Adsorption Structures of Ethylene on Ag (110) and Atomic Oxygen

added Ag-O rows on p(2×1)O-Ag(110) is too narrow to deliver the adsorption sites for the R ..... the closed-packed silver rows along the 〈11h0〉 d...
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J. Phys. Chem. B 1999, 103, 10189-10196

10189

Adsorption Structures of Ethylene on Ag(110) and Atomic Oxygen Precovered Ag(110) Surfaces: Infrared Reflection-Absorption and Thermal Desorption Spectroscopic Studies Masato Akita, Shuji Hiramoto, Naoki Osaka, and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda UniVersity, Shinjuku-ku, Tokyo 169-8555, Japan ReceiVed: June 18, 1999; In Final Form: September 15, 1999

Infrared reflection-absorption spectra in the CH2 out-of-plane wagging (ω(CH2)) vibration region were measured for ethylene (C2H4) adsorbed on Ag(110) as well as on the oxygen-induced p(n×1) reconstructed surfaces of Ag(110) (n ) 6, 4, 3, and 2) at 80 K. C2H4 on Ag(110) gives a main peak at 955 cm-1, whereas C2H4 on p(n×1)O-Ag(110) (n ) 6, 4, 3) gives rise to a 972-976 cm-1 band (R-state) at low exposures, shifting it to 966-970 cm-1 (β-state) at saturation coverage. The adsorption behavior of C2H4 on the p(n×1) surfaces (n ) 6, 4, 3) are explained by assuming that (i) adsorption sites exist between the added Ag-O rows parallel to the 〈001〉 direction; (ii) adsorption sites on both sides of the added Ag-O row form a special pair; (iii) at lower coverages one of the pair is selectively occupied, resulting in the formation of the R state. At higher coverages, where all the sites for the R state are occupied, C2H4 begins to occupy the other site of the pair, forming the β state. Thermal desorption spectra were measured for C2H4 on Ag(110) as well as on the atomic oxygen reconstructed surfaces. The desorption on Ag(110) consists of a state with a peak temperature ) 110 K, whereas those on p(n×1)O-Ag(110) (n ) 6, 4, 3) consist of two states, corroborating the adsorption model on these surfaces derived from the IR spectra. The desorption temperatures at the R states are found to increase as follows: 130 K (p(6×1)) < 145 K (p(4×1)) < 160 K (p(3×1)), which indicates that the stability of the R states increases with the surface coverage of the atomic oxygen. C2H4 on p(2×1)O-Ag(110) does not take either the R or the β state, but exists in an irregular state, giving a broad feature centered at 970 cm-1 for the ω(CH2) band region. This can be explained by considering that the space between the added Ag-O rows on p(2×1)O-Ag(110) is too narrow to deliver the adsorption sites for the R and β states.

Introduction The adsorption structures of ethylene on Ag(110) and its atomic oxygen precovered surfaces have been studied by using various methods including temperature desorption spectroscopy (TDS),1,2,3 low-energy electron diffraction (LEED),1,3,4 highresolution electron energy loss spectroscopy (HREELS),5,6 infrared reflection-absorption spectroscopy (IRAS),7 near-edge X-ray absorption fine-structure measurement,8 and work function measurement.1 These studies have indicated that (i) ethylene adsorbs on the surfaces through a weak π-bonding with the molecular plane nearly parallel to the surfaces, (ii) the adsorption is strongly promoted by the presence of the adsorbed oxygen, and (iii) the adsorption takes place selectively at silver atoms on which a positive charge has been induced by the oxygen atom. Despite these studies, the adsorption sites and the effect of the atomic oxygen on the adsorption behavior of ethylene at the atomic oxygen precovered Ag(110) surfaces have not been clarified yet. It has been known that, when an Ag(110) surface is exposed to increasing amounts of O2 at room temperature, the surface gives a series of reconstructed p(n×1) surface structures with decreasing n from 7 to 2.9 STM studies have demonstrated that the reconstruction proceeds through the formation of the ordered arrangements of the added Ag-O rows parallel to the 〈001〉 direction.10 In the present paper, we applied IRAS and TDS spectroscopies to elucidate the adsorption sites and structures of ethylene on Ag(110) as well as on a series of the reconstructed surfaces, p(n×1)O-Ag(110) (n ) 2, 3, 4, 6). A preliminary

