Desorption of H2 from W(110) and Fe covered W(110) surfaces

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/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 ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s 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.     

Low energy electron diffraction from Na(110) and Na2O(111) surfaces

Surfaces ofNa(110) are grown, investigated and oxidised to give Na₂O(111) surfaces. LEED spectra are taken for these surfaces and compared with theory to determine the surface composition of sodium oxide: the surface terminates the crystal in an integral number of electrically neutral NaONa sandwiches, with a bulk-like inter layer spacing. The effective Debye temperature for the Na(110) surface was found to be 107 K.     

23 excitation of Li+ by electron impact

The 1¹S→2³S excitation of Li⁺ has been studied in the Coulomb-Born-Oppenheimer approximation. The differential and the total cross sections are given at double the threshold energy.     

Electron—impact ionization of Li^＋（ls^2） in the coplanar equal energy—sharing geometry

In this paper, the triple differential cross section for the low-energy electron impact ionization of the Li⁺ ion is considered in the coplanar equal energy-sharing kinematics at an incident energy of 114.083 eV. The emergence of structures in the calculated cross sections is explained in terms of isolated two-body final-state interactions and three-body coupling. The cross section shows two peaks originating from ′classical′ is determined by two-body final-state interactions. In addition, it is demonstrated that the signature of three-body interactions is carried by the magnitude and ratio of these two peaks. The direct and exchange amplitudes are also considered.     

Surface states at the clean surfaces of cleaved Si(111) and GaAs(110)

A combination of ellipsometric data on the electronic transitions from occupied to unoccupied surface states and published photoemission data on the energy distribution of the occupied surface states has been used to construct models of the surface states densities at the cleaved Si (111) and GaAs (110) surfaces.     

Scattering of N2 from Ni(111)

A cold (T_rot<10 K) beam of N₂ with an initial translational energy of 0.40 eV strikes an Ni(111) surface at surface temperatures from 300 to 873 K at several incident angles from 15 to 60°. The rotational energy and angular distributions of the scattered molecules are probed using (2+1) resonance-enhanced multiphoton ionization. Molecules scattered in the specular direction have mean rotational energies that are independent of surface temperature, whereas those scattered at angles far from the specular show a dependence on surface temperature, caused likely by multiple collisions with the surface before escape. A rotational rainbow, seen in systems such as CO–Ni(111) and N₂–Ag(111), is not seen in this system. For molecules that scatter close to the specular direction, approximately 10% of the initial translational energy is converted into rotational energy of the scattered N₂. For surface temperatures above room temperature, the angular distributions indicate that molecules that scatter into low-J states also tend to exit in a broad peak (10–20° FWHM) near the specular, and this peak is broadened with increasing incident angle. The molecules that scatter into high-J states have a much broader distribution, indicating that they are trapped rotationally during the initial collision and suffer multiple collisions before leaving the surface.     

Ionic implantation of N2+ in Ni(110) at 300 K

The present work gives results of a preliminary investigation, carried out by SES, AES, LEED and ELS, on the implantation of nitrogen ions in Ni(110) as a function of ion dose and subsequent surface heat treatment at different temperatures. The fine structure in the SES spectrum is the most sensitive to implantation: peaks at 9, 17.5 and 31.5 eV are shifted towards lower energies by E = 1 eV for the first two peaks and 2.8 eV for the last. At high nitrogen doses a disordered layer is observed by LEED. The p(2 × 3) structure is obtained when the crystal is heated to 750 K. The two electron loss peaks of 4.8 and 10 eV arise from an induced electron N_2p level situated 4.8 eV below the Fermi level.     

