Vibrational analysis of Cs + CO coadsorbed on Ru(0001)

surface s c i e n c e ELSEVIER

Surface Science 352-354 (1996) 1-4

Vibrational analysis of Cs + CO coadsorbed on Ru(0001) Peimo He a, Yabo Xu b, K. Jacobi a,* a Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany b Physics Department, Zheijiang University, Hongzhou 310027, People's Republic of China

Received 5 September 1995; accepted for publication 31 October 1995

Abstract

On the Ru(0001) surface Cs and CO form a very well ordered (Cs + CO)-(2 X 2) compound layer whose structure was determined recently (Cs on-top, CO in 3-fold sites). Here we present a vibrational analysis of the same system using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and low-energy electron diffraction (LEED). Two C - O stretch frequencies are observed depending on whether there are one or two CO molecules in the 2 x 2 cell. Two CO-Ru stretch modes are observed for the first time and are assigned to adsorption in the hcp and fcc hollow sites within the 2 X 2 cell. They are very weak in intensity which is attributed to the threefold-hollow site and some screening in the 2D compound. With CO adsorption a change of the electronic structure is observed: the adlayer loses metallicity and the Cs-Ru stretch becomes visible. Strong changes of the Cs-Ru stretch energies are observed with CO coverage. Keywords: Carbon monoxide; Cesium; Chemisorption; Compound formation; Electron energy loss spectroscopy; Low index single crystal

surfaces The Cs + CO coadsorption system is very interesting because the structure o f the (Cs + CO)-(2 x 2) adlayer has been determined recently [1]. Fig. 1 shows a schematic top view o f the Cs-(2 X 2) adlayer on Ru(0001) the structure of which is also known from an independent study [2]. A s one can see, the 2 × 2 unit cell exhibits two adsorption sites o f high symmetry which are the hcp and fcc threefold-hollow sites. In the (Cs + CO)-(2 X 2) compound CO occupies these two sites giving rise to a 1-CO- or a 2-CO-phase if only the hcp-site or both sites are occupied, respectively [1]. Using high-resolution electron energy loss spectroscopy (HREELS)

* Corresponding author. Fax: +49 30 84135106; E-mail: [email protected].

we have been able to measure the complete set o f dipole active modes consisting of the internal stretch mode o f CO, the C O - R u mode, and the C s - R u mode. W e have found two C O - R u modes which we assign to the two different (hcp and fcc) threefoldhollow sites. Two different C - O stretch m o d e s are found due to the occupation o f the 2 X 2 unit cell by one or two CO molecules. The C s - R u mode shows characteristic differences for the three phases (Cs-(2 x 2), (Cs + CO)-(2 X 2), and (Cs + 2CO)-(2 × 2)). The experimental set up is described elsewhere [3]. H R E E L spectra were taken at a 60 ° angle of incidence with respect to the surface normal and in a specular geometry if not noted otherwise. The primary energy was 1.5 eV and the energy resolution was set to 3.8 meV. Cs was evaporated f r o m a break seal ampoule. Thermal desorption spectroscopy

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P. He et al./ Surface Science 352-354

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(IDS) was used for determination of CO coverage f3CO’ In the following the HREEL spectra will be presented depending on oco which was determined independently by IDS measurements. The IDS signals were calibrated with the help of the Ru(OOO1) CO structure which is known to consist of one CO molecule per 3 Ru surface atoms. The maximum CO coverage found for the Ru(OOOl)-Cs-(2 X 2) surface is two CO molecules per 2 X 2 unit cell. We define this value of two CO molecules per four Ru surface atoms as 6,, = 1.O throughout this contribution. Fig. 2 exhibits a set of HREEL spectra. Between 160 and 240 meV the CO intramolecular stretch vibration is observed. For small 8co there is only one frequency which is 155 meV in the limit 8c, + 0. For &-, > 0.22 two losses are observed. In the spectrum for 8,, = 0.31 and T = 300 K a shoulder at higher binding energy is clearly visible besides the main loss. Following the LEED structure analysis we assign these two losses to CO with one CO molecule and two CO molecules in the Cs-(2 X 2) unit cell. We will call this the l-CO- and 2-CO-phases in the following. We assign the single frequency at small 0co to the l-CO-phase. The regime, where two frequencies are observed, starts already at about 19, = 0.22, i.e., much earlier than at 8,, = 0.5. The latter value is expected for a situation when first a l-CO-phase is formed and later the 2-CO-phase. Quite obviously there is an overlapping regime, i.e., the 2-CO-phase starts forming much earlier than the

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RI, 2. layer RI, 3. Layer hcp /site Fig. 1. Schematic top view of the C&2X 2>Ru(OOOl) surface. The two different threefold-hollow sites in the 2 X 2 unit cell arc indicated.