result of this study has already been published in a separate paper.7 Experimental Section Materials. Ethylene (99.9% purity) was purchased from Takachiho Chemicals Co. Ltd. and used without further purification. Preparation of Substrates. A Ag(110) single crystal (99.999%, 15 mm (φ) × 1 mm) was perchased from Techno Chemics, Inc. The surface was cleaned by Ar sputtering (1 µA/cm2, 500 eV, 15 min at 300 K) and annealing at 700 K. Residual surface carbon was removed by adsorption/desorption cycles of O2. A series of the reconstructed p(n×1) O-Ag(110) surfaces (n ) 2, 3, 4, 6) were prepared by exposing the cleaned Ag(110) surface to appropriate amounts of oxygen. The substrate temperature was kept at 350 K for the preparation of p(3×1) and p(4×1) surfaces and at a room temperature for the preparation of p(2×1) and p(6×1) surfaces. The formation of the reconstructed surfaces were confirmed by observing the anticipated LEED patterns. Measurement of IR Spectra. IR spectral measurements were performed by using an apparatus schematically shown in Figure 1; the apparatus consists of a load-lock chamber and two ultrahigh vacuum (UHV) chambers; one of the UHV chambers containing a four-grid retarding field AES/LEED optics and a quadrupole mass spectrometer was used for preparing the clean and reconstructed Ag(110) surfaces as well as for measuring TDS spectra; the other UHV chamber containing an Fourier

10.1021/jp992025u CCC: $18.00 © 1999 American Chemical Society Published on Web 10/30/1999

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Figure 1. Diagram of the experimental setup used in this paper.

transform infrared absorption spectrometer (Bruker model 66v/ S) was used for IRAS measurements. The temperature of substrates used for IRAS measurements was cooled to 80 K by liquid nitrogen and varied in the 80-1000 K region by a resistive heating method. The base pressure of the two chambers was below 1 × 10-10 Torr. IRAS spectra were recorded in a single reflection mode at an angle of incidence of 85° with a liquid nitrogen cooled MCT detector in the 4000-600 cm-1 region by adding 300 scans at the resolution of 4 cm-1. Ethylene was dosed to precooled substrates at 80 K through a 1/8 in. stainless tube by using a pulse doser. The intensities of the IR bands were found to saturate at a certain amount of exposures to each substrate. Since the exact surface coverage for each spectrum has not been determined, it was assumed that the adsorbate forms a saturation coverage at this state. Measurement of TDS Spectra. TDS spectra were recorded by using a quadrupole mass spectrometer with a heating rate of 1 K/s. The temperature of the Ag(110) substrates, which are the same as that used for the IRAS measurements, was monitored by using a Ni-CrNi thermocouple attached to the backside of the substrates. Results The lower traces of Figures 2 and 3 illustrate the IR spectral changes in the 1020-900 cm-1 region measured for ethylene on Ag(110) and its reconstructed surfaces at 80 K with

increasing exposure up to a saturation level. The upper traces of Figures 2 and 3 exhibit the spectral changes observed by increasing the substrate temperature from 80 K to a desorption temperature around 150 K. Ethylene in the gaseous state gives two CH2 out-of-plane wagging modes (ω(CH2)) at 949 cm-1 (ν7, b1u, IR active) and 943 cm-1 (ν8, b2g, Raman active)11 in the frequency region of Figures 2 and 3. The main features above 950 cm-1 in these figures correspond to the ν7 mode, and the frequency increase of the ω(CH2) band compared to that for the gaseous state suggests that ethylene adsorbs on each substrate through a weak π-bonding interaction. The spectra observed for ethylene on each substrate above 900 cm-1 do not show any bands except for those in the ω(CH2) band region; the absence of in-plane modes in the spectra suggests that the ethylene adsorbs with the molecular plane parallel to the surfaces, as already concluded by Backx et al.5,6 All the spectra in Figures 2 and 3 except for those in Figure 2A give negative bands near 952 cm-1 at lower exposures (the lower traces) and at later steps of desorption (the upper traces). The results suggests the existence of a preadsorbed species which desorbs at the onset of exposure of ethylene to the substrates. This species has not been identified yet. Since the species exists as a minor component, we neglect the negative bands in the following discussion. Spectra of Ethylene on Ag(110). The lower traces of Figure 2A indicate that upon exposing ethylene to Ag(110) the

Adsorption Structures of Ethylene on Ag(110)

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Figure 2. (Lower traces) IR spectral changes in the 1020-900 cm-1 region observed at 80 K for ethylene exposed with increasing amounts to (A) Ag(110), (B) an atomic oxygen precovered Ag(110) (oxygen coverage is less than 0.1), and (C) p(6×1)O-Ag(110). (Upper traces) The IR spectral changes during the temperature increase from 80 K (saturation state) to a desorption temperature around 150 K observed for each substrate.

