Coverage dependent promoter action: K coadsorption and reactions with CO and CO2 on Pd{1 1 0}

The adsorption of CO and CO₂ on K-predosed Pd{1 1 0} at room temperature has been examined via reflection–absorption infrared spectroscopy (RAIRS). CO₂ adsorbs on 0.37 ML K-predosed Pd{1 1 0} with high sticking probability and a reactive chemisorbed intermediate, CO₂⁻, is detected in RAIRS at room temperature. Reaction of this species ultimately yields carbonate. The same high K precoverage induces dissociation of CO at low CO exposure. Carbonate is detected at higher CO exposure and is probably produced via stepwise oxidation of molecularly adsorbed CO. In contrast at low K precoverage (0.11 ML), CO remains intact but the C–O bond is considerably weakened with respect to CO chemisorbed on clean Pd{1 1 0}. These findings illustrate a dual promoter mechanism of K in the adsorption and reaction of CO or CO₂ at high K coverage. The alkali metal induces dissociation of these molecules and directly participates in the formation of a surface compound, K₂CO₃.     

Adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface

The adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface has been studied with scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). At 0.1 ML Cs coverage the whole surface exhibits a mixture of (1 × 2) and (1 × 3) reconstructed structures, indicating that Cs atoms exert a cooperative effect on the surface structures. Real-time STM observation shows that silver atoms on the Cs-covered surface are highly mobile on the nanometer scale at 300 K. The Cs-reconstructed Ag(1 1 0) surface alters the structure formed by dissociative adsorption of oxygen from p(2 × 1) or c(6 × 2) to a p(3 × 5) structure which incorporates 1/3 ML Ag atoms, resulting in the formation of nanometer-sized (10–20 nm) islands. The Cs-induced reconstruction facilitates the adsorption of CO₂, which does not adsorb on unreconstructed, clean Ag(1 1 0). CO₂ adsorption leads to the formation of locally ordered (2 × 1) structures and linear (2 × 2) structures distributed inhomogeneously on the surface. Adsorbed CO₂ desorbs from the Cs-covered surface without accompanied O₂ desorption, ruling out carbonate as an intermediate. As a possible alternative, an oxalate-type surface complex [OOC–COO] is suggested, supported by the occurrence of extensive isotope exchange between oxygen atoms among CO₂(a). Direct interaction between CO₂ and Cs may become significant at higher Cs coverage (>0.3 ML).     

Metastable (2S) helium atom scattering from NiO(100) and Cu(100) surfaces

The scattering of metastable 2³S He atoms (He*) from cleaved NiO(100) as well as from clean and CO-covered Cu(100) surfaces has been studied. For these varied surfaces, which were characterized in situ by ground state He atom scattering, only broad He* angular distributions without any diffraction peaks were observed. For metastable He atoms scattered from the clean Cu(100) surface a total survival probability of 1×10⁻⁶ was determined. For NiO(100) and the CO-covered Cu(100) surface values of about 1×10⁻⁵ were obtained. Time-of-flight spectra of the surviving He* atoms revealed a substantial energetic broadening which increases with the substrate temperature. This behaviour indicates a large well depth for the He*–surface interaction potential and is discussed in terms of an enhanced multiphonon excitation and/or trapping probability upon the scattering.     

Adsorption of oxygen on Au(111) by exposure to ozone

Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O₃ 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 Θ_O=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 O₂ to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O₂ desorption peak at 520 K at low Θ_O, and the peak shifts to higher temperatures for increasing oxygen coverages up to Θ_O=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at Θ_O=1.2 ML. Analysis of the TPD data indicates that the desorption of O₂ from Au(111) can be described by first-order kinetics with an activation energy for O₂ 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.     

(e,2e)反应三重微分截面的计算

给出了利用BBK模型计算电子碰撞电离H,He⁺,He和Li⁺三重微分截面的通用方法,适用于中、高入射能量下的各种几何条件。计算结果与其它理论结果和实验数据进行了比较,符合得很好。     

Two-component exciton model

We have measured the broadening, and in one case also the shift, of alkalinS andnD Rydberg levels by alkali perturbers (Cs by Cs, Rb by Cs and K, and K by Rb) applying the Doppler-free two-photon technique and thermionic detection. Taking into account recent experimental data for the self-broadening of K and Rb Rydberg levels we found that for low principal quantum numbersn the broadening data of the levels are in agreement within the limits of experimental uncertainty in the pure and mixed cases. For highn we could establish that the broadening of the levels is governed by the perturber-valence electron and the perturber-ion core interaction. With increasingn the broadening rates are only dependent on the sort of perturber. As reported recently for Rb-Rb and K-K, oscillations in the broadening data as a function ofn were also observed for Cs-Cs and in the mixed vapours. Our results do not support recent theories which describe the oscillations in terms of resonances in the scattering of the quasifree valence electron on the perturber atom. In a classical absorption experiment oscillations in the self-broadening and shift of the Cs principal series lines discovered by Mazing and Serapinas were found to be due to satellite structures which are merged in non-Lorentzian line wings.     

