with coverage in the range ONo = 0-0.25 ML. Other coverage dependencies (change of order and/or pre-exponential) might be able to explain the peak shape, but a "better" fit does not appear to be sensible. In terms of the physical process involved, we suggest that a local return of the (2x2) honeycomb structure to the O(2x 1) order after NO desorption is operative, which destabilizes the nearby NO molecules, leading to their enhanced molecular desorption. The broad low-T peak, on the other hand, must accompany the restructuring of (2 x 1)O + 2NO to honeycomb O + N O and suggests that the rate determining step in this case is NO desorption. The curves in Figs. 3a and 4a closely follow the reported desorption spectra of Feulner et al. [8], which have already demonstrated such narrow
P. Jakob et al. / Surface Science 370 (1997) L185-L192
TDS peaks. However, the complexity of the layers in their case (containing va- and vz-NO species as well as dissociation products accumulating during the heat-up) made it difficult to present a clear picture of the local structure, mutual stabilization and related processes. In their model a stabilization of v2-NO by vl-NO and by the dissociation products, adsorbed N and O atoms, has been suggested, which is clearly supported by our experiments. This stabilization results in an increase of the vz-NO binding energy with coverage (induced by the opposite electron affinities of v2-NO on the one hand and Vl-NO as well as adsorbed O and N atoms on the other) which has been deduced from the shape of TDS curves. The geometrical structure suggested by Feulner et al., a honeycomb of vl-NO surrounding on-top v2molecules, has very recently been indeed found in LEED-IV analysis [ 11 ]. The arrangement of NO molecules of the saturated NO/O(2 × 1)/Ru(001) layer at T<200 K is much less obvious. TDS implies a NO-coverage of about 0.5ML that, along with the 0.5 ML of oxygen, yields a total coverage of 1 ML. Two possibilities arise: (a) postadsorbed NO is located in the empty trenches of the 0 ( 2 × 1) rows while the oxygen atoms remain in place owing to their negligible mobility at low adsorption temperatures and, (b) NO is not adsorbed on Ru(001) but reacts with the preadsorbed oxygen atoms to form some kind of NO2 species. We can exclude possibility (b) as 180 preadsorption gave virtually identical results as in Fig. 2. Also, the triatomic NOz molecule should exhibit more than a single mode. Possibility (a) must then be realized. However, the relative positions of the oxygen atoms and the NO molecules are still unclear. We repeated the experiment of Fig. 2 by postadsorbing NO at even lower T (50 K) and obtained virtually the same results: a single narrow vibrational mode (not quite as narrow as in Fig. 2) located at 1616cm -1. A rearrangement of the oxygen atoms therefore appears very improbable. More or less random movements of Oads by impact of post-adsorbing NO is unlikely as this would result in considerable layer heterogeneity, contrary to the observations. We conclude that the va (O)-NO species very likely is located in the residual hcp hollow sites between
two oxygen (2 × 1) rows, thereby minimizing repulsion to the neighboring oxygen atoms. The intermolecular distance of the NO molecules in such an arrangement is equal to the substrate lattice constant. The grossly unequal distance of the NO molecules along and perpendicular to the (2 × 1) oxygen rows suggest that a likewise anisotropy of the electronic band structure should exist, i.e. the system will display one-dimensional behavior. Unfortunately its detection is made difficult by the fact that three equivalent 0 ( 2 × 1) domains are in general formed on a flat Ru(001) surface and an ARUPS measurement will always average incoherently over different domain orientations. Low temperature scanning tunnelling spectroscopy would be helpful. Strictly speaking, the NO molecules will not experience the threefold symmetry expected for a molecule located at hcp hollow sites but rather only C s symmetry. This is because the coadsorbed oxygen atoms render the three Ru atoms of the hollow site inequivalent. The only symmetry element therefore will be a plane perpendicular to the 0 ( 2 × 1) rows. It is then possible that even in the absence of intermolecular interactions the NO molecules are tilted away from the surface normal. For strongly interacting molecules alternating tilts may prevail in order to release the repulsive strain between neighboring molecules; there may also be alternating shifts out of the exact threefold sites. In this case the overlayer structure would change from (2 × 1) to (2 × 2) which, however, is indistinguishable from a 3-domain (2 × 1) LEED pattern. A quantitative LEED-IVanalysis is necessary and is under way [ 11 ]. In the XPS spectra of the O Is peak (not shown) of the low T N O + O ( 2 × I ) / R u ( 0 0 1 ) phase we could identify only a single peak at 531.0 eV binding energy. It was larger and slightly broader than the corresponding peak of the 0.5 ML preadsorbed oxygen located at 530.7 eV. According to Umbach et al. [7], the O ls peaks of(triple-) bridge-bonded vl-NO and of Oads strongly overlap whereas linearly bound v2-NO displays a higher binding energy by about 1.5 eV. The interpretation of the 1616 cm -1 vibrational band as a threefold bridge bonded NO species therefore is corroborated by these XPS results. XPS spectra of the