adsorbate at first gives a ω(CH2) band at 964 cm-1, which saturates at a relatively low exposure, and then gives a band at 955 cm-1; on further increase of exposures, the latter band increases in intensity and dominates at the saturation coverage. The 964 cm-1 band may be ascribed to an adsorbate bound at a defect site (adatom, kink and/or step sites); it is often observed that an adsorbate is preferentially trapped by those minority sites.12,13 As can be seen from the upper traces of Figure 2A, upon increasing the substrate temperature, the desorption first takes place at the main site giving the 955 cm-1 band and then at the defect site giving the 964 cm-1 band. Comparison between the IR Spectra of Ethylene on an Atomic Oxygen Precovered Ag(110) Surface with the Oxygen Coverage Smaller Than 0.1 and the IR Spectra of Ethylene on the p(6×1) O-Ag(110) Surface. The substrate of Figure 2B was prepared by exposing an Ag(110) surface to a smaller amount of oxygen than that employed for the preparation of the p(6×1)O-Ag(110) surface. The coverage (θ) of the atomic oxygen on the surface is estimated to be less than 0.1 by assuming θ ) 0.17 for the p(6×1) surface. The lower traces of Figure 2B indicate that the adsorbate gives the ω(CH2) bands at 984, 973, 964, and 955 cm-1 successively with increasing exposures. The electron-withdrawing nature of the atomic oxygen on the silver surface causes induction of positive charges on nearby Ag atoms, resulting in the stabilization of the π-coordination interaction of ethylene with the Ag atoms. It can be postulated that the larger the stabilization effect, the larger the frequency increase of the ω(CH2) band relative to that of the gaseous sample (949 cm-1, vide supra). The 955 cm-1 band, which appears at later steps of exposure corresponds

to that observed for Ag(110) (Figure 2A), indicating that the band can be assigned to an adsorbate at a site on which the atomic oxygen does not exert any influence. Presumably, the 984 cm-1 band in Figure 2B (the lower traces), which saturates at an early stage of exposure, is a counterpart of the 964 cm-1 in Figure 2A and assigned to an adsorbate trapped by a defect site, on which positive charge has been induced by the oxygen atom. A closer inspection of Figure 2B indicates the following facts. (i) The 973 cm-1 band maximizes its intensity at a certain amount of exposures. (ii) Upon further exposure there appears the 964 cm-1 band. (iii) When the 964 cm-1 band maximizes its intensity, the 973 cm-1 band disappears almost completely. (iv) The ratio of the maximum intensity of the 973 cm-1 band to that of the 964 cm-1 one is approximately 1:2.14 The 973 and 964 cm-1 bands are assigned to adsorbates on the silver atoms positively charged by the atomic oxygen. For clarity of the following discussion, the states associated with the 973 and 964 cm-1 bands will be called as “R” and “β states”, respectively, and that associated with the 955 cm-1 band as a “γ state”. From the facts (i-iii), it is clear that the R state is replaced by the β state during the exposure. Comparison of the upper traces of Figure 2B with the lower ones indicates that the spectral changes observed by increasing exposures at 80 K are reversed almost completely by the spectral changes during the desorption process. That is, when the substrate temperature is raised from 80 to 150 K, intensity decrease takes place successively for the IR bands at 955, 964, 973, and 984 cm-1. These results prove the existence of discrete adsorption states with the following order of stability (or the desorption temperature): γ state (955 cm-1) < β state (964

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Figure 3. (Lower traces) IR spectral changes in the 1020-900 cm-1 region observed at 80 K for ethylene exposed with increasing amounts to (A) p(4×1)O-Ag(110), (B) p(3×1)O-Ag(110), and (C) p(2×1)O-Ag(110). (Upper traces) IR spectral changes during the temperature increase from 80 K (saturation state) to a desorption temperature around 150 K observed for the adsorbate on each substrate.

