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1.
The adsorption and reaction of Br2 with Ag(110) was studied with Auger electron spectroscopy, LEED, work function measurements and thermal desorption spectroscopy in the temperature range of 130–1000 K. Depending on Br coverage and crystal temperature, four different adsorption and reaction states could be detected. For fractional monolayer coverages, chemisorbed Br(ad) is found to be the most stable species. This adsorption state saturates for θ(Br) ? 0.75. In the chemisorption stage, two LEED patterns, a p(2 × 1) with θ(Br) ? 0.5 and a c(4 × 2) with θ(Br) ? 0.75, were observed. For higher Br2 exposures and T = 130 K a layer-by-layer growth of AgBr is detected. At higher temperature, T > 190 K, there is evidence for a transformation from a 2D growth mechanism of AgBr into a 3D agglomeration of larger AgBr cluster. Molecularly adsorbed.  相似文献   

2.
The adsorption of oxygen on W(110) for ccverages below 0.5 monolayer has been studied with a number of complementary techniques. Data on adsorption kinetics, LEED intensities, work function changes and desorption kinetics have been used to propose a model combining statistical adsorption and island growth for the formation of the p(2 × 1) structure. On the basis of the measurements it is concluded that the surface is reconstructed for θ < 0.3 monolayer after heating to T > 2000 K, and for θ < 0.1 monolayer for adsorption at 300 K.  相似文献   

3.
We determined the local adsorption structure of disordered oxygen on the Ni(1 1 1) surface by means of diffuse holographic LEED. The measurements have been performed above the critical temperature (Tc=450 K) for the oxygen order-disorder phase transition at 500 K and at a coverage of Θ=0.25 ML. At this temperature we found, in agreement with a previous LEED-IV-analysis [Surf. Sci. 349 (1996) 185], that besides the fcc threefold sites also hcp sites are occupied. In addition, a small amount seems to be located also at top and bridge sites. Reconstructing the holographic wavefield information, the different oxygen adsorption geometries are superimposed in the real space image. Nevertheless, a spatial resolution of 0.5-1 Å was sufficient to clearly distinguish between them. The influence of algorithmic parameters on the image quality was tested.  相似文献   

4.
The adsorption of oxygen on W(100) single crystal surfaces is studied by Auger electron spectroscopy (AES), flash desorption, low-energy electron diffraction (LEED) and retarding field work function measurements with the aim of obtaining a better understanding of the adsorption kinetics and of the structures of the adsorbed layer. The AES results reveal step-wise changes of the sticking coefficients in the coverage range 0 to 1, and activated adsorption at higher coverages. Upon room temperature adsorption a series of complex LEED patterns is observed. In layers adsorbed at 1050 K and cooled to room temperature, the well-known p(2 × 1) structure is the first ordered structure observed. This structure shows a reversible order-disorder transition between 700 K and 1000 K and is characterized by a work function which is lower than that of the clean surface. Heating room temperature adsorbates changes their structure irreversibly. At temperatures below 750 K some new structures are observed. Combining the results obtained in this study with other published work leads to a considerable revision of the previously accepted model of the adsorption of oxygen on W(100).  相似文献   

5.
The adsorption of potassium and the coadsorption of potassium and oxygen on the Pt(111) and stepped Pt(755) crystal surfaces were studied by AES, LEED, and TDS. Pure potassium adlayers were found by LEED to be hexagonally ordered on Pt(111) at coverages of θ = K0.9–;1. The monolayer coverage was 5.4 × 1014K atoms/cm2 (0.36 times the atomic density of the Pt(111) surface). Orientational reordering of the adlayers, similar to the behavior of noble gas phase transitions on metals, was observed. The heat of desorption of K decreased, due to depolarization effects, from 60 kcal/mole at θK <0.1, to 25 kcal/mole at θK = 1 on both Pt(111) and Pt(755). Exposure to oxygen thermally stabilizes a potassium monolayer, increasing the heat of desorption from 25 to 50 kcal/mole. Both potassium and oxygen were found to desorb simultaneously indicating strong interactions in the adsorbed overlayer. LEED results on Pt(111) further indicate that a planar K2O layer may be formed by annealing coadsorbed potassium and oxygen to 750 K.  相似文献   

