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.     

Ab initio study of oxygen point defects on tungsten trioxide surface

The gas response of tungsten trioxide (WO3) based sensors strongly depends on the surface properties. Reconstructed surfaces and oxygen point defects at the surface of the monoclinic WO3 are studied using a self-consistent scheme based on first-principle. The oxygen vacancy is found to be the predominant defect independently of the oxygen partial pressure. Indeed, under rich oxygen atmosphere the formation enthalpies are found to be 1.45 eV in LDA (1.28 eV in GGA) for the oxygen vacancy instead of 2.70 eV (2.42 eV) for the oxygen adatom. When the oxygen partial pressure is lowered, the oxygen vacancy formation enthalpy decreases and becomes exothermic under very O-poor condition (? 1.65 eV in LDA and ? 1.36 eV in GGA). On the other hand, the formation enthalpy of an oxygen adatom rises. Finally, the oxygen vacancy formation acts as a n-doping by introducing negative charge carriers at the bottom of the conduction band. All these results can be very helpful in order to explain the electrical resistivity measurements.     

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.     

Diffusion and desorption of barium and oxygen on tungsten dispenser cathode surfaces

Auger measurements have been made on the concentration of barium and oxygen diffusing out of a pore and slot onto the surface of a simulated tungsten dispenser cathode. Profiles of concentration vs distance from the source were obtained at different temperatures. It is found that at cathode operating temperatures very little gradient of oxygen concentration exists on the surface, and under steady state conditions Ba is the main diffusing species. Ba diffusion distances derived are consistent with previous measurements. However it is found that this distance increases with concentration in spite of the fact that the Ba surface lifetime decreases with concentration. Time dependent measurements made on a clean surface show that the rate of Ba and O build up is limited by the supply rate of oxygen to the surface. This supply rate is not diffusion limited but seems to be limited by the mechanism generating free oxygen. A comparison of emission microscope measurements on a tungsten matrix dispenser cathode with the simulated cathode data indicates that similar oxygen generation processes may be controlling the activation of these cathodes.     

Electron-stimulated desorption of sodium atoms from germanium-coated tungsten

The yield of sodium atoms and energy distribution upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a germanium thin film are measured under variations in the electron energy, the sodium coverage, and the surface temperature by the time-of-flight method with the use of a surface ionization detector. It is revealed that the electron-stimulated desorption of sodium atoms occurs via three channels, namely, a channel involving ionization of adsorbed sodium; the most efficient channel, which is produced by the germanium ionization; and a channel associated with the formation of tungsten excitons, which brings about desorption of NaGe molecules.     

Thermal desorption of silicon from polycrystalline tungsten surfaces

The adsorption of Si on polycrystalline tungsten surfaces was studied for the first time by means of thermal desorption. Instead of the main peak of Si ($\text{m}\text{e}\text{=28}$), the isotope ($\text{m}\text{e}\text{=30}$) contained naturally (～3%) was monitored by mass spectrometry. This method can reduce the contribution from CO to the mass signal of Si. Two desorption peaks were observed at 1480 and 1820 K. The activation energies for the desorption were estimated to be about 90 and 110 kcal/mol, respectively.     

Thermal desorption of metals from tungsten single crystal surfaces

After a short review of the experimental methods used to determine desorption energies E and frequencies v the assumptions underlying the theoretical analysis of the experimental data are discussed. Then recent experimental results on the flash desorption of Cu, Ag and Au from clean, well characterized W {110} and {100} surfaces are presented and analysed in detail, in particular with respect to the coverage dependence. The results obtained this way clearly reveal the limitations of previous analysis methods and of the experimental technique per se (such as structure and phase changes below and in the temperature region in which desorption occurs). Furthermore the need for more theoretical work to understand the large changes of v and E with coverage and the so-called “compensation effect”, i.e. the relation between In v and E, becomes evident.     

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.     

Field desorption of common gases from iridium and tungsten

The technique of field desorption has been used to study adsorption and desorption of gases from field ion tips. The formal procedure of the experiments is quite similar to thermal flash desorption. Values for the desorption field and their change with coverage have been obtained. For H₂/W a field adsorbed state could be detected. Time-of-flight analysis was applied to determine the desorbing species. The gas supply for field ion tips could be measured; it increases exponentially with the field.     

Atomic hydrogen desorption from thin palladium hydride films

It has been proved that hydrogen atoms desorb from the surface of a decomposing thin palladium hydride film. A thin gold film deposited and sintered in situ was used as a selective adsorbent for atomic hydrogen. The TDMS (thermal desorption mass spectrometry) technique was applied to detect the adsorption of hydrogen on gold and to determine the amount adsorbed.     

