Surface diffusion of lead on tungsten

Field electron microscopy is used to study the surface diffusion of lead on tungsten. A simple method to measure rough values of the diffusion coefficient and its dependence on sub-monolayer coverage is described and tested. In the region around (001) the displacement energy found is about 1.30 eV/atom up to 10¹⁵ atoms/cm² where it decreases to 0.78 eV/atom. In the residual region except (110) this energy at 1.5×10¹⁴ atoms/cm² is 1.22 eV/atom, it decreases at 4 × 10¹⁴ atoms/cm² to 0.61 eV/atom and increases at 10¹⁵ atoms/cm² to 0.78 eV/atom. Corresponding values of the diffusion coefficient D and of the preexponential D₀ are given. The dependence of D on submonolayer coverage is discussed.     

Temperature dependence of the work function of hafnium islands on tungsten

Two-dimensional islands of hafnium near {100{ faces of tungsten and their terraces were examined by field electron microscopy. The two-dimensional sublimation energy of the islands was found to be 6.1±0.5 eV. The field electron emission from the islands grows with temperature, the effect being considerably greater than theoretically expected. The negative temperature coefficient of ϕ, α_T, for the islands was estimated at (4–6)×10⁻⁴ eV grad⁻¹ or more, whereas for pure tungsten α_T was found to be 2×10⁻⁵ eV grad⁻¹.     

Dissociative Excitation of Tetramethylsilane (TMS) and Hexamethyldisiloxane (HMDSO) by Controlled Electron Impact

The optical emission spectra in the wavelength region 200–800 nm produced by electron impact on the silicon-organic molecules TMS (tetramethylsilane) and HMDSO (hexamethyldisiloxane) under controlled single-collision conditions have been analyzed. Absolute emission cross sections from threshold to 200 eV impact energy were determined for a variety of emission features. For both targets, the CH(A²Δ → X²Π) emission, the so-called CH ?4300 Å”? band, was found to have the largest emission cross section with values (at 100 eV) of 5.5 × 10^?19 cm² and 6.1 × 10^?19 cm² for TMS and HMDSO, respectively. Relatively high onset energies of 28.0 ± 1.5 eV (TMS) and 33.1 ± 1.5 eV (HMDSO) were measured for these emissions. Weaker emission features in both spectra were identified as CH bands corresponding to the B²∑^? → X²Π transition (the CH ?3900 Å”? system) and the C²∑ → X²Π transition, and as the atomic Si line emissions at 253 nm and 288 nm. Near-threshold studies indicate an onset for the Si emissions of 29.0 ± 2.0 eV (TMS) and 44.6 ± 2.0 eV (HMDSO). Absolute cross sections and appearance energies were also determined for the strongest lines of the hydrogen Balmer series for both targets. The comparatively high onset energies and small emission cross sections for all observed emissions indicate that single-step dissociative excitation processes are unlikely to play a dominant role in low-temperature processing plasmas containing TMS and HMDSO.     

Oxygen diffusion in uranium dioxide in the temperature range of phase transitions

The structure of and oxygen diffusion in UO₂ are studied by the molecular dynamics method in the range of transition to the superionic state (melting of the oxygen sublattice) and near the melting point of UO₂. The temperature dependence of the diffusion coefficient of a doubly charged oxygen ion in UO₂ is constructed. In the crystalline state at temperatures between 1800 and 2600 K, this dependence is described by an exponential dependence with a diffusion activation energy of 2.6±0.2 eV. In the superionic state (2600–3100 K), the activation energy of diffusion of an oxygen anion decreases to 1.88±0.13 eV. In melt (3100–3600 K), the exponential dependence of the diffusion coefficient of O^2- persists but the activation energy of diffusion decreases still further, to 0.8±0.2 eV. Our experimental results agree (within the limits of experimental error) with data on oxygen diffusion in the crystalline phase obtained by other researchers.     