Elemental steps in the growth of thin β-Ga2O3 films on CoGa(1 0 0)

The oxidation of CoGa(1 0 0) at 700 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy (AES). At 700 K, thin well-ordered β-Ga₂O₃ films grow on CoGa(1 0 0). The EEL spectrum of the Ga-oxide films exhibit Fuchs–Kliewer phonons at 305, 455, 645, and 785 cm⁻¹. For low oxygen exposure (<0.2 L), the growth of oxide-islands starts at step edges and on defects. The oxide films have the shape of long, rectangular islands and are oriented in the [1 0 0] and [0 1 0] directions of the substrate. For higher oxygen exposure, islands of β-Ga₂O₃ are found also on the terraces. After an exposure of 200 L O₂ at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin film of β-Ga₂O₃.     

Adsorption of thiophene on Al(1 1 1) and deposition of aluminum on condensed thiophene

The adsorption of thiophene on clean Al(1 1 1) at 90 and 130 K has been studied with X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and work function measurements. Relatively weak chemisorption compared to adsorption on transition metals is indicated by minor changes in the valence spectrum in progressing from monolayer to multilayer thiophene, a modest work function change of −0.50 eV due to saturation dosing, and return of the work function and valence spectrum to that of clean Al(1 1 1) upon annealing at 210 K. The complementary experiment in which aluminum is thermally deposited on multilayer thiophene condensed on gold at 130 K has also been performed. XPS peak area analysis shows that metal doses less than 14×10¹⁵ atoms/cm² result in penetration through the physisorbed thiophene, but higher doses lead to the gradual build up of metal throughout the organic layer. Persistence of the thiophene UPS valence features for metal doses of 50×10¹⁵ atoms/cm² is consistent with penetration and aluminum island formation. For aluminum deposition on thiophene, charge transfer from aluminum is evidenced by metal-induced low binding energy components in the C 1s and S 2p spectra at 282.6 and 162.5 eV, respectively, and a shift in the Al 2p spectrum of 0.5 eV to higher binding energy compared to metallic aluminum. UPS also indicates progression of the frontier orbital toward the Fermi level as aluminum is deposited.     

A new fully quantum-mechanical method used to calculate the collisional broadening coefficients and shift coefficients of Rb D_1 lines perturbed by noble gases He and Ar

In this work, a new full quantum method is proposed to calculate the broadening and shift coefficients of the D_1 line in neutral collision. Based on the variable phase approach and Baranger theory, this method calculates the scattering phase shift instead of scattering matrix elements in order to simplify the calculation. As an illustration, this method is used to calculate the broadening and shift coefficients of the absorption lines of alkali metal atom Rb, as it collides with buffer gas He and Ar, in a temperature range from 150 K to 800 K. With a comparison with other calculations and experiment measurements, the reasonable agreements in all cases demonstrate the validity and simplicity of this method.     

Na||Sb-Pb-Sn液态金属电池电极的价电子结构与热-电性能计算

应用固体与分子经验电子理论系统地研究液态金属池Na||Sb-Pb-Sn电极的价电子结构与热、电性能.研究结果表明:电极合金的价电子结构与其性能密切关联.阴极合金Na_1–xIA_x (IA=K, Rb, Cs)的晶格电子随着掺杂量的增加而减少,诱发合金的熔点、结合能随掺杂量的增加而降低. Na离子输运到阳极,与阳极Sb-Sn-Pb形成产物NaSb_3, NaSn, Na₁₅Sn_4, NaPb.其理论熔点与实验相符. NaSb_3的平均晶格电子数最少,开路电压最高.研究表明:对于Na||Sb-Pb-Sn液态金属电池体系而言,晶格电子扮演重要的角色,可以调控电极的热、电性能.     

