Electron-stimulated cesium atom desorption from the Cs/CsAu/Au/W system

This paper reports on a continuation of the investigation of electron-stimulated Cs-atom desorption from a tungsten surface on which cesium and gold films had been adsorbed at T = 300 K. Earlier studies revealed that Cs atoms start to desorb only after more than one monolayer of gold and more than one monolayer of cesium had been deposited on the tungsten surface. In this case, a coating consisting of a gold adlayer on tungsten, a CsAu compound possessing semiconducting properties, and a cesium monolayer capping CsAu (Cs/CsAu/Au/W) is formed on the tungsten surface at 300 K. The yield of atoms from this system exhibits a resonant dependence on the incident electron energy E _e , with an appearance threshold of 57 eV and a maximum at 64 eV. In this case, Cs atoms desorb in two channels, with one of them involving Cs desorption out of the cesium monolayer, and the other, from the CsAu monolayer. The Cs yield at E _e = 64 eV has been investigated in both desorption channels, with an additional cesium coating deposited on the already formed Cs/CsAu/Au/W layered system, as well as of the effect annealing produces on the yield and energy distributions of Cs atoms. It has been demonstrated that Cs atoms evaporated at 300 K on a layered coating with a cesium monolayer atop the CsAu layer on tungsten capped with a gold adlayer, rather than reflected from the cesium monolayer or adsorbing on it, penetrate through the cesium monolayer into the bulk of CsAu even with one CsAu layer present. The desorption yield does not vary with increasing cesium concentration at 300 K, but falls off gradually at 160 K. Annealing within the temperature range 320 K ≤ T _H ≤ 400 K destroys the cesium monolayer and the one-layer CsAu coating, but the multilayer CsAu compound does not break up in this temperature range even after evaporation of the cesium monolayer. It is shown that Cs atoms escape from the multilayer CsAu compound primarily out of the top CsAu layer.     

The adsorption of xenon by W(111), and its interaction with preadsorbed oxygen

The physisorption of Xe on W(111) and of Xe on partial layers of oxygen chemisorbed on W(111) has been studied using flash desorption and work function methods. It has been found that xenon adsorbs up to monolayer coverages at 104K. Xenon desorbs from W(111) as a single binding state following first order kinetics. At low coverages (θ_Xe < 0.07) the binding energy decreases with increasing coverage possibly because of the presence of high energy adsorption sites due to crystal imperfections and edge effects. For θ_Xe > 0.07 the desorption data fit a first order rate expression with a desorption energy of 9.3 kcal/mol and preexponential ν = 10¹⁵s^?1. The observed work function change of ?1.1 ± 0.1 eV is consistent with monolayer estimates reported in field emission studies of physisorbed xenon on tungsten. The effect of preadsorbed oxygen layers on the physisorption of xenon on this surface is very striking. The energy of desorption shifts as much as 50% higher for a moderate exposure of oxygen. Several physisorption models are explored along with estimates of dispersion and electrostatic interaction contributions.     

Molecular binding states on clean and oxidized surfaces: SiO(g) + W(clean) and SiO(g) + W(oxidized)

The interactions between a molecular beam of SiO(g) and a clean and an oxidized tungsten surface were examined in the surface temperature range 600 to 1700 K by mass spectrometrically determined sticking probabilities, by flash desorption mass spectrometry (FDMS) and by Auger electron spectroscopy (AES). The sticking probability, S, of SiO has been determined as a function of coverage and of surface temperature for the clean and the oxidized tungsten surface. Over the temperature range studied and at zero coverage S = 1.0 and 0.88 for the clean and oxidized tungsten surfaces respectively. The results are consistent with both FDMS and AES. For coverage up to one monolayer there is one major adsorption state of SiO on the clean tungsten surface. FDMS shows that T_m = constant (T_m is the surface temperature at which the desorption rate is maximum) and that desorption from this state is described by a simple first order desorption process with activation energy, E_d = 85.3 kcal mole^?1 and pre-exponential factor, ν = 2.1 × 10¹⁴ sec^?1. AES shows that the 92 eV peak characteristic of silicon dominates. In contrast on the oxidized tungsten surface, T_m shifts to higher temperatures with increasing coverage. The data indicate a first order desorption process with a coverage dependent activation energy. At low coverage (θ ? 0.14) there is an adsorption state with E_d = 120 kcal mole^?1 and ν = 7.6 × 10¹⁹, while at θ = 1.0, E_d = 141 kcal mole^?1. This variation is interpreted as due to complex formation on the surface. AES shows that on oxidized tungsten, in contrast to clean tungsten, the dominant peaks occur at 64 and 78 eV, and these peaks are characteristic of higher oxidation states of silicon. Thus, it is concluded that SiO exists in different binding states on clean and oxidized tungsten surfaces.     