6.
The adsorption of oxygen on Ag(110), (111), and (100) surfaces has been investigated by LEED, Auger electron spectroscopy (AES), and by the measurement of work function changes and of kinetics, at and above room temperature and at oxygen pressures up to 10?5Torr. Extreme conditions of cleanliness were necessary to exclude the disturbing influences, which seem to have plagued earlier measurements. Extensive results were obtained on the (110) face. Adsorption proceeds with an initial sticking coefficient of about 3 × 10?3 at 300 K, which drops very rapidly with coverage. Dissociative adsorption via a precursor is inferred. The work function change is strictly proportional to coverage and can therefore be used to follow adsorption and desorption kinetics; at saturation, ΔΦ ≈ 0.85 eV. Adsorption proceeds by the growth of chains of oxygen atoms perpendicular to the grooves of the surface. The chains keep maximum separation by repulsive lateral interactions, leading to a consecutive series of (n × 1) superstructures in LEED, with n running from 7 to 2. The initial heat of adsorption is found to be 40 kcal/mol. Complicated desorption kinetics are found in temperature-programmed and isothermal desorption measurements. The results are discussed in terms of structural and kinetic models. Very small and irreproducible effects were observed on the (111) face which is interpreted in terms of a general inertness of the close-packed face and of some adsorption at irregularities. On the (100) face, oxygen adsorbs in a disordered structure; from ΔΦ measurements two adsorption states are inferred, between which a temperature-dependent equilibrium seems to exist.  相似文献   

7.
The adsorption of CO on Rh(111) has been studied by thermal desorption mass spectrometry and low-energy electron diffraction (LEED). At temperatures below 180 K, CO adsorbs via a mobile precursor mechanism with sticking coefficient near unity. The activation energy for first-order CO desorption is 31.6 kcal/mole (νd = 1013.6s?1) in the limit of zero coverage.As CO coverage increases, a (√3 ×√3)R30u overlayer is produced and then destroyed with subsequent formation of an overlayer yielding a (2 × 2) LEED pattern in the full coverage limit. These LEED observations allow the absolute assignment of the full CO coverage as 0.75 CO molecules per surface Rh atom. The limiting LEED behavior suggests that at full CO coverage two CO binding states are present together.  相似文献   

8.
The adsorption of alkali metals on transition metals can produce several technologically important effects, but only limited results have been reported on the geometrical structure of such adlayers, especially for adsorption temperatures below 300 K. We have examined the adsorption of Na on Ru(001) as a function of coverage and temperature using LEED to determine the adlayer structure and thermal desorption spectroscopy to characterize binding kinetics and relative Na coverages. The only Na LEED pattern observed following adsorption at 300 K was that of (32 × 32) structure which occurred near saturation of the first layer. However, Na adsorbed at 80 K produces a progression of distinct, ordered LEED patterns with increasing coverage which does not include the (32 × 32) pattern. These patterns result from increasingly compressed, hexagonal arrangements of adsorbate atoms which are uniformly spaced due to mutually repulsive interactions. The order-disorder transition temperature for each structure was also determined by LEED and used to develop a 2D phase diagram for Na on Ru(001). Ordered structures were observed only when Na thermally induced motion was sufficiently limited and the repulsive Na-Na interaction could force the uniform spacing of Na atoms. Thus, low coverage structures only developed where Na mobility was limited by low temperature. High coverage structures were stable to much higher temperatures since motion was inhibited by the high Na density.  相似文献   

9.
The adsorption and desorption of glycine (NH2CH2COOH), vacuum deposited on a NiAl(1 1 0) surface, were investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature-programmed desorption, work function (Δφ) measurements, and ultraviolet photoelectron spectroscopy (UPS). At 120 K, glycine adsorbs molecularly forming mono- and multilayers predominantly in the zwitterionic state, as evidenced by the UPS results. In contrast, the adsorption at room temperature (310 K) is mainly dissociative in the early stages of exposure, while molecular adsorption occurs only near saturation coverage. There is evidence that this molecularly adsorbed species is in the anionic form (NH2CH2COO). Analysis of AES data reveals that upon adsorption glycine attacks the aluminium sites on the surface. On heating part of the monolayer adsorbed at 120 K is converted to the anionic form and at higher temperatures dissociates further before desorption. The temperature-induced dissociation of glycine (<400 K) leads to a series of similar reaction products irrespective of the initial adsorption step at 120 K or at 310 K, leaving finally oxygen, carbon and nitrogen at the surface. AES and LEED measurements indicate that oxygen interacts strongly with the Al component of the surface forming an “oxide”-like Al-O layer.  相似文献   