AES measurements of adsorption isobars for oxygen on tungsten at low pressure and high temperature

Auger electron spectroscopy (AES) has been employed to determine the relative coverage of oxygen on polycrystalline tungsten at high temperatures (1200 ?T ? 2500 K) and low O₂ pressures (5 × 10^?9 ?po₂ ?5 × 10^?6 Torr). We believe that this is the first demonstration that chemical analysis of solid surfaces by AES is possible even at temperatures as high as 2500 K. It is assumed that the relative oxygen coverage is directly proportional to the peak-to-peak amplitude of the first derivative of the 509 eV oxygen Auger peak. The experimental results illustrate the dependence of coverage on temperature and pressure, and it is shown that the results for low coverages may be described reasonably well by a simple first-order desorption model plus a semi-empirical expression for the equilibration probability (or sticking coefficient). On the basis of this approximate model, the binding energy of oxygen on tungsten is estimated as a function of coverage, giving a value of ～ 140 $\text{kcal}\text{mole}$ in the limit of zero coverage.     

Electron-Impact Fragmentographic Fluorescence Spectroscopy of Ethanol,Acetaldehyde, 1- and 2-Propanol and Acetone

Abstract

Gaseous ethanol, acetaldehyde, 1-propanol, 2-propanol and acetone at pressures of 2 × 10^?5 to 5 × 10^?4 torr were irradiated with electrons of energies 0-1000 eV. The visible-region fluorescence of the excited fragments was then measured. Studies included pressure dependence, electron energy dependence, threshold energy measurements, and excitation curve measurements. The aim of the research was to ascertain if molecules of similar structure could be differentiated by their spectra.     

Positron studies of hydrogen-defect interactions in proton irradiated molybdenum

Molybdenum single crystals are irradiated at 20 K with 6 MeV protons. The radiation damage and lattice defect annealing is studied by positron lifetime spectroscopy in the temperature range from 15 to 720 K. Loss of vacancies due to recombination with mobile interstitials is observed at 40 K (Stage I) in agreement with resistivity measurements. This is the first time Stage I is observed by positrons below 77 K. The implanted hydrogen decorates the vacancies around 100 K, which is consistent with a hydrogen migration energy in molybdenum:E _M ^H = 0.3–0.4 eV. Clustering of spatially correlated vacancies takes place in a wide temperature region below the usual vacancy clustering stage (Stage III). Stage III is observed at rather low temperatures (400–480 K) due to the very high vacancy concentration. Hydrogen bound to vacancies and vacancy clusters is released above 540 K, which puts an upper limit to the hydrogen binding energy:E _B ^H 1.4 eV. The present work emphasizes the advantage of employing a vacancy sensitive technique to study hydrogen in metals, where its intrinsic solubility is low. In such metals (as molybdenum) both the effective solubility and the effective mobility of hydrogen are strongly influenced by the presence of vacancies.     

Cosegregation of tungsten and nitrogen on Fe-9%W-N(100)

Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) have been used to study the cosegregation of tungsten and nitrogen on ferritic Fe-9%W-N(100) single crystals with nitrogen contents ranging from c_N = 11 to 51 wt-ppm. Cosegregation occurs at temperatures T≲600°C depending on the nitrogen content. The thickness of the cosegregated surface layer is estimated by means of Ar⁺ ion depth profiling as being less than two atomic layers. The LEED pattern of the tungsten and nitrogen covered Fe-9%W-51ppmN(100) substrate shows a sharp (1 × 1) structure at a low background intensity indicating epitaxial stabilization of the cosegregated tungsten nitride layer on the bcc (100) surface. The tungsten and nitrogen covered Fe-9%W-11ppmN(100) substrate exhibits a c(2 × 2) structure. On Fe-9%W-51ppmN(100) a temperature-driven phase transition between the cosegregated (1 × 1) and c(2 × 2) surface phases is observed.     

