Electron impact desorption of hydrogen on tungsten: II. Hydrogen in mixed adsorption layers

The electron impact desorption (EID) of H⁺ ions and of H₂ molecules from hydrogen coadsorbed with carbon monoxide or oxygen on tungsten has been investigated mass spectrometrically. It is shown that the high EID cross sections for hydrogen on tungsten reported in some earlier investigations must have been due to coadsorbed states. These states have been investigated in some detail with respect to their general adsorptive (relative coverages, sticking coefficients, isosteric heats, and desorption rates) and their EID properties (total and ionic cross sections, threshold energies). The results stress the high specificity of EID for certain (usually weakly bound) adsorption states and its applicability for the investigation of such states even in the presence of other states with much higher populations.     

Electron impact desorption of Xe from the tungsten (110) plane

The electron stimulated desorption of Xe adsorbed on the clean and on oxygen and CO covered tungsten (110) surfaces has been investigated. Only neutral Xe desorption was observed; for Xe on clean W a very small initial regime with cross section 10^?17cm² is followed by a slow decay with cross section 3×10^?19cm². The Xe yield varies nonlinearly with coverage, suggesting desorption from edges of islands or from sites with less than their full complement of nearest neighbor Xe atoms. Desorption from oxygen or CO covered surfaces results in an apparent desorption cross section identical to that of the underlying adsorbate. This results from a kicking off of Xe by electron desorbed O or CO. The true cross sections for these processes are ～10^?14cm² for Xe-0 and ～10^?15 cm² for Xe-CO. Some speculations about the mechanism, particularly the absence of ions are presented.     

Oxygen adsorption on the tungsten (110) plane. Electron impact and thermal desorption at high temperatures

When a layer of oxygen on the (110) plane of tungsten at coverages O/W≦0.5 is heated from 100 K, O⁺ evolution under electron impact becomes almost negligible at 600 K. On further heating, however, a slow, temperature-dependent evolution of O⁺ current is observed atT≳1500 K. For O/W>0.3 there is also desorption under massive bombardment. Once an equilibrium value of O⁺ current has been established, there is rapid adjustment to the appropriate equilibrium value when the temperature changes in the range 1500–1700 K. On cooling toT<1000 K, O⁺ decreases rapidly; on reheating toT>1500 K, O⁺ increases slowly again. Above 1700 K there is thermal desorption which is also reflected in the O⁺ signal. These facts indicate that there is a slow activated evolution of an electron sensitive state above 1500 K, from a reconstructed state formed by heating the low temperature layer toT≧1000 K. The latter state seems to be reformed on cooling below 1500 K.     

Electron stimulated desorption of cesium atoms from germanium-covered tungsten

The electron stimulated desorption (ESD) yield and energy distributions for Cs atoms from cesium layers adsorbed on germanium-covered tungsten have been measured for different Ge film thicknesses, 0.25-4.75 ML (monolayer), as a function of electron energy and cesium coverage Θ. The measurements have been carried out using a time-of-flight method and surface ionization detector. In the majority of measurements Cs is adsorbed at 300 K. The appearance threshold for Cs atoms is about 30 eV, which correlates well with the Ge 3d ionization energy. As the electron energy increases the Cs atom ESD yield passes through a wide maximum at an electron energy of about 120 eV. In the Ge film thickness range from 0.5 to 2 ML, resonant Cs atom yield peaks are observed at electron energies of 50 and 80 eV that can be associated with W 5p and W 5s level excitations. As the cesium coverage increases the Cs atom yield passes through a smooth maximum at 1 ML coverage. The Cs atom ESD energy distributions are bell-shaped; they shift toward higher energies with increasing cesium coverage for thin germanium films and shift toward lower energies with increasing cesium coverage for thick germanium films. The energy distributions for ESD of Cs from a 1 ML Ge film exhibit a strong temperature dependence; at T = 160 K they consist of two bell-shaped curves: a narrow peak with a maximum at a kinetic energy of 0.35 eV and a wider peak with a maximum at a kinetic energy of 0.5 eV. The former is associated with W level excitations and the latter with a Ge 3d level excitation. These results can be interpreted in terms of the Auger stimulated desorption model.     

Electron stimulated desorption of H− and H+ from hydrogen covered tungsten

Electron stimulated desorption from a hydrogen covered polycrystalline tungsten surface has been investigated. The H^? and H⁺ desorption current as a function of incident electron energy was measured in both the low and high energy region. Some threshold-like structures appear in the curve of the desorption current versus incident electron energy; however, no threshold structures have been observed for H^? desorption in the high energy region. The experimental results obtained and the desorption process are discussed in some detail, including initial excitation, charge transfer and ultimate desorption.     

Electron stimulated desorption study of oxygen adsorption on tungsten: Evidence for order—disorder transition

The adsorption of oxygen on a polycrystalline tungsten surface at ~300 K has been studied by means of electron stimulated desorption (ESD) Although precision gas dosing was not employed, the initial sticking probability for dissociative adsorption appears to be essentially unity, while the variation with coverage suggests that a high degree of order exists and that precursor state kinetics are significant. A most noticeable and reproducible discontinuity in ESD parameters occurs at a fractional coverage θ ~ 0.8 (exposure ~ 1.4 × 10¹⁵$\text{molecules}\text{cm}{}^2$ incident) which is interpreted as an order-disorder transition within a single (β₁) chemisorption state, and results in an increase in the ionic desorption cross-section by a factor of ~ 1.26. A discussion of the adsorption kinetics and the disorder transition is given in terms of current models of dissociative adsorption which include the effects of nearest neighbour lateral interactions.     

