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The adsorption of chlorine on clean germanium surfaces was studied in an all-glass system by crushing a single crystal disc of germanium in the presence of the gas and following the pressure changes. Surface areas of the powders were measured by the B.E.T. method using krypton at liquid nitrogen temperature. The experimental techniques evolved for working with chlorine are described. Chlorine was found to adsorb on germanium to monolayer coverage at room temperature and at −78°C. A slow desorption of a fraction of the adsorbed chlorine followed the rapid adsorption. The nature of the adsorption bond and a possible explanation for the slow desorption are discussed. 相似文献
3.
The ellipsometric effects measured upon adsorption in the monolayer range are discussed. In general there are two possible contributions, the effect of the adsorbed layer itself and the effect due to a change in the substrate surface induced by the adsorbed layer. The latter effect occurs mainly in the case of chemical adsorption.The ellipsometric measurements on a number of clean semiconductor surfaces are treated and the results are compared with those obtained by other methods, e.g. electron energy loss spectroscopy and ultra-violet photon spectroscopy. 相似文献
4.
Recent studies of the dissociative adsorption of methane on clean Ni(111), Ni(100), Ni(110), and sulfur-modified Ni(100), as well as ethane, propane, and n-butane on Ni(100) have been carried out under the high incident flux conditions of 1.00 Torr methane, 0.10 Torr ethane, 0.01 Torr propane, and 0.001 Torr n-butane, respectively. It has been found that the activation energies for these processes range from 3.1±1.0 to 13.3±1.5 kcal mol–1. A comparison with the results of corresponding molecular beam studies suggests that the effects of vibrational energy on sticking probabilities must be accounted for and the sticking probabilities of molecules with very low normal kinetic energies must be accurately known when attempting to model high pressure processes using molecular beam techniques. While dissociation of ethane, propane, and n-butane on Ni(100) is believed to proceed primarily via a trapped molecular precursor, the results on sulfur-modified Ni(100) surface indicate that the direct channel to methane dissociation likely dominates and the contribution from the trapped molecular precursor mechanism is likely relatively small, with the sulfur atoms poisoning this reaction by a simple site blocking mechanism. 相似文献
5.
The desorption of CO from clean Pt(111) and (100), and from the same surfaces with partial overlayers of sulfur, was studied by Thermal Desorption Spectroscopy. The method of desorption rate isotherms was employed for data analysis. The desorption of CO from the (111) surface and both surfaces with ordered sulfur overlayers can be described as a first order process with coverage dependent activation energies. The desorption of CO from the clean Pt(100) surface is complicated by the dynamic interaction of the molecule with a thermally activated change of platinum surface structure. On both platinum faces surface sulfur decreases the initial binding energy of CO. As the CO concentration increases, its binding energy decreases very rapidly. This is due to a repulsive interaction which exists between co-adsorbed species. 相似文献
6.
CO adsorption on potassium covered Fe(110) has been studied using UPS, XPS, AES and flash desorption. It was found that CO adsorbs molecularly at room temperature with a larger binding energy than on clean Fe(110). The CO saturation coverage increases and the sticking coefficient decreases with increasing potassium coverage. On heating, the probability of adsorbed CO dissociating increases with the amount of potassium present. The UPS spectra show that the CO 4σ peak is shifted by 0.8 eV to higher binding energies on Fe(110) + K and that at 21.2 eV the peak due to the 1π + 5σ orbitals is split into a double peak. The catalytic relevance of the measurements is discussed with reference to the Fischer-Tropsch synthesis. 相似文献
7.
N.V. Petrova 《Journal of Physics and Chemistry of Solids》2011,72(6):744-748
Structures of carbon monoxide layers on the oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces have been investigated by means of density-functional (DFT) calculations. It is found that CO molecules adsorb at hollow sites on the O/Mo(1 1 0) surface and nearly atop Mo atoms on the O/Mo(1 1 2) surface. The favorable positions for adsorption are shown to be near protrusions of electron density above the Mo surface atoms. The presence of oxygen on the molybdenum surface significantly reduces the binding energy of the CO molecule with the substrate; on the oxygen-saturated Mo(1 1 0) surface, the adsorption of CO is completely blocked. The calculated local densities of states (LDOS) demonstrate that the O 2s peak for O adsorbed on Mo(1 1 0) surface is at −19 eV (with respect to the Fermi level), while for the oxygen atom of an adsorbed CO molecule the related 3σ molecular orbital gives rise to a peak at −23 eV. This difference stems from the bonding of the O atom either with Mo surface for adsorbed O or with C atom in adsorbed CO, and therefore the position of the O 2s peak in photoemission spectra can serve as a convincing argument in favor of either the presence or absence of the CO dissociation on Mo surfaces. 相似文献
8.
