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1.
Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O 3 at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at ΘO=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f 7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O 2 to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O 2 desorption peak at 520 K at low ΘO, and the peak shifts to higher temperatures for increasing oxygen coverages up to ΘO=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at ΘO=1.2 ML. Analysis of the TPD data indicates that the desorption of O 2 from Au(111) can be described by first-order kinetics with an activation energy for O 2 desorption of 30 kcal mol −1 near saturation coverage. We estimate a value for the Au–O bond dissociation energy D(Au–O) to be 56 kcal mol −1. 相似文献
2.
The surface composition and structure of natural diamond (100) surfaces subsequently oxidized with activated oxygen at Tsub≤35°C were investigated with high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy, electron loss spectroscopy (ELS) and low-energy electron diffraction (LEED). Complete surface oxidation (oxygen coverage θ=1 ML) required doses of hundreds of kilolangmuirs of O 2. HREELS vibrational spectra permitted identification of the specific surface oxygen species, and also provided information about the diamond surface states. Most surface sites lost their hydrogen at least once before becoming oxidized. The oxygen coverage θ increased quickly at first, and then more slowly as saturation was approached; different mechanisms or sites may have accounted for the decreased rate. The relative distribution of oxygen species varied with the oxidation conditions. Ether, carbonyl and hydroxyl groups appeared during the initial stages of oxidation, but the hydroxyl groups disappeared at higher coverages. Bridge-bonded ether groups dominated at saturation coverage, although smaller amounts of carbonyl and hydroxyl were still observed. The carbonyl and C---H stretch frequencies increased with oxygen dose due to formation of higher oxidation states and/or hydrogen bonding between adjacent groups. ELS revealed only a low concentration of C=C dimers on the oxidized surfaces, and no evidence of graphitization. Surfaces generated by oxygen addition and then desorption were more reactive than surfaces generated by hydrogen desorption. Oxidized surfaces that were heated in vacuum and then rehydrogenated did not recover the sharp LEED patterns and HREELS spectra of the original plasma-smoothed surface. This effect was presumably due to surface roughening caused by oxygen desorption as CO and CO2, and creation of reactive high-energy sites that quickly bonded to available background gases and prevented large areas of organized surface reconstruction. 相似文献
3.
The adsorption and reaction of methyl nitrite (CH 3ONO, CD 3ONO) on Pt(111) was studied using HREELS, UPS, TPD, AES, and LEED. Adsorption of methyl nitrite on Pt(111) at 105 K forms a chemisorbed monolayer with a coverage of 0.25 ML, a physisorbed second layer with the same coverage that desorbs at 134 K, and a condensed multilayer that desorbs at 117 K. The Pt(111) surface is very reactive towards chemisorbed methyl nitrite; adsorption in the monolayer is completely irreversible. CH 3ONO dissociates to form NO and an intermediate which subsequently decomposes to yield CO and H 2 at low coverages and methanol for CH 3ONO coverages above one-half monolayer. We propose that a methoxy intermediate is formed. At least some C–O bond breaking occurs during decomposition to leave carbon on the surface after TPD. UPS and HREELS show that some methyl nitrite decomposition occurs below 110 K and all of the methyl nitrite in the monolayer is decomposed by 165 K. Intermediates from methyl nitrite decomposition are also relatively unstable on the Pt(111) surface since coadsorbed NO, CO and H are formed below 225 K. 相似文献
4.
Ordered phases of Sr on Mo(112) and their phase transitions have been studied up to one physical monolayer as a function of both coverage and temperature using optical LEED. Starting at a coverage of 0.07, islands of a p(8×1) structure are formed at 100 K, which coexist with a disordered lattice gas. The formation of incommensurate structures with properties of floating solids starts already at coverages slightly above the completed p(8×1) commensurate structure ( θ=0.125). The latter itself behaves like a floating solid and undergoes a depinning transition at T≈125 K, similar to the next commensurate structure, p(5×1), which is formed at θ=0.20. Floating solids are found in the whole coverage range between 0.12 and 0.23. At higher coverage coexistence between p(5×1) and c(2×2) structures is found, which melt by forming intermediate two-dimensional eutectics, i.e. coexistence regions with their melts, with an eutectic point at θ=0.37, Teu=310 K. Close to a coverage of 0.5 a homogenous phase is formed, which disorders by a continuous phase transition, as explicitly tested by determination of the critical exponents β of the order parameter and ν of the correlation length. It is shown that the system belongs to the universality class of the Ising model. An incommensurate phase is again formed at higher coverage due to uniaxial compression of the layers. The behaviour at low coverages can be qualitatively understood assuming lateral interactions along the furrows mainly caused by dipole–dipole interactions and electrostatic screening of the adsorbate induced charge redistribution. 相似文献
5.