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LOSS ENERGY (meV) Fig. 2. HREEL spectra for CO (relative coverage 6’,> adsorbed on the Ru(OOOl)-&(2X 2) surface for two different sample temperatures. The primary energy is indicated.

l-CO-phase has been completed. For CO saturation at Go = 1 one single loss remains at 204 meV. This shows that the CO stretch mode frequency is not influenced by the difference between the hcp and fee hollow site. If there is an influence it is smaller than about 1 meV. The most striking result is the very weak intensity of the CO-Ru stretch mode (see Fig. 2, loss energies between 40 and 60 meV). This is contrary to the CO/Ru(OOOl) system for which the intensity of this mode is as high as for the intramolecular stretch mode [4,5]. Up to a coverage of 6co = 0.33, CO is adsorbed at an on-top site. This is totally different to the Cs + CO compound system in which CO is adsorbed on the threefold-hollow site. The weak intensity of the CO-Ru mode may be due to (a) the shift to the hollow site or (b) to some screening by the neighbouring Cs atoms. We have also used the impact mode in electron scattering to analyze the CO-Ru stretch mode re-

P. He et a l . / Surface Science 352-354 (1996) 1-4

gion. In Fig. 3 we compare the results for specular and off-specular geometry. Actually we observe two different losses for the C O - R u stretch mode which we assign to the hcp and fcc sites. According to the L E E D structure analysis the hcp site is occupied first and we assign the higher lying loss to the hcp site. In agreement with the appearance o f a second line in the C - O stretch mode signal the fcc-site loss is observed only for 0co > 0.2. A s can be seen from Fig. 3 the two losses differ in character; whereas the hcp-site loss is d i p o l e active the fcc-site one is not. W e have already shown that we can resolve the C s - R u vibration Uc~ at about 8 - 1 0 m e V with our spectrometer [6,7]. The C s - R u vibration is observed if the Cs atom is somewhat positively charged which is the case at submonolayer coverages or within a compound layer, i.e., in coadsorption with an electronegative species. I f the Cs monolayer b e c o m e s

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LOSS ENERGY (meV) Fig. 4. HREEL spectra for the CO/Rn(0001)-Cs-(2×2) system in the loss-energy region of the Cs-Ru vibrational mode. Parameter is the relative CO coverage 0co. The sample temperature was 300 K.

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LOSS ENERGY (meV) Fig. 3. HREEL spectra for CO (relative coverage 0co) adsorbed on the Ru(0001)-Cs-(2×2) surface. The measurements were performed at 85 K. The open symbols are for specular detection (dipole scattering, factor 1000); the crosses are 10° off-specular (impact scattering, factor 50). The factor for the 6° off-specular geometry is 400.

metallic, the C s - R u mode Ucs cannot be excited since the delocalized charge screens out any dipole fluctuation. A n y interaction with chemically active gases leads to a reappearance o f the Vc~ mode. In this contribution we have studied the latter effect in detail for the coadsorpti0n with CO. Typical spectra from the low loss-energy region are shown in Fig. 4. The Uc~ mode becomes visible for Oco > 0.2 at which coverage also the CO stretch mode has become observable. The delocalized charge o f the Cs layer screens every dipole-active vibrational mode including the C s - R u mode for coverages 0co < 0.2. The energy o f the C s - R u mode is goes to a m a x i m u m of 10.0 m e V at about 0co = 0.34. This value at is characteristic for the 1-CO-phase as con, cluded from the CO stretch mode results of Fig. 2. At 0co = 1.0 we have only the 2-CO-phase and ~'c~ = 9.0 meV. The intensity o f the R u - C s vibration increases up to 0co - 1.0.

