Diffusion of nickel on the (110) region of a tungsten field emitter

The diffusion of nickel atoms on the (110) plane of a thermally cleaned tungsten crystal has been investigated by a field emission technique. The measurements were carried out for low nickel coverage and substrate temperatures T390 K when the nickel atoms escape from the (110) plane to its surroundings. the value of the activation energy obtained, Q=0.96 eV, is discussed taking into account the conditions under which diffusion occurs.     

Adsorption of oxygen on the (110) plane of tungsten at low temperatures

Isotope labelling experiments have established that the adsorption of O₂ on the W(110) plane at 20 K leads first to the formation of a dissociated atomic layer. A weakly bound molecular species, α-O₂, forms only when the atomic layer is essentially complete (O/W = 0.6). The desorption of α-O₂ was found to be first order with an activation energy of $\text{E = 1.9}\text{kcal}\text{mole}$ and a frequency factor γ = 3 × 10⁹ s^?1. The activation energy is shown to be less than the enthalpy of desorption and the meaning of this result is discussed.     

Adsorption of oxygen on the tungsten (110) plane at low temperatures; Spectroscopic measurements

The adsorption of oxygen on the W(110) plane was carried out at 26 K and investigated by means of ultraviolet and X-ray photoelectron spectroscopy. It was found that atomic oxygen is adsorbed first to essentially saturation coverage (O/W = 0.6) before adsorption of molecular oxygen occurs. The spectrum of the latter is very similar to that of gas phase O₂ but the shift to weaker binding energies is greater for the 1 s level than for the valence orbitals.     

Diffusion and compressibility of sodium on the (110) plane of tungsten

The surface diffusion parameters and the compressibility of sodium on the (110) plane of tungsten have been measured using the field emission fluctuation method for sodium coverages from 0.2 to 3 × 10¹⁴ atoms cm^?2 and for temperatures from 170 to 500 K. Two temperature regimes can be defined. In the high temperature regime (? 300 K) the diffusion is essentially normal with an activation energy ranging from 0.28 to 0.58 eV and a preexponential coefficient D₀ from 10^?8.1 to 10^?2.7 cm² s^?1. In this regime the compressibility increases with temperature indicating an effective repulsive adatoms interaction. In the low temperature regime (? 300 K) the diffusion coefficient decreases with temperature at high coverage and slowly increases with temperature at lower coverage. The transition between both regimes appears on the compressibility versus temperature curve as an inflection point. The comparison of the present results with slow electron diffraction results furnishes strong evidence that the observed transition corresponds to a continuous short-range order-disorder transition.     

Diffusion of hydrogen and deuterium on the (111) plane of tungsten

The diffusion of ¹H and ²H on the (111) plane of a W field emitter has been studied by the fluctuation method at various coverages. Both activated and unactivated diffusion is observed; the latter shows very little isotope effect, suggesting that coupling to the substrate is so strong that mass renormalization makes the effective masses of ¹H and ²H nearly identical. Values of D in the tunneling, i.e. temperature independent, regime are 10^?13?5 × 10^?14 cm²/s depending on coverage. For activated diffusion at high coverages, corresponding to population of the β₁ state E = 2.4?3.2 kcal/mol and D₀ = 2 × 10^?8 ?5 × 10^?7 cm²/s, depending on coverage. For lower coverages, corresponding to β₂ population, E = 7–9 kcal/mol, D₀ = 9 × 10^?6 ?2 × 10^?3 cm²/s, again depending on coverage. Similar values are obtained for ²H, with E and D₀ values slightly reduced. An exponentially decaying correlation signal for clean W was also seen and interpreted in terms of flip-flop of W atoms.     

Work function in field emission — the (110) plane of tungsten

Field emission for the (110) plane of field evaporated and thermally annealed tungsten has been restudied with particular reference to the significance of interpretation of the work function. It is concluded that the results obtained in field emission are consistent with results from non-field emission methods where the important variable of the local field is adequately considered. Possible contribution of other factors to measurement of the clean surface work function is also discussed.     