The chemistry of N 2H 4 on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH 3. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms. 相似文献
6.
We present the results of ab initio calculations of oxygen atomic adsorption in a wide range of coverage on Pt(1 1 1). At θ = 0.25 ML, the O adsorption at fcc hollow site is clearly favoured over the hcp site. At θ = 0.5 ML, the O adsorption energy decreases but the same site is favoured. When experimental or theoretical previously reported data are available, the calculated adsorption energies and site preferences are in good agreement. Among the various configurations and coverages investigated in the present work, no adsorption is stable beyond θ = 0.5 ML, except by occupation of a subsurface tetrahedral site. In that case, a total O coverage of 0.75 ML could be achieved, which is only slightly less stable than the θ = 0.5 ML configuration. The use of thermodynamics permitted to explore the temperature–pressure stability domain corresponding to 0.25 ML, 0.5 ML and 0.75 ML. From this, we conclude that subsurface O species could be stable at temperatures lower than 700 K, with O2 pressures of 1 bar or less. 相似文献
7.
The effects of adsorbed H on the Mo 1−xRe x(110), x=0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x=0.15 alloy only, a c(2×2) LEED pattern is observed at a coverage Θ0.25 ML. A (2×2) pattern is observed for H coverages around Θ0.5 ML from surfaces with x=0, 0.05, and 0.15. Both c(2×2) and (2×2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1×1) pattern is observed. For the alloy surface with x=0.25, only a (1×1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x=0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom. 相似文献
8.
This work is a continuation of our studies of the liquid structure of polyhydric alcohols and their solutions. Dielectric parameters (relaxation time, average cluster dipole moment μ¯c, etc.) have been calculated for 1-hexanol in a wide temperature range in the framework of the cluster model proposed by Dissado and Hill; the values of apparent activation enthalpy Δ HDHexp≠ have been evaluated. These data were compared with the parameters previously determined by us for 2,5-hexanediol and 1,2,6-hexanetriol; the ratio μ¯c/ μ¯v was used for an estimation of the number of molecules in clusters. It was found that at all temperatures in the range 303–393 K, the μ¯c/ μ¯v values for 1-hexanol are significantly larger than the values of average degree of association p¯ calculated from equilibrium dielectric data; the Δ HDHexp≠ values for 1-hexanol are considerably lower than those for 2,5-hexanediol and 1,2,6-hexanetriol. 相似文献
9.
The kinetics of H 2 desorption from H/W(110) and H/Fe 1/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine Δ Hdes results. In the case of H/Fe 1/W(110) this includes a rise from 20 to 30 kcal mol −1 of H 2 at H/Fe = H/W > 0.3. Plots of rate −dθ/d t vs θ (θ being coverage in units of H/W) vary much more steeply than θ 2 at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of Δ Hdes and surface entropy Ss with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence. 相似文献
10.
YBa 2Cu 3O 7−δ (YBCO) films with high critical current density ( Jc) were successfully fabricated on nickel tapes buffered with epitaxial NiO. NiO was prepared on the textured nickel tape by the surface-oxidation epitaxy (SOE) method. We have reported so far a critical temperature ( Tc) of 87 K and Jc=4–6×10 4 A/cm 2 (77 K, 0 T) for the YBCO films on NiO/Ni tapes. To enhance the superconducting properties of the YBCO films on the SOE-grown NiO, depositions of thin oxide cap layers such as YSZ, CeO 2, and MgO on NiO were investigated. These oxide cap layers were epitaxially grown on NiO and provided the template for the epitaxial growth of YBCO films. Substantially improved data of Tc=88 K and Jc=3×10 5 A/cm 2 (77 K, 0 T) and 1×10 4 A/cm 2 (77 K, Hc, 4 T) were obtained for YBCO film on NiO, by using a MgO cap layer with a thickness of 50 nm. The method described in this paper is a simple way to produce long YBCO tape conductors with high- Jc values. 相似文献
11.
Assuming a phenomenological self-energy Im Σ( ω)| ω| β, ( β=1), which becomes gapped below Tc, we derived a new gap equation. The new gap equation contains the effect of the kinetic energy gain upon developing a superconducting order parameter. However, this new kinetic energy gain mechanism works only for a repulsive pairing potential leading to a s-wave state. In this case, compared to the usual potential energy gain in the superconducting state as in the BCS gap equation, the kinetic energy gain is more effective to easily achieve a high critical temperature Tc, since it is naturally Fermi energy scale. In view of the experimental evidences of a d-wave pairing state in the hole-doped copper-oxide high- Tc superconductors, we discuss the implications of our results. 相似文献
12.