Effect of deposition of samarium atoms on the electron-stimulated desorption of cesium and samarium atoms from the surface of tungsten coated by gold and cesium

It has been shown that deposition of Sm atoms on W(100) surface coated by several monolayers of gold and cesium affects noticeably the yield of Cs atoms in electron-stimulated desorption (ESD) from this surface. The measurements have been performed by the time-of-flight method with a surface-ionization detector. The paper reports on the first observation of ESD of Sm atoms from the tungsten surface coated by layers of gold and cesium. The ESD threshold for Sm atoms, E _e = 57 eV, coincides with that for Cs atoms and corresponds to the energy of the Au 5p _3/2 core level. The dependence of the ESD yield of Sm atoms on the bombarding electron energy E _e follows a resonance pattern in the form of a narrow peak located in the range 57 ≤ E _e ≤ 66 eV. Deposition of Sm atoms at room temperature (～300 K) reduces (by a factor of about two) the ESD yield of Cs atoms for 600 s, and deposition of Sm atoms at 160 K reduces the ESD of Cs atoms down to zero already for 270 s. This difference finds explanation in the study of the change the structure of the top layer of the (Au + Cs)/W surface coating undergoes under cooling of the surface from 300 to 160 K.     

Europium adsorption on a tungsten surface with various degrees of oxidation

The kinetics of europium adsorption on a W(100) face with various degrees of oxidation were studied by thermal desorption and Auger electron spectroscopy. The spectrum of Eu atoms desorbed thermally from the W(100) face consists of three successively filling desorption phases whose desorption activation energy decreases from 3 to 2.1 eV with an increase in the surface coverage. The thermodesorption spectrum of Eu atoms from the W(100) face coated with a monatomic oxygen film contains five successively forming desorption phases, with the desorption activation energy increasing to 4 eV for the high-temperature phase. The oxidized W is reduced by europium, and the desorption of the W oxides is replaced by that of EuO. After a monolayer film has formed, the Eu film adsorbed on tungsten starts to grow in the form of three-dimensional crystallites. As the degree of W oxidation increases, the Eu film becomes less nonuniform, until a solid Eu film starts to grow on bulk W oxides and completely screens the tungsten Auger signal.     

Oxidation kinetics of thin titanium films grown on tungsten

Growth of thin Ti films on (100)W and the kinetics of their oxidation are studied using thermal-desorption spectroscopy and Auger electron spectroscopy. Titanium films grow nearly layer by layer on the (100)W face at room temperature. The activation energy for desorption of Ti atoms decreases from 5.2 eV for coverage θ=0.1 to 4.9 eV in a multilayer film. Oxidation of a thin (θ=6) titanium film starts with dissolution of oxygen atoms in its bulk to the limiting concentration for a given temperature, after which the film oxidizes to TiO, with the TiO₂ oxide starting to grow when exposure of the film to oxygen is prolonged. The thermal desorption of oxides follows zero-order kinetics and is characterized by desorption activation energies of 5.1 (TiO) and 5.9 eV (TiO₂).     

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.     

Lithium adsorption on hot oxygen covered tungsten surfaces

The mean residence times τ of lithium particles on oxygen covered tungsten surfaces were measured accurately over a wide temperature range (1200 < T < 1900 K) by the beam modulation technique with a lock-in analyzer. A predominant monocrystalline W(100) structure was obtained by recrystallization of a polycrystalline tungsten ribbon. The residence time was determined as a function of the oxygen coverage θ and the temperatureT of the surface. The desorption energy l and the preexponential factor τ₀, calculated from the Arrhenius equation, are not only dependent on the amount of adsorbed oxygen but also on the oxygen structure. Apparently the desorption parameters l and τ₀ are correlated. An increasing desorption energy l is connected to a decreasing “vibration period” τ₀ whereby the influence on the residence time is partly compensated.     

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.     