10.
Adsorption of CO on W(110) at 100 K produces a number of ordered LEED patterns as coverage increases, culminating in a p(5 × 1) pattern for a full virgin CO layer. The beta-1 layer obtained by heating a virgin layer to 400 K has a p(2 × 1) structure. Absolute coverages, obtained by comparison of XPS intensities (and Auger intensities where feasible) with those of oxygen on tungsten at O/W = 0.5 indicate that CO/W ? 0.8 for the full virgin layer and ? 0.3 for beta-1. These results, together with the LEED data, indicate that low temperature adsorption of virgin CO is not very site specific, and that beta-1 must be dissociated with C and O lying along alternate closepacked rows of W. XPS results for the oxygen 1s peak show that the latter shifts in beta and beta-1 from its position in virgin CO to an energy equal to that seen for pure oxygen on tungsten. A number of electron impact desorption results are also presented, and the nature of the various binding states of CO on this plane is discussed.  相似文献   

11.
The adsorption of oxygen on Rh(111) at 100 K has been studied by TDS, AES, and LEED. Oxygen adsorbs in a disordered state at 100 K and orders irreversibly into an apparent (2 × 2) surface structure upon heating to T? 150 K. The kinetics of this ordering process have been measured by monitoring the intensity of the oxygen (1, 12) LEED beam as a function of time with a Faraday cup collector. The kinetic data fit a model in which the rate of ordering of oxygen atoms is proportional to the square of the concentration of disordered species due to the nature of adparticle interactions in building up an island structure. The activation energy for ordering is 13.5 ± 0.5 kcalmole. At higher temperatures, the oxygen undergoes a two-step irreversible disordering (T? 280 K) and dissolution (T?400K) process. Formation of the high temperature disordered state is impeded at high oxygen coverages. Analysis of the oxygen thermal desorption data, assuming second order desorption kinetics, yields values of 56 ± 2 kcal/ mole and 2.5 ± 10?3 cm2 s?1 for the activation energy of desorption and the pre-exponential factor of the desorption rate coefficient, respectively, in the limit of zero coverage. At non-zero coverages the desorption data are complicated by contributions from multiple states. A value for the initial sticking probability of 0.2 was determined from Auger data at 100 K applying a mobile precursor model of adsorption.  相似文献   

12.
The adsorption of NO on Pd(111) was studied by means of LEED, UPS and thermal desorption measurements. Non-dissociative adsorption is characterized by additional maxima in the photomission spectra at 2.6, 9.2 and 14.6 eV below the Fermi level originating from chemisorption levels which are derived from the highest occupied molecular orbitais of NO. Thermal desorption takes place from three distinct states (α, β and γ) corresponding to binding energies of about 15, 17 and 31 kcal/mole, respectively, with about equal populations. The α-state is associated with a 2 × 2 LEED pattern and the β-state with a c4 × 2 structure, whereas the γ-state corresponds to disordered adsorption at low coverages. Plausible structure models are proposed for the ordered structures with θ = 0.75 for the α-state and θ = 0.5 for the β-state. The strong decrease of the adsorption energy is explained in terms of pronounced short-range repulsive interactions between neighbouring adsorbate molecules.  相似文献   

13.
The adsorption of K on Pt(100) has been followed by thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES); carbon monoxide was used as a probe for the modification of the chemical properties of K promoted surfaces. The role of subsequent adsorption of oxygen on the K modified surfaces has also been measured. For low potassium coverage (θK = 0 to 0.35), the mass-28 TDS peak temperature of adsorbed CO increases continuously with the K coverage, indicating an increase of the adsorption energy of CO which has been explained by a substantial charge donation from K into the 1 orbitals of CO via long range interactions through the platinum substrate. No oxygen uptake was detected after oxygen exposure at room temperature. For high potassium content (θK = 0.45 to 1), the mass-28 TDS peak temperature of coadsorbed CO is very narrow and remains constant at 680 K. We propose the formation of a COKPt surface complex which decomposes at 680 K, since K desorption is detected concomitantly to CO. On such K covered surfaces, the oxygen uptake is promoted, and it cancels the modifications of the surface properties induced by potassium.  相似文献   