cm-1) < R state (973 cm-1) < a trapped state (984 cm-1). As explained later, the order of stability conforms to the result of TDS spectral measurements. The lower traces of Figure 2B indicate that a band centered at 930 cm-1 appears concomitantly with the 964 cm-1 band. The former band may be ascribed to a ω(CH2) mode, which corresponds to an IR-inactive one (ν8, b2g) observed at 943 cm-1 in the gaseous state. As discussed in detail later, this may be ascribed to lowering of a site symmetry of the adsorbate on the substrate. As can be seen from Figure 2C, the spectral changes, caused by increasing both exposure and temperature on p(6×1)O-Ag(110), are similar to the corresponding changes in Figure 2B, except that the spectra in Figure 2C do not give any band at 955 cm-1 assigned to the γ state (the adsorption state, which is not affected by the atomic oxygen.) The 986 cm-1 band, which appears first in the lower traces of Figure 2C, is a counterpart of the 984 cm-1 band in Figure 2B, which was assigned to the adsorbate trapped at a defect site. The 972 and 966 cm-1 bands in the lower traces, which correspond to the 973 and 964 cm-1 bands in Figure 2B, are ascribed to the R and β states, respectively. It is clear from Figure 2C that, as the exposure increases, the R state is replaced by the β state. In addition, the ratio of the maximum intensity of the 972 cm-1 band to that of the 966 cm-1 band is approximately 1:2, as observed for the adsorbate on the substrate of Figure 2B. The spectral changes during the desorption process (the upper traces of Figure 2C) indicate that upon increasing the substrate temperature from 80 to 150 K the desorption takes place in the following order: β state (966 cm-1) f R state (972 cm-1) f trapped state (986

cm-1). The absence of the γ state on the p(6×1) surface suggests that the coverage of the atomic oxygen is too large to deliver Ag atoms as an adsorption site, which is not affected by the oxygen atom (the γ state). The 943 and 930 cm-1 bands in the lower traces of Figure 2C appear concomitantly with the 972 and 966 cm-1 bands; the result indicates that the 943 and 930 cm-1 bands are also ascribable to the R and β states, respectively. As in the case of the 930 cm-1 band in Figure 2B, these bands may be ascribed to an originally IR-inactive band (ν8, b2g), which appears as a result of the site-symmetry lowering (vide infra). IR Spectra of Ethylene on p(n×1)O-Ag(110) Surfaces (n ) 4, 3, 2). The lower traces of Figure 3, parts A, B, and C, illustrate the spectral changes observed for the adsorbate on the p(4×1), p(3×1), and p(2×1) surfaces with increasing exposures to saturation levels at 80 K, and the upper traces show the changes observed during desorption process in the temperature range from 80 to about 150 K. The coverage dependence of the spectra observed for the p(4×1)O-Ag(110) and p(3×1)OAg(110) surfaces (the lower traces of parts A and B of Figure 3) exhibits features similar to those observed for the p(6 × 1)OAg(110) surface (the lower traces of Figure 2C) in the following points: (i) the adsorbates on the p(4×1) and p(3×1) surfaces give rise to a 986 cm-1 band at low exposures, which is the counterpart of the 986 cm-1 band in Figure 2C and assigned to the trapped species by a defect site; (ii) on further increase of exposures, the adsorbates on the p(4×1) and p(3×1) surfaces exhibit spectral changes, giving first a 972-976 cm-1 band and shifting it to 968-970 cm-1 at a saturation coverage; (iii) the intensity ratio of the 976 to 970 cm-1 band at each saturation