P. Jakob et al./ Surface Science 370 (1997) L185-L192
(NO + 2 0 ) ( 2 x 2) honeycomb layer gave approximately a 1:2 ratio of the O ls peaks of v2-NO and adsorbed oxygen at 532.4 and 530.7 eV binding energy, respectively. This result is in good agreement with the structural models presented in this work. The interesting question as to why the earlier IRAS work [ 1] was not able to detect the Vl-NO species may be answered as follows. Similarities of the TDS traces in this work imply not too different adsorbate layers from ours. However, the generally broader features in the TDS and of the vibrational bands indicate a somewhat reduced layer quality in their work. We cannot judge whether this was related to crystal imperfection, surface contaminants and/or a less perfect layer ordering owing to a different preparation procedure. In the present work we did find that the threefold-bridge bonded NO was particularly sensitive to contaminants like hydrogen and oxygen, which broadened the lines drastically, by up to 100 cm -~. For dense layers (like NO adsorbed on the clean Ru(001) surface) such a broadening may occur whenever the contaminant leads to an upward shift of the vibrational frequency. Dynamic dipole coupling and intensity transfer from the main mode to the (blue shifted) contamination bands then enhance their intensities considerably [16]. In HREELS a line broadening of 50-100cm -~ does not seriously affect the spectra as long as the instrumental resolution is of the same order. A narrow, sharp line therefore will have a similar appearance as a broad and weak band of the same integrated intensity. In IRAS this is definitely not the case and detection of broad features suffering from layer inhomogeneity is difficult as these might be superimposed on an unstable baseline or other disturbances. The above mentioned contaminants would be correlated with line shifts of the order of 60 cm 1, which leads us to the conclusion that at least in the HREELS data of Ref. [2] no such contaminations were present. In our case, the much improved stability of the present setup has aided the detection of the various bridge-bonded NO species considerably. Our improved data quality, along with the knowledge of the NO + 2 0 honeycomb structure formed at elevated temperatures allows us to present a comprehensive reinterpretation of the
NO + 2 0 layer properties. Conrad et al. [_ j ....... uted the sole occurrence of va(O)-NO after NO adsorption onto the 0 ( 2 x 1)/Ru(001) layer at low T (while very little v2-NO is observed) to a preferential sticking and occupation of threefold-bridge sites. The energetically favored site, on-top bonded linear v2-NO, was assumed to be formed only after a thermally activated diffusion step of adsorbed NO. Such an immobile NO species at 120 K, however, appears questionable as the ability of chemisorbed molecular species to move a few sites even at very low temperatures has been demonstrated for CO on P t ( l l l ) at around 50 K [17]. The basic idea of our interpretation is that the arrangement of the oxygen atoms decides whether vl(O)-NO or v2-NO is energetically favored; the O-arrangement in turn depends on the NO concentration. The thermally activated step to form v2-NO in our model is related to molecular desorption of vI(O)-NO (thereby reducing the NO-coverage) and the correlated rearrangement of oxygen atoms and of NOads; actually, only one short jump per two oxygen atoms is needed to rearrange the 0 ( 2 x 1) layer into an 0 ( 2 × 2) honeycomb structure [12-14]. In the center of these hexagons there is a single on-top site that can accept one v2-NO per unit cell. No free sites are available thereafter, which naturally explains the zero sticking coefficient regarding NO postadsorption, SNo, reported by Conrad et al. [2] and confirmed in our experiments. In their model, site blocking of supposedly pre-existing vx(O)-sites by v2-NO has been invoked, i.e. v~(O)-NO would have formed if there would be a way to overcome the vanishing SNo. In contrast, our model states that there are no threefold-bridge sites available within the annealed NO + 20/Ru(001) layer. Any site blocking, be it for NO post-adsorption on 0 ( 2 × 1)/Ru(001) (blocking of on-top sites) or on the N O + 2 0 honeycomb structure (blocking of threefold hollow sites), must be attributed to the adsorbed oxygen. A clear explanation of all the observations can be given, once the oxygen layer is not considered static (acting just as a template without mutual interaction with coadsorbed NO) but as an integral part of the coadsorbate layer which adapts dynamically to the respective NO coverage conditions.
P. Jakob et al./ Surface Science 370 (1997) L185-L192
. . . . . . . . . . . . . ~,ement This work was supported by the Deutsche Forschungsgemeinschaft through SFB 338.