Initial stages of the interaction of sodium and cesium with gold

The desorption of Cs and Na atoms from the corresponding layers applied to a gold film deposited on textured tungsten ribbon with a preferred orientation of the (100) surface is studied by thermal desorption spectroscopy with the products of thermal desorption scanned on a pulsed time-of-flight mass spectrometer. The Cs atoms evaporated at T = 300 K are desorbed by two phases, one of which can be identified with the filling of a monolayer and the other can be attributed to the formation of the CsAu compound. The Na atoms evaporated at T = 300 K are desorbed by three phases associated with the formation of a monolayer coating, a sodium compound of with gold, and a multilayer sodium film.     

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.     

Electron-stimulated desorption of rare-earth metal atoms

The yield of europium and samarium atoms in electron-stimulated desorption from layers of rare-earth metals (REMs) adsorbed on the surface of oxidized tungsten has been measured as a function of the incident electron energy, surface coverage by REMs, degree of tungsten oxidation, and substrate temperature. The measurements were performed using the time-of-flight method with a surface-ionization-based detector within the substrate temperature interval 140–600 K. The yield studied as a function of electron energy has a resonance character. Overlapping resonance peaks of Sm atoms are observed at electron energies of 34 and 46 eV, and those of Eu atoms, at 36 and 41 eV. These energies correlate well with the REM 5p and 5s core-level excitation energies. The REM yield is a complex function of the REM coverage and substrate temperature. The peaks due to REM atoms are seen at low REM coverages only, and their intensity usually passes through a maximum with increasing coverage and substrate temperature. The concentration dependence of the REM atom yield is affected by the deposition of slow Ba⁺ ions, but only if they are deposited after the REM adsorption. At higher REM coverages, additional peaks are observed at electron energies of 42, 54, and 84 eV, which originate from excitation of the 5p and 5s tungsten levels and result from desorption of SmO and EuO molecules. The temperature dependence of the intensity of these peaks is explained to be due to the order-disorder phase transition. The desorption of REM atoms is the result of their reversed motion through the adsorbed REM layer, and the SmO and EuO molecules desorb due to the formation of an antibonding state between the REM oxide molecules and the tungsten ions.     

The surface structures growth’s features caused by Ge adsorption on the Au(111) surface

The initial stage of the adsorption of Ge on an Au(111) surface was investigated. The growth and stability of the structures formed at the surface were studied by ultrahigh-vacuum low-temperature scanning tunneling microscopy and analyzed using density functional theory. It was established that the adsorption of single Ge atoms at the Au(111) surface at room temperature leads to the substitution of Au atoms by Ge atoms in the first surface layer. An increasing of surface coverage up to 0.2–0.4 monolayers results in the growth of an amorphous binary layer composed of intermixed Au and Ge atoms. It was shown that the annealing of the binary layer at a temperature of T _s ? 500 K, as well as the adsorption of Ge on the Au(111) surface heated to T _s ? 500 K for coverages up to 1 monolayer lead to a structural transition and the formation of an Au–Ge alloy at least in the first two surface layers. Based on experimental and theoretical data, it was shown that the formation of single-layer germanene on the Au(111) surface for coverages ≤1 monolayer in the temperature range of T _s = 297–500 K is impossible.     

The co-adsorption of copper and oxygen on a tungsten {100} surface

The co-adsorption of Cu on O₂ and a W{100}surface is studied by Auger electron spectroscopy (AES), thermal desorption (TD), low energy electron diffraction (LEED) and by work function change (δø) measurements. It is shown that the presence of Cu on the surface initially decreases s_O, the sticking coefficient of O₂. For longer oxygen exposures and for higher adsorption temperatures, θ_O reaches values larger than those on the clean surface for the same O₂ exposure. Except at the highest θ_O values and temperatures, the sticcking coefficient for copper, s_Cu, is unity and is independent of the oxygen coverage θ_O in the range studied (0 ? θ_O ? 2). Co-adsorption at room temperatures does not produce any long range order while co-adsorption at elevated temperature leads to the ordered structures (1 × 1), p(2 × 1), p(2 × 2) and c(2 × 2). The saturation coverage of the two dimensional co-adsorbate at 800 K is given by the relation $\text{θ}{}_{\mathrm{Cu}}\text{+}\text{8}\text{5}\text{θ}{}_{\mathrm{O}}\text{= 2}$. The work function is a complicated function of θ_O and θ_Cu and is determined predominantly by the temperature at which oxygen is adsorbed. At high temperatures the sequence of adsorption has no influence, in contrast to the room temperature behavior.     