Adsorption Structures of Ethylene on Ag(110) state for ethylene on the p(3×1) surface (Figure 3B) is approximately 1:2, as in the case of the p(6×1) surface (Figure 2C). These common features suggest that the adsorbates on the p(4×1) and p(3×1) surfaces take on adsorption states similar to those (R and β states) on the p(6×1) surface; that is, the 972 and 976 cm-1 bands observed for the p(4×1) and p(3×1) surfaces, respectively, are due to the R state, and the 968 and 970 cm-1 bands for the p(4×1) and p(3×1) surfaces due to the β state. In closer inspection, however, we recognize several discrepancies among the spectral changes. The frequencies of the R and β states depend on the substrates. The ω(CH2) band for ethylene on the p(4×1) surface first appears at 972 cm-1 and gradually shifts to 968 cm-1 with increasing exposures (Figure 3A). On the other hand, ethylene on the p(6×1) and p(3×1) surfaces gives first the higher frequency band (972 cm-1 for p(6×1) and 976 cm-1 for (p(3×1)), which is replaced by the lower frequency band (966 cm-1 for p(6×1) and 970 cm-1 for (p(3×1)) on further exposure; the intensity ratio of the higher to the lower frequency band is about 1:2. The adsorbate on the p(4×1) surface gives rise to 943 and 930 cm-1 bands, which are the counterparts of the bands observed at the same frequencies for the p(6×1) surface (Figure 2C), while the adsorbate on the p(3×1) surface does not give any bands corresponding to the 943 and 930 cm-1 bands. These discrepancies suggest that there exist some differences in adsorption modes among the R and β states on the p(n×1) surfaces (n ) 6, 4, and 3). Then, hereafter we call the R states on the p(6×1), p(4×1), and p(3×1) surfaces as R(6), R(4), and R(3) states, respectively, and the corresponding β states as β(6), β(4), and β(3) states. Comparing the lower traces with the upper ones in Figure 3, parts A and B, we recognize that the spectral changes observed by increasing exposures are completely reversed by the spectral changes observed by raising the substrate temperature. Then, as in the case of the adsorbate on the p(6 × 1) surface (Figure 2C), the desorption takes place in the following order; β(4,3)state f R(4,3)-state f trapped state. As Figure 3C shows, the spectral changes observed for the adsorption and desorption processes of ethylene on p(2×1)OAg(110) are quite different from those observed for the other substrates. The adsorbate at first gives a band near 984 cm-1. Upon increasing exposures, the adsorbate gives rise to a broad band centered at 970 cm-1. Near saturation coverage the adsorbate gives another broad band at 997 cm-1 in addition to the 984 and 970 cm-1 bands. Presumably, the 954 cm-1 band in Figure 3C is due to an artifact, which arises from superposition of the 970 cm-1 band and the negative feature around 955 cm-1 (vide infra). The 984 cm-1 band may be ascribed to an adsorbate trapped by a defect site. The broad features around 997 and 970 cm-1 suggest that the adsorbate on the p(2×1)OAg(110) surface takes on an irregular adsorption state in contrast to the cases on the p(n×1)O-Ag(110) (n ) 6, 4, 3) surfaces. This is consistent with the results of TDS spectral observation explained below. The upper traces of Figure 3C indicate that the spectral changes observed by raising the substrate temperature are completely reversed by the spectral changes observed by increasing exposures. Then, as in the cases of the adsorbates on the other reconstructed surfaces, the desorption on the p(2×1) surface proceeds in a reverse order of the adsorption process. TDS of Ethylene on Ag(110) and p(n×1)O-Ag(110) (n ) 6, 4, 3, 2). Figure 4 exhibits the TDS spectra of ethylene adsorbed on Ag(110) and its atomic oxygen reconstructed surfaces, p(n×1)O-Ag(110) (n ) 6, 4, 3, 2). The thick solid lines for the Ag(110) and p(2×1)O-Ag(110) surfaces show the

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Figure 4. Thermal desorption spectra (TDS) of ethylene adsorbed on Ag(110) and its atomic oxygen reconstructed surfaces. The heating rate was 1 K/s and mass 28 was monitored. Each solid thick line is the TDS spectrum observed for ethylene adsorbed at a saturation level (80 K) on each substrate. Each thin solid line of the TDS spectra for p(n×1)O-Ag(110) (n ) 6, 4, 3) was recorded for the adsorbate which gave the intensity maximum for the IR bands due to the R(n) states (n ) 6, 4, 3). Each dotted line exhibits a difference spectrum calculated by subtracting the thin solid line from the thick solid line.(see text)

spectra of ethylene adsorbed at 80 K on the surfaces at saturation levels. The thick solid lines of the spectra observed for the p(n×1) surfaces (n ) 6, 4, 3) were recorded on the adsorbates which gave the intensity maxima for the IR bands associated with the β(n) states (n ) 6, 4, 3) at 80 K, whereas the thin solid lines were recorded on the adsorbates which gave the intensity maxima for the IR bands associated with the R(n) states (n ) 6, 4, 3) at 80 K. The dotted lines show the calculated spectra obtained by subtraction of the thin solid lines from the thick solid ones. The TDS measurements on the p(n×1)O-Ag(110) surfaces (n ) 6, 4, 3) demonstrate that the desorption of the adsorbates at saturation levels consists of two states; one of the states corresponds to the dotted lines with peak temperatures of 110120 K and the other to the thin solid lines with peak temperatures of 130-160 K. The results of the IR spectral measurements on the p(n×1) surfaces (n ) 6, 4, 3) indicate that upon raising the substrate temperature the desorption first takes place at the β state and then at the R state. Therefore, the TDS peaks at 110-120 K are associated with the β states and those at 130-160 K with the R states. It should be noted that the desorption temperatures of the R states increase on the order of 130 K (p(6×1)) < 145 K (p(4×1)) < 160 K (p(3×1)), as can be seen from Figure 4. This result indicates that the stability of the R states increases with the surface coverage of the atomic oxygen. On the other hand, the desorption temperatures of the β states do not change explicitly, although a slight increase with the oxygen coverage is recognized.