References [1] B.E. Hayden, K. Kretzschmar and A.M. Bradshaw, Surf. Sci. 125 (1983) 366. [2] H. Conrad, R. Scala, W. Stenzel and R. Unwin, Surf. Sci. 145 (1984) 1. [3] G.E. Thomas and W.H. Weinberg, Phys. Rev. Lett. 41 (1978) 1181. [4] P.A. Thiel and W.H. Weinberg, J. Chem. Phys. 73 (1980) 4081. [5] P.A. Thiel, W.H. Weinberg and J.T. Yates, Jr., J. Chem. Phys. 71 (1979) 1643. 1-6] P.A. Thiel, W.H. Weinberg and J.T. Yates, Jr., J. Chem. Phys. Lett. 67 (1979) 403.
[7] E. Umbach, S. Kulkarni, P. Feulner and D. Menzel, Surf. Sci. 88 (1979) 65. [8] P. Feulner, S. Kulkarni, E. Umbach and D. Menzel, Surf. Sci. 99 (1980) 489. [9] B.N.J. Persson, F.M. Hoffmann and R. Ryberg, Phys. Rev. B 34 (1986) 2266. [10] P. Jakob and D. Menzel, in preparation. [ 11 ] M. Stichler and D. Menzel, ICSOS-5 (Aix-en-Provence) abstract Fr 104P, to be published. [12] F.M. Hoffmann, M.D. Weisel and C.H.F. Peden, Surf. Sci. 253 (1991) 59. [13] K.L. Kostov, H. Rauscher and D. Menzel, Surf. Sci. 278 (1992) 62. [14] B. Narloch, G. Held and D. Menzel, Surf. Sci. 317 (1994) 131. [15] P. Feulner and D. Menzel, J. Vac. Sci. Technol. 17 (1980) 662. [16] B.N.J. Persson and F.M. Hoffmann, J. Electron Spectrosc. Relat. Phenom. 45 (1987) 215. [17] J.V. Nekrylova and I. Harrison, J. Chem. Phys. 101 (1994) 1730.
surface s c i e n c e ELSEVIER
Surface Science 370 (1997) L185-L192
Surface Science Letters
The adsorption of NO on Ru(001) and on 0 ( 2 x 1)/Ru(001) revisited P. Jakob
*, M . S t i c h l e r , D . M e n z e l
Physik-Department E 20, Technische Universitat Mfinchen, D-85747 Garching, Germany
Received 21 May 1996; accepted for publication 27 August 1996
Abstract
The infrared absorption spectra of NO adsorbed on Ru(001) and on 0(2 x 1)/Ru(001) have been re-examined to clarify several open questions in these systems, including an old puzzle regarding the insensitivity of IRAS to detect NO/Ru(001) on threefold bridge sites despite its clear presence in (specular) HREELS spectra. We find no indication for such an inconsistency in vibrational spectra as we observe both, the multiply coordinated vl-NO and the linear v2-NO species; we are able to detect their internal stretching modes with signal-to-noise ratios up to 1000: 1. We attribute this to better instrumentation as well as to improved crystal perfection and layer homogeneity. These improvements reflect themselves in unparalleled 3 cm- 1 linewidths of the vibrational bands and very narrow peaks in thermal desorption spectroscopy. As in the case of CO/O(2 x 1)/Ru(001), a (2 x 2) honeycomb structure must form for NO/O(2 x 1)/Ru(001) after annealing to 400 K. A consistent new picture of the processes occurring upon coadsorption and annealing can be obtained. Keywords: Infrared absorption spectroscopy; Low index single crystal surface; Nitrogen oxides; Oxygen; Ruthenium; Surface structure, morphology, roughness, and topography; Thermal desorption spectroscopy; Vibrations of adsorbed molecules
T h e ability to sensitively a n d reliably detect i n d i v i d u a l surface species is a crucial p o i n t for the a p p l i c a b i l i t y of the v a r i o u s surface spectroscopies as general analytic tools. W h e r e a s each of the m a n y techniques has its strengths a n d weaknesses it is p a r t i c u l a r l y d i s t u r b i n g w h e n e v e r two qualitatively equivalent m e t h o d s such as specular H R E E L S a n d infrared a b s o r p t i o n s p e c t r o s c o p y ( I R A S ) exhibit strikingly different results. Two articles in the early 1980s are of p a r t i c u l a r relevance to the t o p i c of this Letter: an I R A S a n d T P D s t u d y of v a r i o u s N O a n d N O + O layers on * Corresponding author. Fax: +49 89 289 123 38; e-mail: [email protected]
Ru(001) b y H a y d e n et al. [ 1 ] a n d a H R E E L S w o r k on the same subject b y C o n r a d et al. [ 2 ] . B o t h articles are b a s e d on earlier w o r k on this a d s o r p t i o n system b y W e i n b e r g ' s g r o u p [ 3 - 6 ] a n d at this institute [7,8]. T h e m o t i v a t i o n for r e - e x a m i n i n g the infrared a b s o r p t i o n spectra of a d s o r b e d N O / R u ( 0 0 1 ) was a r e m a r k m a d e in Ref. [ 1 ] t h a t despite strenous a t t e m p t s to observe the triple-bridge b o n d e d v l - N O - s p e c i e s (expected at 1 3 8 0 - 1 6 0 0 c m -1) n o t h i n g was f o u n d in this region. H R E E L S s p e c t r a at i m p r o v e d r e s o l u t i o n a n d sensitivity [ 2 ] , on the o t h e r hand, c o u l d resolve the related v i b r a t i o n a l feature w i t h o u t p r o b l e m s , in a g r e e m e n t with the earlier ( H R E E L S ) w o r k in Refs. [ 3 - 6 ] . I n a d d i t i o n , C o n r a d et al.