Thermal desorption of Ag from oxidized W

Thermal desorption spectra (TDS) of Ag condensates deposited at two substrate temperatures T_s = 300 K and T_s = 779 K have been obtained. A shift of the temperature T_m of the maximum of the desorption flux R_e was observed. It was established that the shift of the maximum depends on the value of the initial coverage σ₀. A significant difference was found to exist between TDS of silver condensates deposited on oxidized and clean W substrates due to differences in the mechanism of condensation. Silver condensates were deposited on oxidized W at different initial conditions (T_s, impingement rates R_i, etc.) but equal σ₀. The corresponding TDS were compared and a conclusion has been drawn that the shift of T_m is due to the different structure of Ag condensates. TDS of Ag condensates deposited at room temperature (T_s = 300 K) were interpreted using the method of Bauer et al. [J. Appl. Phys. 45 (1974) 5164: Surface Sci. 53 (1975) 87]. The dependence of the desorption flux R_e on the substrate temperature T_s and coverage σ was treated on the basis of the Polanyi-Wigner equation and some parameters of the condensation process were evaluated.     

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO₄ tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO₄ tetrahedra are readily formed even at room temperature. Nevertheless, the SiO₂ thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ～400°C), silicon diffusion towards the surface is promoted and a thicker SiO₂ layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.     

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 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.     

Adsorption,coadsorption, and reactivity of sodium and nitric oxide on Ag(111)

The adsorption, desorption, and surface structural properties of Na and NO on Ag(111), together with their coadsorption and surface reactivity, have been studied by LEED, Auger spectroscopy, and thermal desorption. On the clean surface, non-dissociative adsorption of NO into the a-state occurs at 300 K with an initial sticking probability of ~0.1, saturation occurring at a coverage of $\text{~}\text{1}\text{20}$. Desorption occurs reversibly without decomposition and is characterised by a desorption energy of E_d ~ 103 kJ mol^?1. In the coverage regime 0 < θ_Na < 1, sodium adsorbs in registry with the Ag surface mesh and the desorption spectra show a single peak corresponding to E_d ~ 228 kJ mol^?1. For multilayer coverages (1 < θ _Na < 5) a new low temperature peak appears in the desorption spectra with E_d ~ 187 kJ mol^?1. This is identified with Na desorption from an essentially Na surface, and the desorption energy indicates that Na atoms beyond the first chemisorbed layer are significantly influenced by the presence of the Ag substrate. The LEED results show that Na multilayers grow as a (√7 × √7) R19.2° overlayer, and are interpreted in a way which is consistent with the above conclusion. Coadsorption of Na and NO leads to the appearance of a more strongly bound and reactive chemisorbed state of NO (β-NO) with E_d ~ 121 kJ mol^?1. β-NO appears to undego surface dissociation to yield adsorbed O and N atoms whose subsequent reactions lead to the formation of N₂, N₂O, and O₂ as gaseous products. The reactive behaviour of the system is complicated by the effects of Na and O diffusion into the bulk of the specimen, but certain invariant features permit us to postulate an overall reaction mechanism, and the results obtained here are compared with other relevant work.     

Metal-support interactions in systems palladium deposited on oxidized tungsten surfaces

The morphology of the palladium (Pd) overlayers on oxidized tungsten (W) tips has been studied by Field Emission Microscopy (FEM). The effect of thermal treatment on the interaction of Pd with the support and chemisorption of CO on variously treated Pd-containing samples has been investigated. The results are discussed in relation to complementary macroscopic experiments by synchrotron radiation excited photoelectron spectroscopy (SRPES) and thermally programmed desorption (TPD) of carbon monoxide (CO) on a polycrystalline W foil. A distinct influence of support pre-oxidation on the Pd layer growth has been demonstrated. Two types of oxidized supports have been used: tungsten with oxygen pre-adsorbed at room temperature (RT) and then heated to 700 K (WO_x/W (RT) system) and tungsten oxidized at 1300 K (WO_x/W (1300 K) system) in situ. The surface of WO_x/W (1300 K) sample is fully oxidized in contrast to WO_x/W (RT), where the presence of un-oxidized patches has been demonstrated by SRPES measurements. A Pd layer grows on the WO_x/W (RT) surface mostly on the densely populated planes (1 1 0) and (2 1 1) of the W tip. Heating of this system up to 700 K results in disaggregation of the original Pd layer. Pd clusters on the tungsten tip oxidized at 1300 K are localized on the atomically rough (1 1 1) plane. The observed differences in CO adsorption on the aforementioned types of investigated samples can be attributed to differences in the chemical nature of their surfaces.