Electron-stimulated desorption of europium atoms from an oxidized tungsten surface

The yield of europium atoms in electron-stimulated desorption from Eu layers adsorbed on the surface of oxidized tungsten was studied with a surface-ionization detector as a function of the incident-electron energy, surface coverage by europium, and degree of tungsten oxidation. The yield of Eu atoms measured as a function of electron energy exhibits a distinct resonant character with peaks at electron energies corresponding to europium and tungsten core-level ionization energies. The peaks associated with the europium ionization reach a maximum intensity at europium coverages less than 0.1 and decrease subsequently to zero with increasing coverage, while the peaks due to tungsten ionization pass through the maximum intensity at a monolayer europium coverage. The coverage corresponding to the maximum europium atom yield grows with increasing tungsten oxidation. The results obtained are accounted for by the formation of the europium and tungsten core excitons. In the first case, the particles desorb in the reverse motion toward the surface of the oxidized tungsten; in the second, they desorb as a result of repulsion between the tungsten core exciton and the EuO molecule.     

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.     

Electron stimulated desorption of oxygen from nickel

Electron Stimulated Desorption (ESD) of O⁺ ions from oxygen-covered Ni(100) has been investigated at 390 K and 500 eV primary energy. The ion energy distribution is found to peak at 7.5 eV and to extend to 11 eV, over our whole exposure range (0–1000 L). The 7.5 eV peak height as a function of exposure shows that desorption takes place both in the chemisorption and the oxidation region. Emission of O⁺ occurs preferentially along the surface normal, with a base width of ≈ 60°. No azimuthal structure is observed. Additional electron energy dependent measurements clearly show a threshold near the oxygen 2s level.     

Ion induced desorption of oxygen from platinum

Ion induced desorption has been used to study the initial uptake of oxygen on polycrystalline platinum. Rapid increase is observed in the O^? secondary ion signal with oxygen exposure. The dependence of the O^? signal on sample temperature is observed to exhibit a peak near the temperature corresponding to the main thermal desorption state.     

Samarium atom yield under electron-stimulated desorption from oxidized tungsten surface

The yield of samarium (Sm) atoms under electron stimulated desorption from Sm layers adsorbed on the surface of oxidized tungsten was studied as a function of incident electron energy, surface coverage by samarium, degree of tungsten oxidation, and substrate temperature. The measurements were conducted by the time-of-flight technique with a surface ionization detector in the substrate temperature interval from 140 to 600 K. The yield vs. incident electron energy function has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, which may be related to excitation of the Sm 5p and 5s levels. The Sm yield is a complex function of samarium coverage and substrate temperature. Sm atom peaks occur only in the Sm coverage range from 0 to 0.2 monolayers (ML), in which the yield passes through a maximum. The shape of the yield temperature dependence is a function of Sm coverage. For low Sm cover-ages (<0.07 ML), the yield decreases slowly with the temperature increasing to 270 K, after which it drops to zero at temperatures above 360 K. At higher coverages, the Sm yield passes through a maximum with increasing temperature and additional peaks appear at electron energies of 42, 54, and 84 eV, which can be assigned to excitation of the tungsten 5p and 5s levels. These peaks are most likely associated with desorption of SmO molecules, whose yield reaches a maximum at an Sm coverage of about 1 ML.     

Electron-stimulated desorption of europium atoms from the surface of oxidized tungsten

An analysis of the yield q of europium atoms is made, and scenarios of electron-stimulated desorption are put forward. Expressions are obtained for the dependence of q on the coverage of oxidized tungsten by europium atoms.     

Electron-stimulated desorption of atomic oxygen from polycrystalline Ag

Emission of neutral oxygen atoms from an oxygen-charged polycrystalline Ag wire has been examined by using electron-stimulated desorption (ESD) in conjunction with threshold ionization detection (appearance potential). A quadrupole mass spectrometer (QMS) was used to detect the neutral oxygen atoms, but the ionizer was operated in the appearance potential (AP) mode to make it possible to distinguish the oxygen atoms from products formed by collision of the oxygen atoms with the walls. Loss of the reactive oxygen atoms was also minimized by enclosing the ESD chamber in fused silica. With a primary beam energy of 100 eV, the ESD cross section for oxygen atom desorption was found to be 7 × 10^-19cm² at 100 °C, and an ESD threshold was found to exist below 34 eV. Flash desorption of oxygen allowed estimation of the bulk diffusivity of oxygen through polycrystalline Ag. A value of 2.64 × 10^-6cm²s^-1 at 500 °C was obtained.