0039-6028/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0039-6028(96)01176-4
P. Jakob et al. / Surface Science 370 ( 1997) L185-L192
also studied NO postadsorbed on a O ( 2 x l ) / R u ( 0 0 1 ) layer for which a modified bridge-bonded NO-state (v~ (O)-NO) was identified which Hayden et al. likewise had failed to detect. No further IRAS studies have been devoted to this system despite the quoted contradiction. In view of the interesting question of whether dipolecoupled HREELS and IRAS possess different sensitivities for certain modes, we reinvestigated this adsorbate system with an IRAS system we recently constructed, for which it constitutes an excellent test system. Our new data also give access to an improved understanding and a reinterpretation of the rather complex N O and N O + O layers. Our U H V system consists of a highly stable setup for infrared absorption measurements in reflection geometry (incidence angle 84 ° from the surface normal) which was designed to allow high pressure measurements as well. A MCT-detector is used in general to detect the reflected p-polarized light. The standard settings of data collection are coaddition of 1000 scans at 2 cm -~ resolution which takes around 5 min, resulting in a noise level (transmission spectra) of 6 x 10 .5 at 2000cm -~. Besides IRAS, the apparatus contains LEED, XPS and TDS facilities. Surface cleanliness from carbon is monitored by TDS of desorbing CO during heat-up of an oxygen covered layer: any traces of desorbing CO above 5 0 0 K indicate residual carbon contaminants. Clean Ru(001) is prepared by flashing the sample to 1560 K twice (maintaining it at that temperature for 10-20 s) to remove adsorbed oxygen. Contaminants other than carbon were detected by XPS. Also, the shape of TDS curves and of vibrational bands has been found to be sensitive to surface contaminants, as is well known. The N O layers were prepared at l 1 5 K (NO/Ru(001)) and 150 K ( N O / O ( 2 x 1)/Ru(001)) via a microcapillary dosing array mounted directly above the sample in the IRAS measurement position. For the latter layer, oxygen was first adsorbed at 400 K up to saturation; a subsequent anneal to 700 K ensured proper ordering of the O-layer. Care has been taken to prevent possible contaminants in the N O reservoir (N20, NO2, etc.) to reaching the sample, by passing the N O gas through a liquid nitrogen cold trap.
In Fig. 1 we show infrared absorption spectra of NO/Ru(001) which had been adsorbed at 115 K and then annealed to 200 K while keeping the N O gas flowing. In this manner perfect ordering into the (2 x 2) superlattice is achieved. We note that exposure at 200 K would lead to partial N O dissociation at the beginning of the dosage when the N O layer is non-saturated [8]. Once saturated the NO/Ru(001) layer can be heated up to 350 K without inducing any irreversible changes in the spectra; only after the beginning of molecular N O desorption does dissociation of residual N O become possible. In Fig. la we clearly resolve NO bands at 1504.5 and 1829.9cm -1 which can be attributed to the internal stretch modes of the triply-coordinated vl-NO and the linear v2-NO species, respectively. This assignment follows the earlier work in Refs. [ 2 - 6 ] . The v2-NO and vl-NO modes give 5.9 and 1.1% changes in reflectivity at linewidths of 4.3 and 6.5cm -~, respectively, at 89 K. Elevated temperatures cause negative line shifts of the v2-NO vibrational bands, in parallel with a slight broadening. For vl-NO this shift is opposite in sign and the line broadening is much I
I
1504.5
I
I
I
I
(a)
N
(b) ~
"~
1510.3
E t,'-
i-.-q
NO/Ru(O01)
/ 1823.8
tQ
14so 1;oo
"3 1829.9
liOO 1 5o 1;oo ldso 19oo Wavenumber cm~
Fig. 1. Infrared absorption spectra of the vN~o-bands of a saturated NO/Ru(001) layer. The spectra were obtained at a sample temperature of 89 K (a) and 300 K (b) and they are offsetfor clarity of presentation. Spectralresolutionwas 2 cm- 1 and no smoothing or baseline correction has been applied.