10194 J. Phys. Chem. B, Vol. 103, No. 46, 1999 The TDS spectrum of ethylene on Ag(110) (Figure 4) consists of a single peak at about 110 K, which is ascribable to the desorption at the γ state. The TDS spectrum of ethylene on the p(2×1) surface gives a broad feature accompanying a peak at 100 K. The broadness of the TDS peak conforms to the results of IR spectral measurement (Figure 3C), which suggest that the adsorbate exists in an irregular state. Discussion The IR and TDS spectra of ethylene on Ag(110) prove that the adsorbate takes on mainly a single state (the γ state). According to Kru¨ger and Benndorf,1 a saturation coverage state of ethylene on Ag(110) at 95 K exhibits a LEED pattern ascribable to a c(2×2) overlayer structure. The grooves between the closed-packed silver rows along the 〈11h0〉 directions on Ag(110) are too narrow to accommodate ethylene with the parallel orientation. So, we tentatively conclude that the adsorbate sits directly on the closed-packed row with the molecular plane parallel to the surface. This kind of adsorption has been proved for ethylene on Cu(110) by STM observation.15 The IR and TDS spectral measurements demonstrate that ethylene on p(n×1)O-Ag(110) (n ) 3, 4, 6) takes on two kinds of adsorption states, R(n) and β(n) states (n ) 3, 4, 6). If the adsorption sites of ethylene in the R and β states exist on the Ag atoms forming the Ag-O added rows, the intensity ratio of the IR bands associated with the R(3), R(4), and R(6) states and that of the β(3), β(4), and β(6) states should be equal to the ratio of the surface coverages of the atomic oxygen on the p(3×1), p(4×1), and p(6×1) surfaces, which is 4:3:2 (see Figure 5B). This is not the case, because, as Figures 2C and 3, parts A and B, show, the intensities of the IR bands due to the R(3,4,6) states at saturation levels are similar to each other and the same is true for the bands due to the β(3,4,6) states. Thus, the adsorption sites do not exist on the Ag atoms forming the added Ag-O rows but on the Ag atoms, which are located between the added rows. As Figure 5A shows, Ag atoms forming the adsorption sites can be classified as Ag1 and Ag2; i.e., Ag1 exists in a row parallel to the 〈001〉 direction, which is the nearest to the added Ag-O row, and Ag2 in a neighboring row, which is the second nearest to the added row. Ag1 forms the adsorption site on both the p(3×1) and p(4×1) surfaces. As for the p(6×1) surface, Ag1 is more favorable than Ag2 for adsorption, because the positive charge of Ag1 induced by the atomic oxygen should be larger than that of Ag2 due to the proximity of Ag1 to the oxygen. Figure 5C schematically shows the p(n×1) surfaces (n ) 3, 4, 6), where vertical bars exhibit the Ag-O added rows. In the figure, dotted squares represent adsorption sites on the surfaces. Upon exposure to the surfaces ethylene first occupies one of the sites (a hatched square in Figure 5C), forming the R states. Although the similarity in frequency of the ω(CH2) bands due to the R states, i.e., 972 cm-1 (R(6)), 972 cm-1 (R(4)), and 976 cm-1 (R(6)), suggests that all the states take a common mode of interaction with the surfaces, it is not clear whether the R states take on an on-top site or a bridge site. STM study should be done to elucidate this point. In the following, however, we tentatively conclude that they take the on-top site, because, if they take the bridge site, the frequency increase of the ω(CH2) bands relative to the gaseous state should be much larger than the observed ones, i.e., 23 cm-1 (R(6)), 23 cm-1 (R(4)), and 27 cm-1 (R(6)). The positive charge on the Ag1 atom induced by the atomic oxygen increases as the oxygen coverage increases. This means that the π-bonding interaction of each adsorption state increases in the following order, R(6) < R(4)