Phase transformations and vacancy formation energies of transition metals by positron annihilation

The technique of the coincidence count rate at the peak of the angular correlation curve (CCR) in positron annihilation has been applied to the investigation of vacancy formation energies in thermal equilibrium in nickel, cobalt, and iron. The monovacancy formation energyE ^1v/F has been determined to (1.55±0.05) eV and (1.34±0.07) eV for nickel and cobalt, and (1.60±0.10) eV for α-iron, and (1.40±0.15) eV for γ-iron, respectively. The structural phase transformations in cobalt (693 K) and iron (1183 K, 1663 K) are exhibited by discontinuities of the CCR. In the case of cobalt the CCR follows exactly the change of the thermal expansion at the transition temperature. The temperature dependence of the CCR in the prevacancy region is found to be proportional to the thermal expansion for all metals investigated.     

Positron trapping and self-diffusion activation energies in chromium

The activation energy for self-diffusion in chromium is found to be 3.51±0.13 eV from positron trapping measurements. The 4.51 eV activation energy seen in tracer diffusion work is therefore ascribed to divacancies, theQ ₂/Q ₁ ratio being typical of the bcc refractory group. The positron data give 0.55 for the ratioH _1v ^M /H _1v ^F of vacancy migration and formation energies, in agreement with quenching data for tungsten.     

Subthreshold sputtering at high temperatures

The sputtering of tungsten from a target at a temperature of 1470 K during irradiation by 5-eV deuterium ions in a steady-state dense plasma is discovered. The literature values of the threshold for the sputtering of tungsten by deuterium ions are 160–200 eV. The tungsten sputtering coefficient measured by the loss of weight is found to be 1.5×10^?4 atom/ion at a deuterium ion energy of 5 eV. Previously, such a sputtering coefficient was usually observed at energies of 250 eV. The sputtering is accompanied by a change in the target surface relief, i.e., by the etching of the grain boundaries and the formation of a wavy structure on the tungsten surface. The subthreshold sputtering at a high temperature is explained by the possible sputtering of adsorbed tungsten atoms that are released from the traps around the interstitial atoms and come to the target surface from the space between the grains. The wavy structure on the surface results from the merging of adsorbed atoms into ordered clusters.     

The mass composition of cosmic rays according to the data obtained using the TUNKA-25 setup

The data obtained using the Cherenkov TUNKA-25 extensive air shower (EAS) installation and the refined method were used to obtain the energy dependence of the average EAS depth maximum. The EAS distributions over the depth of the maximum were obtained for the energies of 5 × 10¹⁵ and 2 × 10¹⁶ eV. An analysis of the shape of distributions indicates that the model of interaction of the nuclei that yields the highest position of the maximum (QGSJET-01) is preferable. If this model is used to account for the maximum’s position, one finds that 〈InA〉 = 1.9±0.2 and helium is dominant in the mass composition at the energy of 5 × 10¹⁵ eV. A sharp increase in the mean mass of the composition is observed at energies in excess of 10¹⁶ eV.     

Photoionization and double excitation spectrum of Be2+

The photoionization cross-section of Be²⁺ has been measured in the range from threshold at 154 eV up to 420 eV. The value at threshold of [1.4 ± 0.2] × 10^?18 cm² is in good agreement with theoretical predictions. The method used was that of absorption using two laser produced plasmas. The doubly excited resonance 2s2p ¹p⁰ at 281.25 ± 0.07 eV has been observed and its Fano profile measured.     