14.
Jakub Drnec 《Surface science》2009,603(13):2005-2014
The adsorption of Cs on Pt(1 1 1) surfaces and its reactivity toward oxygen and iodine for coverages θCs?0.15 is reported. These surfaces show unusual “anomalous” behavior compared to higher coverage surfaces. Similar behavior of K on Pt(1 1 1) was previously suggested to involve incorporation of K into the Pt lattice. Despite the larger size of Cs, similar behavior is reported here. Anomalous adsorption is found for coverages lower than 0.15 ML, at which point there is a change in the slope of the work function. Thermal Desorption Spectroscopy (TDS) shows a high-temperature Cs peak at 1135 K, which involves desorption of Cs+ from the surface.The anomalous Cs surfaces and their coadsorption with oxygen and iodine are characterized by Auger Electron Spectroscopy (AES), TDS and Low Electron Energy Diffraction (LEED). Iodine adsorption to saturation on Pt(1 1 1)(anom)-Cs give rise to a sharp LEED pattern and a distinctive work function increase. Adsorbed iodine interacts strongly with the Cs and weakens the Cs-Pt bond, leading to desorption of CsxIy clusters at 560 K. Anomalous Cs increases the oxygen coverage over the coverage of 0.25 ML found on clean Pt. However, the Cs-Pt bond is not significantly affected by coadsorbed oxygen, and when oxygen is desorbed the anomalous cesium remains on the surface.  相似文献   

15.
The absolute coverage of deuterium adsorbed on Ni(110) at temperatures below 170 K to the formation of a (1 × 2) LEED pattern has been determined by nuclear microanalysis (NMA). The result, θD = 0.96 ± 0.08, is consistent with a saturation coverage of one full monolayer. Heating the crystal above ~ 190 K is shown to result in a gradual loss of deuterium from the system, accompanied by streaking of the LEED pattern, with complete desorption above ~ 340 K. The low-temperature (2 × 1)-D phase was found to correspond to θD = 0.64 ± 0.05 monolayers. The results are expected to be valid also for the equivalent phases obtained by hydrogen adsorption.  相似文献   

16.
The adsorption of Xe on a Ni(100) surface has been studied in UHV between 30 and 100 K using LEED, thermal desorption spectroscopy (TDS), work function (Δφ) measurements, and UV photoemission (UPS). At and below 80 K, Xe adsorbs readily with high initial sticking probability and via precursor state adsorption kinetics to form a partially ordered phase. This phase has a binding energy of ~5.2 kcal/mole as determined by isosteric heat measurements. The heat of adsorption is fairly constant up to medium coverages and then drops continuously as the coverage increases, indicating repulsive mutual interactions. The thermal desorption is first order with a preexponential factor of about 1012 s?1, indicative of completely mobile adsorption. Adsorbed Xe lowers the work function of the Ni surface by 376 mV at monolayer coverage. (This coverage is determined from LEED to be 5.65 × 1014 Xe molecules/cm-2.) For not too high coverages, θ, Δφ(θ) can be described by the Topping model, with the initial dipole moment μ0 = 0.29 D and the polarizability α being 3.5 × 10?24 cm3. In photoemission, the Xe 5p32 and 5p12 orbitals show up as intense peaks at 5.56 and 6.83 eV below Ef which do not shift their position as the coverage varies. Multilayer adsorption (i.e. the filling of the second and third layers) can be seen by TDS. The binding energies of these α states can be estimated to range between 4.5 and 3.5 kcal/mole. The results are compared and contrasted with previous findings of Xe adsorption on other transition metal surfaces and are discussed with respect to the nature of the inert-gas-metal adsorptive bond.  相似文献   

17.
Thermal desorption spectra taken after adsorption of carbon monoxide at room temperature on W(210) show sequential formation with increasing coverage of strongly bound β2 and β1 binding states, correlated to the sequential formation of P(2 × 1) and (1 × 1) adsorbate structures as observed by LEED. Adsorption at room temperature gives a poorly ordered arrangement of adsorbed CO molecules, but well-ordered structures are produced by subsequent anneal. For adsorption without anneal the work function increases monotonically with coverage to a maximum of Δφ = + 0.70 eV at saturation coverage of 1 monolayer. For adsorption followed by anneal the work function dependence upon coverage is less simple, with even a decrease of work function at coverages less than a quarter monolayer. LEED intensity-voltage measurements from P(2 × 1)CO and P(2 × 1)N structures suggest that CO molecules occupy the sites of 4-fold symmetry upon which nitrogen is believed to be adsorbed. The distinction between the β2 and β1 states of adsorbed CO is attributed to heterogeneity induced by the reduction in binding energy of a CO molecule when its nearest-neighbor sites are occupied.  相似文献   