Akita et al. < R(3), which is corroborated by the results of the TDS measurement. As Figure 4 shows, the desorption temperatures associated with the R states increase in the following order; 130 K (p(6×1)) < 145 K (p(4×1)) < 160 K (p(3×1)). The on-top coordination of ethylene on an isolated Ag atom may result in the C2V site symmetry of the adsorbate. As can be seen from Figure 5C, the R(3) state can keep the C2V site symmetry, since the corresponding adsorption site occupies a symmetric position with respect to the neighboring added Ag-O rows. On the other hand, the site symmetry of the R(4) and R(6) states is reduced to a lower one, because the sites for these states occupy asymmetric positions with respect to the neighboring Ag-O rows (see Figure 5C). The lowering of the site symmetry of the R(4) and R(6) states relative to C2V of the R(3) state may explain the results of the IR spectral measurements, which indicate that the R(4) and R(6) states give rise to the IR band at 943 cm-1 in addition to the 972 cm-1 band, while the R(3)-state gives only the 976 cm-1 band. As the exposure to the p(n×1) surfaces (n ) 3, 4, 6) increases, ethylene occupies the adsorption sites successively, keeping the separation between the occupied sites as large as possible. This is mainly due to a steric repulsion between the adsorbates. At certain exposure levels, the ω(CH2) band intensities due to the R states maximize, and upon further exposure the bands are replaced by the ω(CH2) bands associated with the β states. As for the p(6×1) and p(3×1) surfaces, the ratio of the maximum intensities of the ω(CH2) bands due to the R states to those due to the β states are roughly 1:2. These results can be explained by the following assumptions. (i) The adsorption sites on both sides of the Ag-O added row form a special pair. (ii) At lower exposures ethylene occupies selectively one of the paired sites, keeping the other site unoccupied. The adsorption at this stage forms the R states. (iii) After all the sites for the R state are occupied, as schematically shown in Figure 5C, ethylene begins to adsorb at the other site of the pair, resulting in the formation of the β-states. Once one of the paired sites is occupied by ethylene, the positive charge of Ag1 forming the other site is reduced appreciably. This explains the preferential occupation at one of the paired sites by ethylene at lower exposures. The TDS measurements indicate that the desorption temperatures at the β states are appreciably lower that those at the R states. In the β states, the positive charge induced by an atomic oxygen is shared by two ethylene molecules, resulting in the weaker π-bonding interaction (or the smaller stability) compared to the R states. The ω(CH2) band for ethylene on p(4×1)O-Ag(110) shows gradual intensity increase and frequency shift from 972 to 968 cm-1 upon increasing exposures (see Figure 3A). Although the frequency shift suggests that ethylene on the p(4×1) surface also undergoes an R to β state conversion, the absence of the discrete spectral change indicates that the adsorption process on the p(4×1) surface is appreciably different from those on the p(6×1) and p(3×1) surfaces. It is recognized from Figure 5, parts A and B, that the ratio of the surface densities of the Ag1 atoms on the reconstructed surfaces is as follows: 2 (p(6×1)):3 (p(4×1)):2 (p(3 × 1)). This means that, if all the Ag1 atoms in each reconstructed surfaces constitute the adsorption sites, the intensity of the ω(CH2) band due to the β(4) state is larger than those due to the β(6) and β(3) states. This is not the case, as can be seen from Figure 2C and parts A and B of Figure 3. Presumably, as an adsorption model for the β(4) state in Figure 5D shows, an adsorption site lying next to an occupied site in the 〈11h0〉 direction within the same groove between the Ag-O added rows on the p(4×1) surface remains unoccupied

Adsorption Structures of Ethylene on Ag(110)

J. Phys. Chem. B, Vol. 103, No. 46, 1999 10195

Figure 5. Schematic representations of the reconstructed surfaces, p(6×1)O-Ag(110), p(4×1)O-Ag(110), and p(3×1)O-Ag(110) (A,B) and adsorption states, R(n) and β(n) states (n ) 3, 4, 6) (C and D). The larger shaded and open circles and the solid smaller circles in A and B denote silver atoms and oxygen atoms, respectively. A space-filling model of ethylene is inserted at the bottom of the figures in A and B for reference. The shaded vertical bars in C and D denote schematically the added Ag-O rows along the 〈001〉 direction. The open dashed squares and shaded squares in C and D denote unoccupied and occupied adsorption sites, respectively (see text).

because, if both sites were occupied, there should be a large repulsive interaction between the adsorbates. This kind of lateral interaction may exclude the clear distinction between the R and β states and cause the gradual frequency change during the formation. The TDS measurements, however, do not always corroborate the explanation, because, as the dotted lines in Figure 4 show, the width of the TDS peak associated with the desorption at the β(3) state is broader than that of the desorption at the β(4) state, suggesting that there exists a larger lateral interaction between the β(3) adsorbates than that between the β(4) adsorbates. To clarify this point, however, we should perform more detailed TDS measurements by using a smaller Ag(110) substrate than that used in this study (see Experimental Section);

one of the main reasons for the broadness of the TDS spectra in Figure 4 is due to the large substrate used in this study. The broad features observed for the IR bands of ethylene on p(2×1)O-Ag(110) (Figure 3C) indicate the absence of specific adsorption states. In accordance with this, the TDS spectrum gives a broad band containing a main peak at about 100 K (see Figure 4). The existence of the state with the peak temperature appreciably lower than those observed for the adsorbates on Ag(110) and p(n×1)O-Ag(110) (n ) 3, 4, and 6) proves that in order to stabilize the adsorption state of ethylene on the atomic oxygen precovered Ag(110) surfaces it is necessary to have proper spaces (for the R and β states) in addition to the positive charges on the Ag atoms induced by the atomic oxygen.