The microstructures of a Tl 0.8Pb 0.2Bi 0.2Sr 1.6Ba 0.4Ca 2Cu 3O 9+δ/Ag tape (tape I) with Jc of 17,600 A/cm 2 at 77 K and 0 T and three Tl 0.8Pb 0.2Bi 0.2Sr 1.8Ba 0.2Ca 2.2Cu 3O 9+δ/Ag tapes with Jc's of 9300 (tape II), 16,700 (tape III) and 25 200 A/cm 2 (tape IV) prepared using the powder-in-tube method and an in-situ reaction method, were investigated using high-resolution transmission electron microscopy. In the tape preparation, an intermediate rolling process was incorporated during final heat-treatment for the last tape, but not for the rest of the tapes. Tl-1223 grains are in a thin plate-like shape. Tendency of directional grain-alignment increased in an order of tapes I, II, II and IV. In tape IV, Tl-1223 grains are clearly textured at least partly. In lattice defects, while stacking faults were prevalent in the former composition, dislocations and holes were frequently observed in the latter. Also impurity phases were appeared to be more abundant in the former than in the latter. The relationship between Jc and the microstructure in the tapes was explained in a term of grain-linking. 相似文献
13.
XPS, HREELS, ARUPS and Δø data show that furan chemisorbs non-dissociatively on Pd{111} at 175 K, the molecular plane being significantly tilted with respect to the surface normal. Bonding involves both the oxygen lone pair and significant π interaction with the substrate. The degree of decomposition that accompanies molecular desorption is a strong function of coverage: 40% of the adsorbate desorbs molecularly from the saturated monolayer. Decomposition occurs via decarbonylation to yield CO a and H a followed by desorption rate limited loss of H 2 and CO. It seems probable that an adsorbed C 3H 3 species formed during this process undergoes subsequent stepwise dehydrogenation ultimately yielding H 2 and C a. 相似文献
14.
This paper extends a previous study (Pehrsson and Mercer, submitted to Surf. Sci.) on unheated, hydrogenated, natural diamond (100) surfaces oxidized with thermally activated oxygen (O *2). In this paper, the oxidation is performed at substrate temperatures from Tsub=24 to 670°C. The diamond surface composition and structure were then investigated with high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), electron loss spectroscopy (ELS) and low energy electron diffraction (LEED). The oxygen coverage (θ) increased in two stages, as it did during oxidation at T<80°C. However, there are fundamental differences between the oxidation of nominally unheated and heated diamond surfaces. This difference is attributed to simultaneous adsorption and rapid desorption of oxygen species at higher temperatures; the desorption step is much slower without heating. The initial oxidation rates were similar regardless of the substrate temperatures, but the peak coverage (θ) was lower at higher temperatures. For example, θ plateaued at 0.4±0.1 ML at 600°C. The lower saturation coverage is again attributed to oxygen desorption during oxidation. Consistent results were obtained on fully oxidized surfaces, which when heated in vacuum to Tsub=600°C, lost 60% of their adsorbed oxygen. ELS revealed few C=C dimers on the oxidized surfaces, and more graphitization than on unheated surfaces. Oxidation at elevated temperatures also increased the carbonyl to ether ratio, reflecting etching-induced changes in the types of surface sites. The carbonyl and C–H stretch frequencies increased with oxygen dose due to formation of higher oxidation states and/or hydrogen bonding between adjacent groups. The oxygen types did not interconvert when the oxidized surfaces were heated in vacuum. Oxygen desorption generated a much more reactive surface than heating-induced dehydrogenation of the smooth, hydrogenated surface. 相似文献
15.
The adsorption of H 2 and D 2 has been studied on clean and K-promoted Pd(100) surfaces using thermal desorption, work function changes, ultraviolet photoelectron and Auger spectroscopy. The potassium adlayer significantly lowers the sticking coefficient (from 0.6 to 0.06 at θ k = 0.2), and the uptake of hydrogen, but increases the desorption energy for H 2 desorption. Calculation showed that each potassium adatom blocks approximately 4–5 adsorption sites for H 2 adsorption. Atomization of hydrogen led to an increase of hydrogen uptake. The adsorption of potassium on the H-covered surface caused a significant decrease in the amount of hydrogen adsorbed on the surface (as indicated by less desorbing hydrogen below 500 K) and promoted the dissolution of H atoms into the bulk of Pd. The dissolved hydrogen was released only above 600–650 K. In the interpetation of the results the extended charge transfer from K-dosed Pd to the adsorbed H atoms and the direct interaction between adsorbed H and K adatoms are taken into account. 相似文献
16.