Oxygen adsorption on clean Mo (100) surfaces has been studied by LEED, AES, work function changes and energy loss spectroscopy. At room temperature, the oxygen uptake as determined by AES is linear up to one third of the saturation value. Data obtained with CO adsorption have been used to determine the oxygen coverage. With increasing oxygen exposure LEED shows three stages: a c (2 × 2) phase growing simultaneously with a (6 × 2) structure, a stage with (110) microfacets covered by two-dimensional structures and finally a p (3×1) structure together with a p (1×1) structure, probably due to an oxide phase. Even in the low temperature range (370–500 K) remarkable effects are observed: adsorption at 370 K produces a disordered c (4×4) structure which is followed by a (√5 × √5)?R 26° 33 structure. The same occurs when the inital c (2 × 2) structure formed at 295 K is heated above 370 K. Measurements of the work function indicate a minimum at the end of the c (2×2) structure, then a rapid increase and at saturation a value of about 1.5 V above that of the clean surface. Energy loss spectroscopy measurements point to an increase of the surface plasmon energy during the faceting stage. New transitions are observed which are due to new electronic levels induced by the adsorption. They are comparable with photoemission results on W and Mo. 相似文献
9.
H. Ibach 《Surface science》1975,53(1):444-460
Surface reconstruction and steps alter the electron affinity and ionization potential of surfaces by a few tenth of an eV. In an simple potential model these variations are related to the changes in the activation energy for chemisorption. Without introducing fitted parameters the model describes semiquantitatively the influence of surface re-construction, step density and doping on the oxygen sticking coefficient on silicon and gallium arsenide. Possible applications of the model to stepped metal surfaces and to the influence of steps on catalysis in general are discussed as well. 相似文献
10.
We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (1 1 0) and (1 0 0) surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically “downhill” while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at 1 atm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (1 1 0) than on (1 0 0). On the (1 1 0) surface, it is much easier to form a second vacancy on the doped surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (1 0 0) is comparable to that of forming the first vacancy on the undoped surface. Thus doping makes the (1 1 0) surface a better oxidant but it has a small effect on the oxidative power of the (1 0 0) surface. On the (1 1 0) surface CO adsorption results in formation of a carbonate-like structure, similar to the undoped surface, while on the (1 0 0) surface direct formation of CO2 is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation. 相似文献
11.
《Surface science》1986,175(3):445-464
The adsorption and reaction of acetonitrile (CH3CN) on clean and oxygen covered Ag(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), isotope exchange, chemical displacement reactions and high resolution electron energy loss spectroscopy (EELS). On the clean Ag(110) surface, CH3CN was reversibly adsorbed, desorbing with an activation energy of 10 kcal mol-1 at 166 K from a monolayer state and at 158 K from a multilayer state. Vibrational spectra of multilayer, monolayer and sub-monolayer CH3CN were in excellent agreement with that of gas phase CH3CN indicating that CH3CN is only weakly bonded to the clean Ag(110) surface. On the partially oxidized surface CH3CN reacts with atomic oxygen to form adsorbed CH2CN, OH and H2O in addition to forming another molecular adsorption state with a desorption peak at 240 K. This molecular state shows a CN stretching frequency of 1840 cm-1, which is indicative of substantial rehybridization of the CN bond and is associated with side-on coordination via the π system. The CH2CN species is stable up to 430 K, where C-H bond breaking and reformation begins, leading to the formation of CH3CN at 480 K and HCN at 510 K and leaving only carbon on the surface. In the presence of excess oxygen atoms C-H bond breaking and reformation is more facile leading to additional desorption peaks for CH3CN and H2O at 420 K. This destabilizing effect of O(a) on Ch2CN(a) is explained in terms of an anionic (CH2CN-1) species. Comparison of the vibrational spectra from CH2CN(a) and CD2CN(a) supports the following assignment for the modes of adsorbed CH2CN: ν(Ag-C) 215: δ(CCN) 545; ϱt(CH2) 695; ϱw(CH2) 850; ν(C-C) 960; ϱr(CH2) 1060; δ(CH2) 1375; ν(CN) 2075; and ν(CH2) 2940 cm-1. These results serve to further indicate the wide applicability of the acid-base reaction concept for reactions between gas phase Brönsted acids and adsorbed oxygen atoms on solver surfaces. 相似文献
12.
The adsorption of H2O on the surface of a single-crystal sphere of silver with exposed (111), (100) and (112) facets has been examined using ESDIAD (electron stimulated desorption ion angular distribution), LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The purpose of the study was (a) to examine the influence of substrate geometry for adsorption of H2O on a metal surface for which the adsorbate-substrate interaction is weak, and (b) to study the influence of a surface impurity, oxygen, on the surface chemistry and local bonding structure of H2O on Ag. We have found no evidence for either long-range or short-range local bonding order for adsorbed H2O at 80 K on any of the surfaces studied. This appears to be a consequence, in part, of the lattice mismatch between the Ag crystal structure and the two-dimensional H2O ice crystal structure. Adsorbed H2O reacts with preadsorbed oxygen to form OH species which are bonded with the molecular axis perpendicular to Ag(111) and (100) but “inclined” on (112) surfaces, as identified using ESDIAD. The “inclined” OH species are associated with atomic steps on the (112) surface. 相似文献
13.