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P. He et al./Surface Science 352-354 (1996) 1 - 4

Our results and conclusions can be summarized as follows: Using HREELS we collected a complete set of the main (dipole active) vibrational modes of the well ordered (Cs + CO)-(2 × 2) 2D compound layer. We also studied the evolution of the vibrational spectra with CO coverage. The different vibrational modes exhibit characteristic changes in energy and intensity with coverage.

completing the 2-CO-phase it decreases again to 9.0 meV. The bond of Cs to the Ru surface is strengthened by the interaction with CO. Per single CO molecule this strengthening is larger in the 1-COphase. In the 2-CO-phase the lateral bond network seems to weaken the individual Cs bond to the Ru substrate.

3. The C O - R u mode 1. The intermolecular CO stretch mode For 0co ~ 0 there is one single loss peak observed at 155 meV. This energy is strongly reduced compared to the gas phase (265 meV) or to chemisorbed CO (0co ~ 0) on Ru(0001) (245 meV). The Cs atoms lower the local potential so that strong backdonation into the 2 7 * orbital becomes possible. For 0co ~ 1 the loss energy goes to 183 meV. For 0co >__0.22 a second frequency is observed varying from 177 to 204 meV. We assign the two loss energies to the I-CO- and 2-CO-phases. The occurrence of two frequencies at a given 0co value is a clear indication that the interaction of CO with Cs and Ru is basically a local process. The strong change in energy, which is observed for both frequencies with coverage, indicates that the backdonated charge per individual molecule is reduced with coverage. This is the non-local part of the interaction between Cs, CO, and Ru. For 0co = 1 there remains only one line at 204 meV which means that the difference of the two adsorption sites (hcp and fcc threefold hollow) influences the intramolecular stretch mode energy by less than 1 meV. From modelling of the CO stretch mode intensities we conclude the following: For 0co < 0.22 the adlayer is mobile to such a degree that only the 1-CO-phase grows. At about Oco = 0.22 the adlayer becomes immobile so that the 2-CO-phase grows in addition. This finding differs from the interpretation of the LEED result in terms of a sequential formation of the 1-CO-phase and the 2-CO-phases [1].

2. The C s - R u mode The C s - R u mode becomes observable for 0co > 0.22 and increases to 10.0 meV at 0co = 0.3 where the 1-CO-phase prevails. For 0co ~ 1.0, i.e., in

The ratio of the C - O and the C O - R u stretch mode intensities is about 20, i.e., larger than any ratio found for CO species adsorbed on different sites at a given surface. We argue that both the weak dipole moment at the threefold-hollow sites and some screening in the compound layer explain the large ratio. We observe for the first time different frequencies for the different threefold-hollow sites. Finally, we note that the CO bonding has both local and non-local character. The observation of two C - O stretch mode frequencies visualizes the local nature of the interaction between Cs, CO, and Ru. The smooth changes of all the modes with 0co, on the other hand, indicate some non-local character. Acknowledgements The support of Professor G. Erfl and the technical assistance of P. Geng and M. Richard are gratefully acknowledged. We thank H. Bludau, H. Dietrich and Dr. H. Over for fruitful discussions. P. He is grateful to the Max-Planck-Gesellschaft for a postdoctoral fellowship. Y. Xu thanks the Alexander-vonHumboldt Stiftung for a stipend. References [1] H. Over, H. Bludan, R. Kose and G. Ertl, Surf. Sci. 331-333 (1995) 62. [2] H. Over, H. Bludau, M. Skottke-Klein, G. Ertl, W. Moritz and C.T. Campbell, Phys. Rev. B 45 (1992) 8638. [3] H. Shi, K. Ja¢obi and G. Ertl, Surf. Sci. 269//270 (1992) 682. [4] G.E. Thomas and W.H. Weinberg, J. Chem. Phys. 70 (1970) 1437. [5] P. He, H. Dietrich and K. Jacobi, Surf. Sci. 345 (1996) 241. [6] K. Jacobi, H, Shi, M. Gruyters and G. Ertl, Phys. Rev. B 49 (1994) 5733. [7] K. Jacobi, H. Shi, H. Dietrich and G. Ertl, Surf. Sci. 331-333 (1995) 69.