P. Jakob et al./ Surface Science 370 (1997) L185-L192 Table 1 Vibrational frequencies of adsorbed NO (Vr~o) of the NO/Ru(001) and the NO/O(2 x 1)/Ru(001) layers in Figs. 1 and 2; the sample temperature during data acquisition is denoted as Tin; measured linewidths of the individual bands are quoted in parentheses; for details of the layer preparation see text Layer
T~ (K)
Vibrational frequencies (cm-1) vl-NO
NO/Ru(001) NO/Ru(001) N O + O(2 x 1)/Ru(001)
89 300 150
NO+O(2 × 1)/Ru(001)~
150
vl (O)-NO
1504.5 (6.5) 1510.3 (14.1) 1615.8 (3.8)
vz-NO 1829.9 (4.3) 1823.8 (5.4) ~1835 ( ~ 7 ) 1857.5 (3.7)
a Annealed to 400 K.
stronger, in agreement with the expectation from theory of dephasing [9]. This behavior is demonstrated by curve (b) of Fig. 1 which was recorded at 300 K. The observed modifications are completely reversible and they are summarized in Table 1 together with the results shown in Fig. 2 I
I
I
~"
(b)
I
I
(a)
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3.7
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1615.8 wt
NO/O(2xl)/Ru(001) T m = 150 K
q
15 o
1 oo
1;so 1 oo 1 5o
1857.5
1 oo
19oo
Wavenumber cm "~ Fig. 2. Infrared absorption spectra of the vN~-bands of a N O + O ( 2 × l ) layer on Ru(001). The spectrum in (a) was taken after adsorption of NO at 150K; spectrum (b) was obtained after annealing the layer to 430 K. Both spectra were measured at 150 K and they are offset for clarity of presentation. Spectral resolution was 2 cm- 1 and no smoothing or baseline correction has been applied. The two inserts display the structural arrangement of oxygen atoms (full circles) and NO molecules (open circles) on the Ru(001) substrate for the two layers in (a) and (b).
(obtained from a N O / O ( 2 x 1)/Ru(001) layer, see below). The origin of these changes of the vibrational line shape is the anharmonic coupling of the internal N O stretch mode (VN_o) to thermally excited low frequency modes of N O (frustrated translations or rotations) or to Ru-phonons, and will be presented in more detail in a separate article [ 10]. We note that we also were able to detect the low coverage bands at 1130 and 1400 cm -1 which disappear again at higher ¢9NO. The effect of oxygen coadsorption on N O adsorption is displayed in Fig. 2. N O postadsorption onto a well ordered 0 ( 2 x 1)/Ru(001) layer (O =0.5 ML) at 150 K gives rise to a surprisingly narrow feature at 1615.8cm -1 with a 3.8 cm-1 linewidth and a 7.7% change in reflectivity. Deconvolution with the instrumental resolution of 2 c m - ~ yields an intrinsic width of only 2.8 cm -~. In addition, some intensity is found in the region of the v2-NO species at 1835 cm -1 which we attribute to N O located at O ( 2 x 1) domain boundaries. Heating the layer to 350440 K leads to a dramatic change of the spectrum: the 1615.8 cm -~ band is completely gone and an even sharper ( F = 3 . 7 cm-1; 2.6 cm -1 after deconvolution) and more intense mode ( A T = 13.6%) has grown at 1857.5 c m - 1 (v2-NO) (Fig. 2b). The sharp (2 x 2) overstructure is retained after N O postadsorption at low T and upon heating to 320 K when the spots become slightly more diffuse. Above 400 K they sharpen again, especially after recooling to low temperatures. In a very recent LEED-IVanalysis [11] of the layer after annealing to 430 K, equal occupation
P. Jakob et al. /Surface Science 370 (1997) L185-L192
o! lacp and fcc hollow sites by the oxygen atoms in a honeycomb arrangement has been found, while the NO molecules are located at on-top positions in the center of the hexagons, in agreement with the observation of the linear v2-NO species in IRAS. Note that for the very similar coadsorbate system C O + 2 0 / R u ( 0 0 1 ) a ( 2 x 2 ) honeycomb structure is also formed after moderate annealing, to 360 K [12-14]. This structural similarity also shows up in thermal desorption spectroscopy where a very narrow peak of desorbing NO has been found (see below). The masses 2, 14, 28 and 30 have been followed in parallel in a thermal desorption experiment while ramping the sample temperature linearly at 1 K/s. A glass envelope (with an entrance hole of 4 mm in diameter) around the ionization cage is used to enlarge the desorption signal as well as to improve the signal dynamics [15]. In Figs. 3 and
4000
,,,,,,,,,I
....