Li~+,Zn~(2+),Mg~(2+)掺杂Lu_2O_3∶Er~(3+)荧光粉的制备及发光特性

用微波高温固相法合成了Er~(3+)单掺Lu_2O_3,Li~+与Er~(3+)共掺Lu_2O_3及Li~+,Zn~(2+),Mg~(2+)掺杂Lu_2O_3∶Er~(3+)的荧光粉。实验表明金属离子Li~+、Zn~(2+)、Mg~(2+)、Er~(3+)掺杂Lu_2O_3,不影响Lu_2O_3的立方晶相。扫描电子显微镜测量表明,Li~+掺杂可以有效改善粉体的分散性和形貌,Li~+,Zn~(2+),Mg~(2+)共掺杂获得的粉体颗粒分布更加均匀,粒径范围为80～100 nm。379 nm激发下,Li~+与Er~(3+)共掺样品发光较单掺Er~(3+)样品在565 nm处的发光增强了4.5倍,而Li~+、Zn~(2+)、Mg~(2+)与Er~(3+)共掺样品较其发光增强5.3倍。980 nm激发下,Li~+与Er~(3+)共掺样品,Li~+、Zn~(2+)、Mg~(2+)与Er~(3+)共掺样品的发光分别比单掺Er~(3+)样品在565 nm处发光增强23倍与39倍,在662 nm处发光强度分别增强20倍与43倍。379 nm激发下,较单掺Er~(3+)的样品,掺杂Li~+的样品和Li~+,Zn~(2+),Mg~(2+)和Er~(3+)共掺的样品荧光寿命均有所增加,而Zn~(2+)、Er~(3+)共掺及Mg~(2+)、Er~(3+)共掺样品的荧光寿命则有所缩短。     

H++CO2散射在30电子伏能量下电荷转移和非电荷转移的非绝热分子动力学模拟

在大气化学、天体物理学和癌症质子疗法中都涉及到高能H⁺+CO₂散射反应.本文在最简电子-核动力学（SLEND）基础上系统研究在30 eV下的H⁺+CO₂散射.SLEND用经典力学描述核,用单行列式Thouless波函数描述电子.本文模拟了CO₂在42个取向共3402条轨线,为理解H⁺+CO₂散射中的各个反应过程和机制提供了系统描述：非电荷转移散射（NCTS）,电荷转移散射（CTS）和C=O双键的断裂,这些关于反应的有用信息不能完全从实验中获取.本文提供了散射的详尽细节,包括随CO₂取向不同主彩虹角和次彩虹角的出现和合并.SLEND NCTS和CTS的微分散射截面通过高等半经典方法计算,显示NCTS散射截面对所有散射角都同实验符合得很好,而CTS散射截面只大散射角时同实验结果符合得很好而在小角散射时稍差.无论是CTS还是NCTS SLEND都预言了主彩虹角的特征,这同实验完全一致.     

SiO production from Si(100) and (111) surfaces by reaction with O2 beams

Desorption kinetics of SiO in the reaction of O₂ with Si(100) and (111) surfaces were investigated at surface temperatures between 1000 and 1300 K by using a pulsed molecular beam technique. The gaseous SiO product was detected by a mass spectrometer above 1000 K. At temperatures lower than 1050 K, the SiO signal appeared with an induction time after the O₂ beam irradiation onto the surface, which means that the desorption occurs via sequential steps. The rate constants for two steps were obtained by a computer simulation of the relaxation waveforms of the SiO signal. The values obtained are in the same range as those of D'Evelyn et al. However, they are one order of magnitude larger than those of Yu and Eldridge. The activation energies for the two steps are 2.8 and 2.4 eV for both Si(100) and (111) surfaces. No significant difference between the two kinds of surfaces was found from these values. However, many etch pits were observed on the Si(100) surface after the reaction, while no such etch pits were formed on the Si(111) surface. The planes of the etch pits formed on the (100) surfaces consisted mostly of the {111} facets.     

Li+低能(e,2e)反应角分布

利用修正的BBK理论，考虑入射道的库仑相互作用及出射电子的交换对称性，在共面-等能分享-垂直角度碰撞几何中，分别计算了能量为79.6，105.6，227.6和375.6eV的入射电子碰撞Li⁺(1s²)(e，2ｅ)反应三重微分截面(TDCS).结果表明：在接近阈能的碰撞中，两出射电子连线平行于入射电子方向时，TDCS最大；两出射电子连线垂直于入射电子方向时，TDCS最小；入射电子能量达5倍的电离阈能时，主要为单次双体碰撞，而且入射道库仑场对决定低能碰撞的TDCS起着重要作用. 关键词：     

Adsorption of H2S,H2O and O2 on Si(111) surfaces

Electron energy-loss spectroscopy has been applied to the study of Si(111) surfaces covered with H₂S, H₂O and O₂ at room temperature and the surfaces annealed at ～ 600°C. The experimental results strongly suggest that H₂S and H₂O adsorb in the molecular states at room temperature. It is proposed that O₂ is first adsorbed in a molecular state, then adsorbs as atoms, and finally oxidizes forming SiO₂.     