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.     

Diffusion of tungsten clusters on tungsten (110) surface

Using molecular dynamics simulation and modified analytic embedded-atom method, we have investigated the self-diffusion of clusters on a tungsten (110) surface. As compared to the linear-chain configuration, the close-packed islands for tungsten clusters containing more than nine adatoms have been predicted to be more stable with the relatively lower binding energies. The migration energies show an interesting and oscillating behavior with increasing cluster size. The tetramer, hexamer and octamer have obviously higher migration energies than the others. The different atomic configurations and diffusion mechanisms have been determined during the diffusion processes. It is clear that the dimer-shearing mechanism occurs inside the hexamer, while it occurs at the periphery of heptamer. The successive hopping mechanism of individual atom is of critical importance in the migration of the clusters containing five or fewer adatoms. In addition, the diffusion of a cluster with nine adatoms is achieved through the changes of the cluster shape.     

Diffusion of oxygen on tungsten (110)

The surface diffusion of oxygen on W(110) was studied in the temperature range 760–880°C and for coverages up to one monolayer. Coverage-distance plots after diffusion runs were determined via corresponding work function profiles measured by a newly developed, laterally resolving vibrating capacitor method. The coverage profiles were evaluated in terms of a concentration dependent diffusion coefficient (DC) using the Boltzmann-Matano analysis. The temperature dependence of the DC can be described by a constant activation energy of 27 ± 2 kcal/mole in the coverage range 0.4 < θ < 0.9. The concentration dependence of the DC shows a maximum near θ = 0.45and a decrease beyond θ = 0.6. This behavior is correlated with observed LEED structures and interpreted by a rather abrupt change of the chemical potential near θ = 0.5 and a decreasing hopping probability for higher coverages.     

Diffusion behavior of tungsten clusters on the tungsten (110) planes

Diffusion of tungsten diatomic clusters on the W (110) exhibits comparable prefactor (1.6×10^-4 cm²/sec) as single W atoms, but with slightly higher activation energy. Clusters with three atoms are unstable with respect to diffusion.     

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.     

Periodic field-dependent photocurrent from a tungsten field emitter

We report the observation of a periodic electric-field-dependent component in the photo-current from a tungsten field emitter illuminated by the focused beam of a krypton ion laser tuned to a photon energy of ~3.51 eV. Over the field range we have investigated, the phase φ of the oscillations in the photocurrent can be related to the electric field strength F by the power law φ ∝ F^α, where $\text{α ≈ ?}\text{1}\text{2}$. In this preliminary study, we describe the frequency, amplitude, and harmonic structure of oscillations detected with the probe hole centred over the (510) region of the field emission pattern. A Fowler-Nordheim plot of the photocurrent is presented, and the nonoscillatory component is analysed on the basis of a simple theory of barrier penetration. The origin of the oscillatory component of the photocurrent is discussed.     

Thermoelectric measurements on tungsten at ultra low temperatures

Measurements are reported of the thermoelectric ratio G for three zone-refined tungsten samples at temperatures down to 45 mK. For the purest sample $\text{(}\text{R}{}_{\mathrm{300K}}\text{R}{}_{\mathrm{0K}}\text{= 44 000)}$, G behaves simply and in accord with expectations. For two less pure samples $\text{(}\text{R}{}_{\mathrm{300K}}\text{R}{}_{\mathrm{0K}}\text{= 44 000 and 22 000)}$, G behaves anomalously, becoming increasingly more negative with decreasing temperature down to at least 45 mK. This anomalous behavior is tentatively attributed to trace amounts of iron.     