4 we show the signals of 28 and 30 amu which correspond to desorbing N 2 and NO, respectively. Whereas the pure NO/Ru(001) layer (curve a) exhibits the well-known double peak structure (30 amu) [8] a single state is observed in this temperature range when NO is post-adsorbed onto a 0 ( 2 × 1)/Ru(001) layer (curve b). However, in the latter case a broad desorption peak at 270 K (with relatively steep increasing slope and a long tail on the high temperature side) is observed. The total amount of desorbing NO is slightly less (80%) for the coadsorbate layer compared to the pure NO/Ru(001) layer. Dissociation, as indicated by the 28 amu signal (traced by the 14 amu signal), on the other hand, is considerably reduced (down to 1/5 of the original level) for the N O + O layer. A quantitative analysis (taking into account the relative signal strength for NO (30 amu) and N2 (28 amu) of 1 : 3.5) shows that 2/3 of the saturated
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.....
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Thermal desorption spectra 3500 (n t-
(a) NO/Ru(001 ) (b) NO/O(2xl)/Ru(O01)
o R
3000
dT/dt = 1 K/s
.6 2500
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500
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600
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650
700
Temperature [K] Fig. 3. Thermal desorption spectra of the mass 28 amu signal featuring desorbing N 2. Curves (a) and (b) were obtained from pure NO/Ru(001) and the mixed N O + 0 ( 2 x 1)/Ru(001) layers, respectively.
P. Jakob et aL / Surface Science 370 (1997) L185-L192 1.2
I ' ' ' * ' * ' ' ' l ' ' ' ' ' J ' ' ' l ' ' ' ' ' ' ' ' ' l ' ' ' ' ' ' ' ' ' l ' ' ' ' ' ' ' ' ' l ' * ' ' ' ' ' ' ' r ' ' ' ' ' ' ' ' ' l ' ' l l r l ' '
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Thermal desorption spectra
1.o 0.9
(a) NO/Ru(001) (b) N O / O ( 2 x l ) / R u ( 0 0 1 )
xi"~
0.8
dT/dt = 1 K/s
,---,
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500
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550
600
Temperature [K] Fig. 4. Thermaldesorption spectra of the mass 30 amu signal featuringdesorbing NO. Curves (a) and (b) were obtained from the pure NO/Ru(001) and the mixedNO + 0(2 x 1)/Ru(001) layers,respectively.
NO/Ru(001) layer desorb molecularly while the residual 0.25 ML (assuming a saturation coverage of 0.75 ML) is dissociated at higher temperatures. For NO + 0(2 x 1)/Ru(001) the dissociation of NO therefore amounts to only 0.05 ML. This small remnant may well occur at defects and domain boundaries of the honeycomb structure. The extremely narrow TDS lineshape of desorbing NO at around 470 K with only 12 K (NO) peak width (after annealing the layer to 400 K for some time) indicates a high pre-exponential factor Ko for the desorption kinetics, which can be explained by an immobile adsorbate species, as has also been found for CO+20/Ru(001) [13]. However, even for k o = l x 1018 s -1 the TDS peak would be about twice as broad as observed in our experiment. A reasonable fit of the experimental data can be obtained by the assumption of an increase of the
NO binding energy of 4% (EB(ONo=0)= 168 kJ/mol) with coverage in the range ONo = 0-0.25 ML. Other coverage dependencies (change of order and/or pre-exponential) might be able to explain the peak shape, but a "better" fit does not appear to be sensible. In terms of the physical process involved, we suggest that a local return of the (2x2) honeycomb structure to the O(2x 1) order after NO desorption is operative, which destabilizes the nearby NO molecules, leading to their enhanced molecular desorption. The broad low-T peak, on the other hand, must accompany the restructuring of (2 x 1)O + 2NO to honeycomb O + N O and suggests that the rate determining step in this case is NO desorption. The curves in Figs. 3a and 4a closely follow the reported desorption spectra of Feulner et al. [8], which have already demonstrated such narrow
P. Jakob et al. / Surface Science 370 (1997) L185-L192
TDS peaks. However, the complexity of the layers in their case (containing va- and vz-NO species as well as dissociation products accumulating during the heat-up) made it difficult to present a clear picture of the local structure, mutual stabilization and related processes. In their model a stabilization of v2-NO by vl-NO and by the dissociation products, adsorbed N and O atoms, has been suggested, which is clearly supported by our experiments. This stabilization results in an increase of the vz-NO binding energy with coverage (induced by the opposite electron affinities of v2-NO on the one hand and Vl-NO as well as adsorbed O and N atoms on the other) which has been deduced from the shape of TDS curves. The geometrical structure suggested