氦氢离子协同注入对锆膜形貌及物相结构的影响

 采用磁控溅射法制备了Zr-Mo膜,随后在低能静电加速器上分别采用剂量为2.80×10¹⁷~1.12 ×10¹⁸ions·cm^-2的He⁺、H⁺离子辐照Zr-Mo膜,利用光学透镜、扫描电镜、原子力显微镜和X射线衍射研究He⁺、H⁺离子协同注入效应对Zr-Mo膜微观结构的影响。实验结果表明：原始Zr-Mo膜表层晶粒清晰可见,尺寸约为200nm;辐照效应可导致Zr-Mo膜表层产生微观损伤区域,在注He⁺基础上注H⁺导致Zr-Mo膜出现更为严重的损伤现象;离子注入的表面溅射效应可使膜面晶粒边界逐渐刻蚀退让,导致膜面更加光滑、细致;He⁺、H⁺ 离子协同注入可使Zr-Mo膜晶格发生畸变,注入期间未使Zr-Mo膜发生吸H相变生成氢化物。     

Spektroskopische Untersuchung unelastischer Stöße zwischen Ionen oder metastabilen Atomen der Edelgase und Alkalimetallatomen bei thermischen Stoßenergien

The present work deals with the spectroscopic investigation of Penning ionization and nonresonant charge exchange due to thermal-energy collisions of metastable atoms or ions of the inert gases He, Ne with atoms of alkali metals K, Rb, Cs in a low temperature plasma of the corresponding binary mixtures. The rate constants of these reactions and partial rate constants of exciting several alkali metal ion levels were determined by the kinetics of the decay of ionized and excited particles in the afterglow of a pulsed discharge. Measured Penning and charge exchange constants are in order of 10^10-9 cm³ sec^?1. Partial Penning constants in the He? Cs system exhibit a small dependence of the energy defect of reaction, whereby an important part of collisions results in excited Cs⁺-states. On the contrary, energy transfer by charge exchange in He? Rb, Cs systems distinguishes by a significant energy defect dependence. Charge exchange from He⁺ to Rb₀ leads above all to He(2³S₁)-atoms and normal Rb⁺-ions. It follows from the metal ion spectrums, that Penning ionization and charge exchange of the investigated systems under utilized conditions represent as processes of selective excitation of alkali metal ions in afterglow and stationary discharge.     

Low-energy D scattering from Cs and CsCl adsorbed on the Si(100)2 × 1 surface

On the basis of neutralization and inelastic scattering of low-energy D⁺ ions, the bond nature of Cs and CsCl adsorbed on the Si(100)2 × 1 surface has been investigated. In a low-coverage regime ( < 0.2 ML), Cs is adsorbed ionically on Si(100) and subsequent oxygen adsorption readily causes the ionic Cs---O bond. CsCl is dissociatively adsorbed on the surface in a low-coverage regime due to the preferential reaction of Cl with the Si dangling bond and the resultant Cs is ionically bonded to Si.     

Adsorption of NO on Au atoms and dimers supported on MgO(1 0 0): DFT studies

The adsorption of NO on single gold atoms and Au₂ dimers deposited on regular O²⁻ sites and neutral oxygen vacancies (F_s sites) of the MgO(1 0 0) surface have been studied by means of DFT calculations. For Au₁/MgO the adsorption of NO is stronger when the Au atom is supported on an anionic site than when it is on a F_s site, with adsorption binding energies of 1.1 and 0.5 eV, respectively. In the first case the spin density is mainly concentrated on the metal atom and protruding from the surface. In such a way, an active site against radicals such as NO is generated. On the F_s site, the presence of the vacancy delocalizes the spin into the substrate, weakening its coupling with NO. For Au₂/MgO, as this system has a closed-shell configuration, the NO molecules bonds weakly with Au₂. Regarding the N–O stretching frequencies, a very strong shift of 340–400 cm⁻¹ to lower frequencies is observed for Au₁/MgO in comparison with free NO.     

Electrical resistivity of intercalated molybdenum disulfide

The electrical resistivity of hexagonal synthetic single crystals and natural crystals of MoS₂ intercalated with alkali metals (K, Rb and Cs) has been measured. The average room-temperature values is 0.004 ω-cm, and the best single crystal data on K_0.4MoS₂ showed at T^1.5 temperature behaviour characteristic of acoustical phonon scattering for a quasi-isotropic metal. The metallic behaviour and scattering mechanism are discussed.     