The valence-band structure and the vibrational modes of CO adsorbed on nickel-promoted TiO 2(110) surfaces as a function of CO exposure have been studied by means of ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy-loss spectroscopy (HREELS). It is found that CO exists in molecular form at room temperature on the nickel-promoted TiO 2(110) surfaces and most likely binds to the Ni atoms or nickel-affected sites rather than to the substrate atoms. At saturation coverage, CO molecules adsorb simultaneously on the 2-fold bridge sites and terminal sites on the (111)-oriented Ni islands deposited upon TiO 2(110). The occupation of the edge sites of Ni islands gives rise to an anomalously low frequency of the C---O stretching vibration. This frequency, indicative of a weakened C---O bond, suggests existence of a precursor to the dissociated state. 相似文献
17.
Pr concentration dependence of the superconducting transition temperature Tc in the Ho 1−xPr xBa 2Cu 3O 7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y 1−xPr xBa 2Cu 3O 7−δ reference system. Both systems have the same value of the critical concentration xc=0.58, in accordance with nearly equal ionic radii of Ho 3+ and Y 3+ ions. It has been shown that the Tc( x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr IV-ions, if one takes also into account that the number of these ions changes with x. 相似文献
18.
We have investigated the microwave response at 45 GHz in an organic superconductor λ-(BEDT-TSF) 2GaCl 4 with Tc = 4.8 K. We determine the μ0Hc2– T phase diagram from microwave loss and find that the superconducting state is in the pure limit ( l/ ξGL 10). Although the real part of the complex conductivity (= σ1 + i σ2) does not show a coherence peak just below Tc, the London penetration depth completely saturates at low temperatures down to T/ Tc = 0.2, which may provide an evidence for a conventional s-wave pairing. In the metallic state below about 50 K, (parallel to the c-axis) deviates downward from , while σ2, which should be zero in a conventional metal, increases exponentially toward Tc. In spite of the fact that the Hagen–Rubens limit is well satisfied as far as the dc conductivity is concerned, a Drude model is unable to explain the large positive σ2. In order to explain such anomalies in the metallic state, we propose a possible existence of so-called a pseudogap near a Fermi level. The anomalous increase of the positive σ2 may be attributed to an appearance of pre-formed electron pairs in the pseudogap state. This appearance can be regarded as a precursor to the superconducting transition. Such a precursory phenomenon has been observed also in the isostructural FeCl 4 salt with the anomalous metallic states, which shows a negative σ2 in contrast to the GaCl 4 salt. Just the opposite of ground states in between the GaCl 4 and FeCl 4 salts may result in the contrasting anomalous metallic states with different precursory phenomena with opposite signs of σ2. 相似文献
19.
Magnetization measurements on the Fe 60Mn 5Ni 35 and Fe 50Mn 15Ni 35 alloy samples were carried out in the temperature range 80 T300 K and in magnetic fields up to 8 kOe. The Fe 60Mn 5Ni 35 was found to order ferromagnetically with a Curie temperature, Tc, above 300 K. From the temperature dependence of the spontaneous magnetization, Ms, it was concluded that the magnetic behavior of Fe 60Mn 5Ni 35 follows Wohlfarth theory of weak itinerant ferromagnet. The Fe 50Mn 15Ni 35 sample exhibits a magnetic phase transition from ferromagnetism to paramagnetism at Tc=242 K. The critical amplitudes and critical exponents ( β, γ and δ) have been determined by using Arrott plots, Kouvel–Fisher method and scaling plots of the reduced magnetization and reduced magnetic field. The values of β, γ and δ are discussed and compared with the results obtained for various theoretical models and also with the experimentally determined values for related systems obtained by others. 相似文献
20.
The polycrystalline sample of LiFe 1/2Ni 1/2VO 4 was prepared by a standard solid-state reaction technique and confirmed by X-ray diffractometry. LiFe 1/2Ni 1/2VO 4 has orthorhombic crystal structure whose dielectric and electric modulus properties were studied over a wide frequency range (100 Hz–1 MHz) at different temperatures (296–623 K) using a complex impedance spectroscopy (CIS) technique. The frequency and temperature dependence of dielectric constant ( εr) and tangent loss (tan δ) of LiFe 1/2Ni 1/2VO 4 are studied. The variation of εr as a function frequency at different temperatures exhibits a dispersive behavior at low frequencies. The variation of the εr as a function of temperature at different frequencies shows the dielectric anomaly in εr at 498 K with maximum value of dielectric constant 274.49 and 96.86 at 100 kHz and 1 MHz, respectively. Modulus analysis was carried out to understand the mechanism of the electrical transport process, which indicates the non-exponential type of conductivity relaxation in the material. The activation energy calculated from electric modulus spectra is 0.38 eV. 相似文献
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