18.
Atomic hydrogen chemisorbed on a Ni (111) surface forms at coverages between 0.3 and 0.6 an ordered 2 × 1-structure as observed by low energy electron diffraction (LEED). The intensity of the fractional-order LEED spots was measured at different coverages as a function of temperature. Continuous order-disorder transitions are found, the maximum transition temperature (270 K) being at θ = 0.5. The phase diagram, however, is asymmetric with respect to this coverage and can therefore presumably not be explained on the basis of (independent) pairwise interactions.  相似文献   

19.
The interaction of oxygen with a Pt(110) crystal surface has been investigated by thermal desorption mass spectroscopy, LEED and AES. Adsorption at room temperature produces a β-state which desorbs at ~800 K. Complete isotopic mixing occurs in desorption from this state and it populates with a sticking probability which varies as (1 ? θ)2, both observations consistent with dissociative adsorption. The desorption is second order at low coverage but becomes first order at high coverage. The saturationcoverage is 3.5 × 1014 mol cm?2. The spectra have been computer analysed to determine the fraction desorbing by first (β1) and second (β2) order kinetics as a function of total fractional coverage θ using this fraction as the only adjustable parameter. The β1 desorption commences at θ ~ 0.25 and β1 and β2 contribute equally to the desorption at saturation. The kinetic parameters for β1 desorption were calculated from the variation of peak temperature with heating rate as ν1 = 1.7 × 109 s?1 and E1 = 32 kcal mole?1 whereas two different methods of analysis gave consistent parameters ν2 = 6.5 × 10?7 cm2 mol?1 s?1 and E2 = 29 and 30 kcal mole?1 for β2 desorption. The kinetics of desorptior are discussed in terms of the statistics for occupation of near neighbour sites. While many fea tures of the results are consistent with this picture, it is concluded that simple models considering either completely mobile or immobile adlayers with either strong or zero adatom repulsion are not completely satisfactory. The thermal desorption surface coverage has been correlated with the AES measurements and it has been possible to use the AES data for PtO as an internal standard for calibration of the AES oxygen coverage determination. At low temperature (170 K) oxygen populates an additional molecular α-state. Adsorption into the α- and β-states is competitive for the same sites and pre-saturation of the β-state at 300 K excludes the α-state. This, together with the AES observation that the adsorption is enhanced and faster at 450 than 325 K suggests a low activation energy for adsorption into the β-state.  相似文献   

20.
The adsorption, decomposition, and desorption of NO on the close packed Ni(111) surface have been investigated by XPS, XPS satellites, XAES, UPS, and LEED between 125 and 1000 K. At adsorption temperatures below 300 K a single molecular species (v) is formed with about unit sticking coefficient, which is interpreted as bridge-bonded; its saturation coverage is about 85% of that of CO, i.e. 0.5 relative to surface Ni atoms. Adsorption at 300 to 400 K yields dissociative adsorption (β) followed by molecular adsorption; above 400 K only dissociated species are formed. Upon heating, a full molecular layer dissociates only after some NO desorption (at 380–400 K), while dilute layers (below half coverage) dissociate already above 300 K without NO desorption. Together with quantitative findings this shows that for dissociation of one v-NO, the space of two is required. N2 desorption from the β-layer occurs above 740 K; the oxygen staying behind diffuses into the crystal above 800 K. Readsorption of NO onto a β-layer or onto an oxygen precoverage at 125 K leads, besides to an α1-state similar to v-NO, to another molecular state (α2) which is interpreted as linearly bound. The resulting total coverage is considerably higher than in a virgin layer. This shows that the blocking of dissociation in a full v-layer is probably not due to β requiring the same sites, but to kinetic hindrance; an influence of β-induced surface reconstruction cannot be excluded, however. The LEED results agree with a previous report and are well compatible with the other results.  相似文献   

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