The selective adsorption of 4He on in-situ cleaved LiF surfaces has been studied under improved resolution. The main results are as follows: (1) There are four bound states in the surface potential well, at energies of ?5.8, ?2.2, ?0.6 and ?0.1 meV. The lowest three levels were reported previously. (2) Most of the structure previously designated as “fine structure” is due either to transitions to these four levels via various small reciprocal lattice vectors or to the opening of diffraction channels. (3) The transitions involving the [01] and [01?] reciprocal lattice vectors (i.e., the ones nearly perpendicular to the incident wave vector) are strong; as much as 85% of the specular intensity may be removed. Transitions via the other small reciprocal lattice vectors are much weaker. (4) The widths of the lines are consistent with the velocity distribution, which has a half-width of about 2%. (5) The observed energies agree fairly well with those calculated by Tsuchida for a zeta-function potential, but are not consistent with a Morse potential. 相似文献
14.
Séverine Le Moal Marco Moors Jan Markus Essen Conrad Becker Klaus Wandelt 《Surface science》2010,604(19-20):1637-1644
CO adsorption on clean and oxidized Pt3Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt3Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt3Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt3Ti(111) surfaces (< 135 L O2 exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt3Ti(111) surfaces (> 220 L O2 exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed. 相似文献
15.
Ultraviolet photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS) and Auger (AES) measurements were used to study oxygen adsorption on sputtered an annealed GaAs(111)Ga, (1̄1̄1̄)As, and (100) surfaces. Two forms of adsorbed oxygen are seen in UPS. One of them is associatively bound and desorbs at 400–550 K mainly as molecular O2. It is most probably bound to surface As atoms as indicated by the small amounts of AsO which desorb simultaneously. The second form is atomic oxygen bound in an oxidic environment. It desorbs at 720–850 K in the form of Ga2O. Electron irradiation of the associatively bound oxygen transforms it into the oxidic form. This explains the mechanism of the known stimulating effect of low energy electrons on the oxidation of these surfaces. During oxygen exposure a Ga depletion occurs at the surface which indicates that oxygen adsorption is a more complex phenomenon then is usually assumed. The following model for oxygen adsorption is proposed: oxygen impinges on the surface, removes Ga atoms and thus creates sites which are capable of adsorbing molecular oxygen on As atoms of the second layer and are surrounded by Ga atoms of the first layer. This molecular oxygen is stable and simultaneously forms the precursor state for the dissociation to the oxidic form. 相似文献
16.
Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a × LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θo = θco = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with Oad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with COad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies. 相似文献
17.
The adsorption of acetic acid is studied on clean and oxygen-covered Au/Pd(100) alloys as a function of gold content by temperature-programmed desorption and reflection–absorption infrared spectroscopy. Au/Pd(100) forms ordered alloys such that, for gold coverages above ~ 0.5 monolayers, only isolated palladium atoms surrounded by gold nearest neighbors are present. Predominantly molecular acetic acid forms on Au/Pd(100) alloy surfaces for gold coverages greater than ~ 0.56 ML, and desorbs with an activation energy of ~ 59 kJ/mol. Heating this surface also forms some η1-acetate species which decompose to form CO and hydrogen. On alloy surfaces with palladium–palladium bridge sites, η1-acetate species initially form, but rapidly convert into η2-species. They thermally decompose to form CO and hydrogen, with a small portion rehydrogenating to form acetic acid between 280 and 321 K depending on gold coverage. The presence of oxygen on both Pd(100) and Au/Pd(100) alloys facilitates acetate dehydrogenation so that only η2-acetate species form on these surfaces. The presence of oxygen also serves to stabilize the acetate species. 相似文献
18.
Atomic beam scattering was used to study the gas-surface interaction between helium and ionic crystals. In particular, the binding energies of the selectively adsorbed states were measures for two isotopes of helium on the (001) surfaces of NaF and LiF cleaved in vacuum. Consistent results were obtained by avoiding incident conditions where band structure effects invalidate the free particle approximation. The energy levels can be fit to a 9-3 model potential. 相似文献
19.
Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E0 < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π1(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E0 < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme. 相似文献
20.
Low-energy electrons have a particularly important role in many of the techniques of surface science. In some experiments, such as low-energy electron diffraction and characteristic loss spectroscopy, they are scattered either elastically or inelastically; in others, such as electron-stimulated desorption, they are used to produce excitations of the surface; and in still others, the resident electrons are excited to energies where they may escape from the material, as in photoemission, Auger, and ion neutralization spectroscopy. The reason for this central role is that, of all the physical probes, the electron is the simplest one that interacts strongly enough to be sensitive to the last few layers of atoms. 相似文献