Electron reflection measurements on W(001) and W(110) between 0 and 40 eV

High resolution low energy electron reflection measurements from W(001) and W(110) in the range 0–40 eV are reported. Spectra recorded for a variety of angles of incidence in several different azimuths are presented. In particular, some fine structure features associated with beam thresholds are observed for all incidence conditions. However, it is found that other threshold effects are either absent or very weak. It is shown that this phenomenon is probably a consequence of the limited azimuthal resolution of the spectrometer used. Because of the variety of incidence conditions investigated, the measurements comprise a suitable data base for testing LEED calculation schemes in the very low energy range.     

Work function and LEED study of Na and O2 covered surfaces of W(112)

Coadsorption of Na and O on W (112) surfaces has been studied with LEED and work function measurement. The composition and sequence of deposition of the adsorbates have been varied systematically. The results suggest that Na and O form a W-O-Na double layer, regardless of the sequence of deposition. It is found that the underlying O layer causes increased interaction among the Na atoms which improves the long range older of the Na array. An expansion of the Na layer due to O adsorption has been observed.     

The vectorial photoelectric effect on cesiated GaAs (110) and Si (111) surfaces

Polarization dependence of photoelectric emission at 45° incidence was investigated on cesiated Si (111) and GaAs (110) surfaces. Yield and energy distributions from surfaces covered with submonolayers of cesium did not depend on polarization of the light. Energy distributions and emitted current depend on the plane of polarization when more than a monolayer of cesium was deposited onto cooled samples. These phenomena can be explained in terms of radiation density calculated from Fresnel's equation and bulk optical properties of the semiconductor; no polarization dependence of the optical excitation need be invoked.     

共面双对称几何中Li+(1s2)(e,2e)反应的理论研究

利用Berakdar理论,考虑了入射道的库仑相互作用,在共面双对称碰撞几何中,计算了能量为251eV入射电子碰撞Li⁺(1s²)(e,2e)反应的三重微分截面(TDCS),讨论了不同散射振幅对截面的贡献,分析了干涉效应及入射道库仑场对截面的影响.结果表明,它们对决定TDCS的角度分布和大小起着重要作用. 关键词：     

Oxidation of W(110) studied by LEED and STM

The oxidation of W(110) was studied over a temperature a range of 1000 K to 1600 K at 1 × 10^? 6 Torr oxygen. The subsequent oxide structure was then characterized using Low Energy Electron Diffraction (LEED) and Scanning Tunneling Microscopy (STM). It was found that the resulting structure was remarkably similar to that of Mo(110) oxidized under similar conditions. Using the Mo(110) oxide structure as our basis, along with atomic resolution STM images, we have constructed a model for the surface oxide of W(110).     

Chemical effects in the N7 VV Auger spectra from W(100) and W(110) surfaces

Fine structure in the N₇ VV Auger spectra from clean W(100) and W(110) surfaces has been interpreted by Lander's band model for the doubly ionized final state. It is shown that the energies of the prominent emissions in the spectra are similar for the two surfaces and furthermore are consistent with the self convolution of a bulk density of states for tungsten. An additional feature in the spectrum from the W(100) surface has been attributed to emission from an intrinsic surface state at ?0.4 eV. The localization of this state at the surface was confirmed by its sensitivity to adsorbates (H₂, CO, O₂ and I₂). During the interaction of these gases with the surface the Auger spectra always retained the features attributed to the bulk density of states which were modified only by a shift in the background intensity profile. New emission features in this part of the spectra were not seen except for the example of hydrogen adsorption when a single new emission could be seen on each of the two tungsten surfaces. However, each adsorbate produce either one (H₂) or two (CO, O₂ and I₂) new emissions at lower energies which were attributed to emissions from new adsorbate derived levels which reside at energies below the prominent features of the tungsten valence band. The location of these new adsorbate levels is compared and contrasted with the equivalent ultraviolet photoelectron spectroscopic determinations.