Electrical and dielectric properties of β-BaB2O4 (BBO) and CsLiB6O10 (CLBO) crystals

In barium borate (BBO) crystals, sodium and potassium ions, inherited due to the preparation technique, are dominant charge carriers. The conductivity between layers is higher; the conductivity activation energy and the conductivity at 350 °C being equal to 1.01±0.05 eV and (1.3±0.2)×10⁻⁸ S/cm, respectively. The conductivity activation energy and the conductivity at 350 °C along the channels are equal to 1.13±0.05 eV and to (4±0.2)×10⁻⁹ S/cm, respectively. Relative static permittivity is almost isotropic, and equal to 7.65±0.05. Upon storing of cesium–lithium borate (CLBO) crystals, pre-heating to 600 °C eliminates the influence of surface humidity. At 500 K, the ionic conductivity ranges from 4×10⁻¹² to 2×10⁻¹⁰ S/cm; the conductivity activation energy ranges from 1.01 to 1.17 eV. Relative static permittivity is equal to 7.4±0.3.     

Ar+, He+，S+离子轰击n-InP单晶表面的机理研究

通过X射线光电子能谱和低能电子衍射实验研究了10～180 eV的Ar⁺、 He⁺、S⁺离子轰击n-InP(100)表面, 发现S⁺离子轰击可以产生In-S组分,减轻离子轰击对表面的物理损伤.对于Ar⁺离子轰击后的表面,经过S⁺离子处理和加热过程以后,表面损伤得到了修复,最终得到了2×2的InP表面,进一步验证了S⁺离子对InP表面的修复作用.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH_x(a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.     

Direct evidence for a very low atom displacement threshold energy around the 〈110〉 directions in copper

The dependence of the displacement cross section of Cu on electron energy and crystal orientation has very accurately been determined by high-voltage electron microscopy. The minimum displacement threshold energy of (9.5 ± 0.5) eV occurs at an angle of 10° from 〈110〉 directions.     

Optical properties of dangling-bond states at cleaved silicon surfaces

The spectral dependence of surface photovoltage and surface photoconductance both under continuous illumination as well as LEED I/V spectra were studied with cleaved Si(111)-2 × 1 surfaces at 130 K. Between 0.23 and 0.5 eV a doubly peaked absorption band was found with opposite sign compared to the SPV and SPC signals at higher photon energies. This band is due to electronic transitions from occupied to empty dangling-bond states located at the raised and the lowered rows of atoms in the 2 × 1 reconstruction, respectively. This absorption shows a pronounced dependence on the polarization of the incident light which correlates with the spatial symmetry of the dangling-bond states. Anneals at up to 500 K remove the low-energy absorption peak and equalize the 2 × 1 reconstruction: The homogeneous Si(111)-2 × 1 structure exhibits a buckling of 0.3 Å and a dangling-bond absorption with a threshold at 0.42 eV and a maximum at 0.47 eV. An anneal at 750 K, forming the 7 × 7 structure, destroys the peak of opposite sign in SPV and SPC and only leaves a broad tail with a threshold of 0.32 eV.     

Une détermination du coefficient d'autodiffusion de surface en présence d'une couche d'adsorption à l'aide de pointes àémission de champ (nickel sur tungsténe)

The tip blunting technique to measure the surface self-diffusion of clean metals (A. Piquet, Vu Thien Binh, H. Roux, R. Uzan and M. Drechsler) is extended to study the influence of an adsorption layer on diffusion. The system studied is nickel on tungsten. The increase of the apex radius is measured by means of FEM characteristics. In the temperature range used (1200–1500 K), the nickel monolayer (1.16 × 10¹⁵ atoms/cm²) is maintained by compensation of desorbed Ni atoms with a continual flux from an evaporation source. The adsorption life time between 1350 and 1500 K decreases from 850 to 16 s. The conservation of the degree of coverage leads to a method to determine the desorption activation energy of nickel (E_d = 4.56 eV/atom). The surface self-diffusion data of tungsten with a nickel monolayer are found to be D₀ = 3 × 10^?3cm/²s and Q_s = 1.9 eV/atom, compared to the clean tungsten data D₀ = 1 cm²/s and Q_s = 3.1 eV/atom. The Ni monolayer increases the surface self-diffusion coefficient by a factor 160 at 1200 K and 20 at 1500 K. The results are discussed with respect to nickel activated sintering of tungsten powders.