by Feulner et al., a honeycomb of vl-NO surrounding on-top v2molecules, has very recently been indeed found in LEED-IV analysis [ 11 ]. The arrangement of NO molecules of the saturated NO/O(2 × 1)/Ru(001) layer at T<200 K is much less obvious. TDS implies a NO-coverage of about 0.5ML that, along with the 0.5 ML of oxygen, yields a total coverage of 1 ML. Two possibilities arise: (a) postadsorbed NO is located in the empty trenches of the 0 ( 2 × 1) rows while the oxygen atoms remain in place owing to their negligible mobility at low adsorption temperatures and, (b) NO is not adsorbed on Ru(001) but reacts with the preadsorbed oxygen atoms to form some kind of NO2 species. We can exclude possibility (b) as 180 preadsorption gave virtually identical results as in Fig. 2. Also, the triatomic NOz molecule should exhibit more than a single mode. Possibility (a) must then be realized. However, the relative positions of the oxygen atoms and the NO molecules are still unclear. We repeated the experiment of Fig. 2 by postadsorbing NO at even lower T (50 K) and obtained virtually the same results: a single narrow vibrational mode (not quite as narrow as in Fig. 2) located at 1616cm -1. A rearrangement of the oxygen atoms therefore appears very improbable. More or less random movements of Oads by impact of post-adsorbing NO is unlikely as this would result in considerable layer heterogeneity, contrary to the observations. We conclude that the va (O)-NO species very likely is located in the residual hcp hollow sites between
two oxygen (2 × 1) rows, thereby minimizing repulsion to the neighboring oxygen atoms. The intermolecular distance of the NO molecules in such an arrangement is equal to the substrate lattice constant. The grossly unequal distance of the NO molecules along and perpendicular to the (2 × 1) oxygen rows suggest that a likewise anisotropy of the electronic band structure should exist, i.e. the system will display one-dimensional behavior. Unfortunately its detection is made difficult by the fact that three equivalent 0 ( 2 × 1) domains are in general formed on a flat Ru(001) surface and an ARUPS measurement will always average incoherently over different domain orientations. Low temperature scanning tunnelling spectroscopy would be helpful. Strictly speaking, the NO molecules will not experience the threefold symmetry expected for a molecule located at hcp hollow sites but rather only C s symmetry. This is because the coadsorbed oxygen atoms render the three Ru atoms of the hollow site inequivalent. The only symmetry element therefore will be a plane perpendicular to the 0 ( 2 × 1) rows. It is then possible that even in the absence of intermolecular interactions the NO molecules are tilted away from the surface normal. For strongly interacting molecules alternating tilts may prevail in order to release the repulsive strain between neighboring molecules; there may also be alternating shifts out of the exact threefold sites. In this case the overlayer structure would change from (2 × 1) to (2 × 2) which, however, is indistinguishable from a 3-domain (2 × 1) LEED pattern. A quantitative LEED-IVanalysis is necessary and is under way [ 11 ]. In the XPS spectra of the O Is peak (not shown) of the low T N O + O ( 2 × I ) / R u ( 0 0 1 ) phase we could identify only a single peak at 531.0 eV binding energy. It was larger and slightly broader than the corresponding peak of the 0.5 ML preadsorbed oxygen located at 530.7 eV. According to Umbach et al. [7], the O ls peaks of(triple-) bridge-bonded vl-NO and of Oads strongly overlap whereas linearly bound v2-NO displays a higher binding energy by about 1.5 eV. The interpretation of the 1616 cm -1 vibrational band as a threefold bridge bonded NO species therefore is corroborated by these XPS results. XPS spectra of the
P. Jakob et al./ Surface Science 370 (1997) L185-L192
(NO + 2 0 ) ( 2 x 2) honeycomb layer gave approximately a 1:2 ratio of the O ls peaks of v2-NO and adsorbed oxygen at 532.4 and 530.7 eV binding energy, respectively. This result is in good agreement with the structural models presented in this work. The interesting question as to why the earlier IRAS work [ 1] was not able to detect the Vl-NO species may be answered as follows. Similarities of the TDS traces in this work imply not too different adsorbate layers from ours. However, the generally broader features in the TDS and of the vibrational bands indicate a somewhat reduced layer quality in their work. We cannot judge whether this was related to crystal imperfection, surface contaminants and/or a less perfect layer ordering owing to a different preparation procedure. In the present work we did find that the threefold-bridge bonded NO was particularly sensitive to