Electron impact excitation of high-lying states in the tungsten atom

The extended crossed-beam method has been used to study excitation of high-lying levels in the tungsten atom. 40 excitation cross sections of WI spectral lines have been measured for exciting electron energy 50 eV. Two optical excitation functions were recorded in the electron energy range 0–200 eV. The results obtained were used to calculate the total excitation cross sections of 24 odd WI energy levels. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 283–288, May–June, 2007.     

Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.     

Field desorption of a potassium-gold film on tungsten

The formation and destruction of alloys and chemical compounds of gold with potassium occurring in their coadsorption on tungsten have been studied by continuous-mode field-desorption microscopy, field-emission microscopy, and time-of-flight mass analysis of the products of field-induced desorption. The effect of an electric field on these processes was investigated. Monatomic and cluster ions of potassium and its compounds with gold were found to appear in fields too weak for field desorption to set in. Chemical reactions give rise to autooscillations in the field desorption of potassium.     

Vibration states of hydrogen on tungsten

The vibrational excitation of hydrogen adsorbed on the low-index faces of tungsten have been investigated using electron energy-loss spectroscopy. The results indicate the same vibrational force constant in “on-top” sites independent of substrate-atom geometrical environment.     

Adsorption and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.     

钨中氢同位素热脱附实验的速率理论模拟研究

本文采用基于速率理论的模拟方法研究钨材料中氢同位素氘的热脱附谱. 热脱附数据来源于520 K下受等离子体辐照的多晶钨, 入射离子能量为40 eV, 剂量为1× 10²⁶ D/m². 通过调节速率理论中的俘获能、俘获率等参数, 最终获得与实验相符合的热脱附拟合谱. 拟合结果表明, 钨中俘获的氘存在于三种俘获态, 俘获能分别为1.14 eV, 1.40 eV和1.70 eV, 相应脱附温度峰值为500 K, 600 K和730 K. 这三个俘获能分别应对应于第一原理计算得到的空位俘获第3–5个氢原子的俘获能(含零点振动能修正)、空位俘获第1–2个氢原子的俘获能, 空位团簇对氢原子的俘获能. 模拟结果表明, 在本辐照实验条件下, 钨中空位及空位团簇是氘在钨中的主要俘获态.     

The displacement of hydrogen by carbon monoxide on the (100) face of tungsten: A photoemission and thermal desorption study

Photoelectron spectra (hv = 21.22 eV) and thermal desorption data were obtained for CO and H coadsorbed on W(100) at 80 K. When the clean surface is exposed to a saturation dose of H₂, subsequent exposure to CO results in the formation of a state whose emission spectrum is similar to that of molecular α-CO. Upon heating to ~280 K, a structural rearrangement occurs in which most of the adsorbed CO is converted to the strongly bound β form as the hydrogen is simultaneously desorbed. These data plus the observation that H₂ cannot be adsorbed to any significant degree on a saturated layer of β-CO suggest that adsorbed β-CO and H occupy the same atomic sites on the W(100) surface. The distinction between long and short range repulsive COH interactions is discussed. For CO adsorbed on clean W(100), the range of activation energies for vigin to β conversion is calculated from the UPS data to be 45–62 kJ/mol.     

Thermal desorption of dysprosium from tungsten microcrystal

Activation energy for thermal desorption of dysprosium from a tungsten microcrystal of about 300 nm diameter was determined by means of the field-emission method. The desorption was detected from the whole W emitter surface in the temperature range 1490-1665 K for dysprosium average coverage θ < 0.06 monolayer. The average activation energy was determined to be 4.09 ± 0.06 eV/atom and the frequency factor to be about 10¹¹ s⁻¹. The energy may mainly concern the desorption from the atomically rough regions of the microcrystal.     

氩离子原子过程参数的系统计算与评估：I 电子碰撞激发

利用准相对论扭曲波玻恩近似加交换方法,在组态平均近似下,系统地计算了类氢、类氦、类锂氩离子n≤6的各组态之间的碰撞激发过程截面,并和已有的理论结果进行了详细的对比分析。计算结果和相对论扭曲波近似的计算结果符合得很好,相对偏差一般都小于10%。由于没有考虑共振效应,计算的结果与强耦合方法的计算结果在入射电子能量较低的情况下有较大偏差,其他情况则符合较好,相对偏差一般在15%以内。该方法可以方便地计算大量应用所需的原子过程参数。     

氩离子原子过程参数的系统计算与评估：Ⅰ电子碰撞激发

利用准相对论扭曲波玻恩近似加交换方法，在组态平均近似下，系统地计算了类氢、类氦、类锂氩离子n≤6的各组态之间的碰撞激发过程截面。并和已有的理论结果进行了详细的对比分析。计算结果和相对论扭曲波近似的计算结果符合得很好，相对偏差一般都小于10％。由于没有考虑共振效应。计算的结果与强耦合方法的计算结果在入射电子能量较低的情况下有较大偏差，其他情况则符合较好，相对偏差一般在15％以内。该方法可以方便地计算大量应用所需的原子过程参数。