10196 J. Phys. Chem. B, Vol. 103, No. 46, 1999 Conclusion The IR spectral measurements indicated that upon increasing exposures of ethylene to the p(n×1)O-Ag(110) (n ) 3, 4, 6) surfaces at 80 K the adsorbate takes on discrete states successively, i.e., at first the R(n) states and then β(n) states (n ) 3, 4, 6). The results could be explained by postulating that (i) adsorption sites exist between the neighboring added Ag-O rows; (ii) two sites on both side of the added row forms a special pair. At lower exposures, ethylene takes on the R(n) states by occupying one of the special pairs selectively and upon further exposure ethylene occupies both of the paired sites, resulting in the formation of the β(n) states. According to the TDS measurements, the R(n) states are more stable than the β(n) states and the relative stability of the R(n) states is as follows: R(6) < R(4) < R(3). The selective occupation of one of the special pair at lower exposures and the relative stability of the adsorption states could be explained by considering that, the larger the positive charge induced by the precovered atomic oxygen on Ag atoms consisting of the adsorption site, the larger the π-bonding interaction (or the stability) of ethylene with the substrates. In contrast to the case of the p(n×1) surfaces (n ) 3, 4, 6), ethylene on p(2×1)O-Ag(110) takes on an irregular adsorption state. The TDS measurement indicated that the stability of the main adsorbate on the latter surface is lower than that of the adsorbates on the former surfaces. The result proved that in order to stabilize ethylene on the atomic oxygen precovered Ag(110) surfaces there should be the proper spaces of interaction (for the R and β states) in addition to the positive charge induced by the atomic oxygen.

Akita et al. Although the adsorption models proposed by the present paper should be verified directly by STM observation and the explanation of the relative stability of the adsorption states in terms of the positive charge on Ag atoms should be substantiated by ab initio calculations, the results of the present paper provide new insights into the adsorption characteristics of ethylene on Ag(110) and its atomic oxygen reconstructed surfaces. References and Notes (1) Kru¨ger, B.; Benndorf, C. Surf. Sci. 1986, 178, 704. (2) Barteau, M. A.; Madix, R. J. Surf. Sci. 1981, 103, L171. (3) Campbell, C. T.; Paffett, M. T. Surf. Sci. 1984, 139, 396. (4) Rovida, G.; Pratesi, F.; Ferroni, E. Appl. Surf. Sci. 1980, 5, 121. (5) Backx, C.; de Groot, C. P. M.; Biloen, P. Appl. Surf. Sci. 1980, 6, 256. (6) Backx, C.; de Groot, C. P. M. Surf. Sci. 1982, 115, 382. (7) Akita, M.; Osaka, N.; Hiramoto, S.; Itoh, K. Surf. Sci., in press. (8) Solomon, J. L.; Madix, R. J. J. Chem. Phys. 1990, 93, 8379. (9) Engelhardt, H. A.; Menzel, D. Surf. Sci. 1976, 57, 591. (10) Taniguchi, M.; Tanaka, K.; Hashizume, T.; Sakurai, T. Surf. Sci. 1992, 262, L123. (11) Chough, S. H.; Panchenko, Y. N.; Bock, C. W. J. Mol. Struct. 1992, 272, 179. (12) Tu¨shaus, M.; Schweizer, E.; Hollins, P.; Bradshaw, A. M. J. Electron Spectrosc. Relat. Phenom. 1987, 44, 305. (13) Stranick, S. J.; Kamma, M. M.; Weiss, P. S. Surf. Sci. 1995, 338, 41. (14) A curve resolution procedure was performed on the spectra in Figure 2B by assuming Gaussian components at 984, 973, 964, and 955 cm-1. Although the existence of the negative band at 952 cm-1 made the estimation of the intensities of the components less quantitative, the results corroborated the facts (i-iv). (15) Doering, M.; Buisset, J.; Rust, H.-P.; Briner, B. G.; Bradshaw, A. M. Faraday Discuss. 1996, 105, 163.