contaminants like hydrogen and oxygen, which broadened the lines drastically, by up to 100 cm -~. For dense layers (like NO adsorbed on the clean Ru(001) surface) such a broadening may occur whenever the contaminant leads to an upward shift of the vibrational frequency. Dynamic dipole coupling and intensity transfer from the main mode to the (blue shifted) contamination bands then enhance their intensities considerably [16]. In HREELS a line broadening of 50-100cm -~ does not seriously affect the spectra as long as the instrumental resolution is of the same order. A narrow, sharp line therefore will have a similar appearance as a broad and weak band of the same integrated intensity. In IRAS this is definitely not the case and detection of broad features suffering from layer inhomogeneity is difficult as these might be superimposed on an unstable baseline or other disturbances. The above mentioned contaminants would be correlated with line shifts of the order of 60 cm 1, which leads us to the conclusion that at least in the HREELS data of Ref. [2] no such contaminations were present. In our case, the much improved stability of the present setup has aided the detection of the various bridge-bonded NO species considerably. Our improved data quality, along with the knowledge of the NO + 2 0 honeycomb structure formed at elevated temperatures allows us to present a comprehensive reinterpretation of the
NO + 2 0 layer properties. Conrad et al. [_ j ....... uted the sole occurrence of va(O)-NO after NO adsorption onto the 0 ( 2 x 1)/Ru(001) layer at low T (while very little v2-NO is observed) to a preferential sticking and occupation of threefold-bridge sites. The energetically favored site, on-top bonded linear v2-NO, was assumed to be formed only after a thermally activated diffusion step of adsorbed NO. Such an immobile NO species at 120 K, however, appears questionable as the ability of chemisorbed molecular species to move a few sites even at very low temperatures has been demonstrated for CO on P t ( l l l ) at around 50 K [17]. The basic idea of our interpretation is that the arrangement of the oxygen atoms decides whether vl(O)-NO or v2-NO is energetically favored; the O-arrangement in turn depends on the NO concentration. The thermally activated step to form v2-NO in our model is related to molecular desorption of vI(O)-NO (thereby reducing the NO-coverage) and the correlated rearrangement of oxygen atoms and of NOads; actually, only one short jump per two oxygen atoms is needed to rearrange the 0 ( 2 x 1) layer into an 0 ( 2 × 2) honeycomb structure [12-14]. In the center of these hexagons there is a single on-top site that can accept one v2-NO per unit cell. No free sites are available thereafter, which naturally explains the zero sticking coefficient regarding NO postadsorption, SNo, reported by Conrad et al. [2] and confirmed in our experiments. In their model, site blocking of supposedly pre-existing vx(O)-sites by v2-NO has been invoked, i.e. v~(O)-NO would have formed if there would be a way to overcome the vanishing SNo. In contrast, our model states that there are no threefold-bridge sites available within the annealed NO + 20/Ru(001) layer. Any site blocking, be it for NO post-adsorption on 0 ( 2 × 1)/Ru(001) (blocking of on-top sites) or on the N O + 2 0 honeycomb structure (blocking of threefold hollow sites), must be attributed to the adsorbed oxygen. A clear explanation of all the observations can be given, once the oxygen layer is not considered static (acting just as a template without mutual interaction with coadsorbed NO) but as an integral part of the coadsorbate layer which adapts dynamically to the respective NO coverage conditions.
P. Jakob et al./ Surface Science 370 (1997) L185-L192
. . . . . . . . . . . . . ~,ement This work was supported by the Deutsche Forschungsgemeinschaft through SFB 338.
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[7] E. Umbach, S. Kulkarni, P. Feulner and D. Menzel, Surf. Sci. 88 (1979) 65. [8] P. Feulner, S. Kulkarni, E. Umbach and D. Menzel, Surf. Sci. 99 (1980) 489. [9] B.N.J. Persson, F.M. Hoffmann and R. Ryberg, Phys. Rev. B 34 (1986) 2266. [10] P. Jakob and D. Menzel, in preparation. [ 11 ] M. Stichler and D. Menzel, ICSOS-5 (Aix-en-Provence) abstract Fr 104P, to be published. [12] F.M. Hoffmann, M.D. Weisel and C.H.F. Peden, Surf. Sci. 253 (1991) 59. [13] K.L. Kostov, H. Rauscher and D. Menzel, Surf. Sci. 278 (1992) 62. [14] B. Narloch, G. Held and D. Menzel, Surf. Sci. 317 (1994) 131. [15] P. Feulner and D. Menzel, J. Vac. Sci. Technol. 17 (1980) 662. [16] B.N.J. Persson and F.M. Hoffmann, J. Electron Spectrosc. Relat. Phenom. 45 (1987) 215. [17] J.V. Nekrylova and I. Harrison, J. Chem. Phys. 101 (1994) 1730.