Laser-stimulated field desorption of molecular ions from a tungsten emitter

We report here on a experimental observation of photon-stimulated field emission of molecular anthracene ions from the surface of a layer adsorbed on a tungsten field-emitter tip. When the tip is irradiated with laser pulses 249, 308, and 400 nm in wavelength falling within the absorption bands of anthracene, the stimulated ion signal is proportional to the pulse fluence. The efficiency of the process decreases with the increasing laser wavelength. Photon stimulation is believed to be due to the resonance excitation of the anthracene molecules, followed by the field ionization of the excited molecules.     

Absolute coverage measurements: CO and oxygen on the (110) plane of tungsten

An effusion source, calibrated with a vibrating quartz microbalance, has been used to determine absolute coverages of CO and of oxygen adsorbed on a tungsten (110) plane by an extension of the field emitter detector method: The amount of gas reflected from the substrate is measured as a function of the absolute amount impinged per unit area; maximum coverage in the chemisorbed layer can be obtained very directly from this information. Work function increments vs. absolute coverages were measured in the same apparatus by the vibrating condenser method. Results were as follows: For virgin CO, adsorbed at 100 K, the maximum coverage obtained was CO/W = 0.71 ; this leads to a maximum coverage for beta or beta-precursor CO of CO/W = 0.28. The maximum work function increment for virgin CO was 0.8 eV. For oxygen, adsorption at 100 or 300 K a decrease in sticking coefficient by several orders of magnitude occurs when the coverage is O/W = 0.5. Adsorption at 20 K leads to the molecular precursor noted by Leung and Gomer, which converts to atomic oxygen at <50 K. By adsorbing at 20 K the O atom coverage can be increased to O/W = 0.62. Work function versus coverage data for this system are also presented.     

High field magnetoresistance at low temperatures

To explain recent experiments on n-InSb we calculate the transverse and longitudinal electrical conductivity in strong magnetic fields. We consider a free electron-point impurity system in the generalized Born approximation, and in the self-consistentt-matrix approximation. The position, shape and amplitude of the Shubnikovde Haas oscillations and the quantum limit behaviour are in good agreement with experiment.     

A calculation of surface diffusion coefficients of adsorbates on the (110) plane of tungsten

An attempt to calculate the prefactors of the diffusion coefficients of tungsten, xenon and oxygen atoms on the W(110) plane in the zero-coverage limit was made, using a Fokker-Planck formalism. Pairwise substrate-substrate and adsorbate-substrate atom interactions were assumed and expressed by appropriate Morse potentials. The vibrations of the substrate were approximated by an Einstein model of independent oscillators. When Morse potentials were so adjusted as to give good agreement between experimental and calculated activation energies, a prefactor in excellent agreement with experiment was calculated for W/W(110). For Xe/W(110) and O/W(110) the calculated values were ～10^?4 cm²/s, i.e. essentially “normal”, while the experimental values are much lower. Possible reasons for the discrepancies are discussed.     

Diffusion and phase transitions in barium monolayers on the (011) plane of tungsten

The diffusion parameters of barium on W(011) are shown to correlate with phase transitions in the adlayer. A suprisingly strong difference is found both in absolute values and in the coverage dependence of the diffusion parameters for barium on the (011) plane of W and Mo. A considerably lower mobility of barium on molybdenum is attributed to a larger contribution of covalent bonding on this substrate. It is concluded that the electron structure, together with the atomic structure of a surface, plays an important role in surface diffusion.     

Increase in the power of arc cesium-oxygen thermionic converters with tungsten electrodes at elevated emitter temperatures

The properties of experimental thermionic converters with emitters made of oxygen-containing single-crystal tungsten are studied. The tungsten is produced by the chloride CVD method. Such emitters not only perform their basic function in the converter but also serve as an “internal” oxygen source. An attempt is made to raise the emitter temperature in order to increase the maximal output of the converter using the new emitter material. The efficiency of applying an oxygen-free single-crystal tungsten collector is contrasted with that when the collector is made of polycrystalline tungsten or Cb-1 niobium alloy.