碱金属在GaAs(110)表面上的吸附

本文讨论Li,Na,K,Cs在GaAs(110)表面上的吸附,考虑理想表面和弛豫表面两种情况。计算采用集团模型,用电荷自洽的ExtendedHucheltheory(缩写为EHT)方法进行。结果表明,吸附后表面原子趋向于理想位置,碱金属原子位于垂直于表面沿[001]方向横跨表面Ga原子的对称平面上。碱金属吸附后的费密能级在价带顶以上约0.7eV处,是由表面Ga原子与碱金属原子间的相互作用决定的。而在价带中碱金属原子主要与表面As原子成键。 关键词：     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

Positron annihilation studies of defects in helium-irradiated silicon

Positron annihilation studies have been carried out on Si-n irradiated with He⁺ ions at the V.U.B. cyclotron, to a dose of 4×10¹⁷ He/cm². No temperature dependence on the S-parameter and lifetimes is seen below the irradiation temperature. The positron lifetime associated to the created defects is 290 ps. During the isochronal annealing, this lifetime stays constant up to 700 K. It is attributed to the annihilation of positrons from large vacancy-clusters filled with He atoms. From the isochronal annealing results, only one annealing stage is seen. This annealing stage which extends over a long range of temperature 700–1000 K, is ascribed to the degassing of helium atoms from defects and the growth of vacancy-clusters. The lifetime of positrons in those defects reaches a value of about 530±30 ps at 1000 K, indicating that the vacancy-clusters formed have a mean size of more than 8 vacancies.     

Adsorption of carbon monoxide on Pt{100} surfaces: dependence of the CO stretching vibrational frequency on surface coverage

We have used the ab initio cluster model approach to study the dependence of the CO stretching frequency on CO surface coverage. We have also investigated the relative importance of the various factors that can affect the position of the CO stretching band as coverage increases. Two effects can change the CO stretching frequency: the adsorbate–adsorbate dipole coupling, which is a purely physical effect, and the changes in the 2π^* CO molecular orbitals, due to the different chemical environment at higher coverages. From our vibrational analysis, we conclude that CO–CO dipole coupling is the main cause of the upward shift of the CO stretching band when the CO coverage is increased. The population of the 2π^* CO molecular orbitals does not change at any coverage within the region considered. We have also estimated the ¹²CO–¹³CO dipole coupling, which previous studies have assumed to be weak. Our results demonstrate that the ¹²CO–¹³CO dipole coupling is indeed weak compared with the ¹²CO–¹²CO dipole coupling. At a CO surface coverage of 0.5 monolayers (ML), we have calculated a band shift of 40 cm⁻¹ to higher frequency. However, we should point out that when one ¹²CO molecule is surrounded by a ¹³CO environment, the ¹²CO stretching band shifts 10 cm⁻¹ upwards. We have also computed the heat of adsorption of CO on Pt{100}-(1×1) as a function of CO coverage. The initial heat of adsorption is calculated to be about 192 kJ mol⁻¹ and then drops to 180 kJ mol⁻¹ at 0.5 ML. These results agree quite well with recent calorimetric measurements. Besides that, we have estimated that the CO–CO interaction energy at 0.5 ML is repulsive and has a value of 5 kJ mol⁻¹.     

In-situ TEM observation of the evolution of helium bubbles in Mo during He+ irradiation and post-irradiation annealing

The evolution of helium bubbles in purity Mo was investigated by in-situ transmission electron microscopy (TEM) during 30 keV He⁺ irradiation (at 673 K and 1173 K) and post-irradiation annealing (after 30 keV He⁺ irradiation with the fluence of 5.74×10¹⁶ He⁺/cm² at 673 K). Both He⁺ irradiation and subsequently annealing induced the initiation, aggregation, and growth of helium bubbles. Temperature had a significant effect on the initiation and evolution of helium bubbles. The higher the irradiation temperature was, the larger the bubble size at the same irradiation fluence would be. At 1173 K irradiation, helium bubbles nucleated and grew preferentially at grain boundaries and showed super large size, which would induce the formation of microcracks. At the same time, the geometry of helium bubbles changed from sphericity to polyhedron. The polyhedral bubbles preferred to grow in the shape bounded by {100} planes. After statistical analysis of the characteristic parameters of helium bubbles, the functions between the average size, number density of helium bubbles, swelling rate and irradiation damage were obtained. Meanwhile, an empirical formula for calculating the size of helium bubbles during the annealing was also provided.