Clean Tungsten Surface Research Articles

Theoretical models of the negative ionization of hydrogen on clean tungsten, cesiated tungsten and cesium surfaces at low energies

The formation of negative hydrogen ions by scattering protons from a metal surface is described with two models: a probability model and an amplitude model. In both models the electron motion is described quantum mechanically and the nuclear motion classically. However, in the probability model the time evolution of the ionization probability is considered, while in the amplitude model the time evolution of the corresponding wave function amplitude is considered. The electron affinity level of an atom close to the metal is lowered by means of image forces and broadened due to resonant transition of an electron between te conduction band of the metal and the valence shell of the atom. The calculated position of the affinity level and the transition rate in both models give rise to maximum negative ionization efficiencies of 4% on W(110), 40% on cesiated tungsten and 15% on cesium.

Reactions of acetaldehyde and ethanol on W(100) and W(100)−(5 × 1)C surfaces

The reactions of acetaldehyde and ethanol were studied on clean and carburized W(100) surfaces using temperature-programmed reaction spectroscopy (TPRS). On a clean tungsten surface the room-temperature adsorption of either molecule resulted in complete dissociation into adsorbed hydrogen, carbon, and oxygen atoms at low exposures. With increasing exposures acetaldehyde, ethanol, ethylene, and methane were observed to form as additional reaction products via an ethoxy surface intermediate. Passivation of the tungsten surface, either by the initial breakup of the parent molecule or by the deposit of carbon from cracking ethylene, resulted in markedly different product distributions. In particular, methane formation from CC bond scission was eliminated by carbide formation, CO bond rupture was reduced somewhat, and the reduction of surface oxygen to form water was enhanced. In the case of ethanol the dehydrogenation/dehydration selectivity was increased by a factor of 5 on the carbide surface. Formation of surface oxygen or surface alkoxide groups promoted a lower energy pathway for decomposition to all products.

Decomposition of ammonia on a tungsten surface. Effects of carbon and oxygen

In this work the effects of carbon and oxygen on the decomposition of ammonia on a tungsten surface have been studied. The composition of the surface was examined by Auger electron spectroscopy (A.e.s.) and the surface structure by low-energy electron diffraction (l.e.e.d.). The rate of the reaction, the extent of nitrogen adsorption during the reaction, and the extent to which molecular nitrogen is adsorbed, all decreased linearly with increasing amount of carbon on the surface, finally approaching zero at full coverage. The carbon on the surface thus decreased the number of effective active sites for ammonia decomposition and nitrogen adsorption on the surface. On the other hand, as oxygen coverage increased, the decomposition rate increased initially and then decreased. This is interpreted as an activation of the N2 desorption rate by the presence of oxygen on the surface. From the l.e.e.d. studies, a C(2 × 2) structure under reaction conditions was observed on the clean tungsten (100) surface. A superposition structure of C(2 × 2)–N and (4 × 1)–O or C(2 × 2)–N and (2 × 1)–O was obtained in the steady state of the reaction on the oxygen preadsorbed surface.

Secondary ions from hydrogen ion bombardment of metal surfaces

The production of negative ions at metal surfaces under ion bombardment is of theoretical and practical interest. In this paper data for H+ and H− production at clean and hydrogen-covered tungsten surfaces under H+n bombardment are given, and the data interpreted. In the experimental apparatus primary ion beams of H+n are impacted on a tungsten target, from which secondary H+ and H− ions are emitted, and mass and energy are analyzed. It is found that the secondary H+ spectrum from H+n bombardment is a broad peak, reaching an Emax which is a function of the primary ion energy, independent of the state of hydrogen coverage of the tungsten surface. The H− yield has a peak and tail distribution, the peak being a function of the degree of surface coverage. The mechanism whereby the high-energy peak arises and the amount of hydrogen trapped in the lattice is related to multiple scattering process in the target. The low-energy peak of H− ions arises from surface atoms sputtered off the surface as H− ions. A new theory explaining the secondary H− production in terms of a two electron Auger process followed by resonant capture of an electron from a normally unoccupied state in the metal is presented.

Reaction mechanism of ammonia decomposition on tungsten

The mechanism of ammonia decomposition on a clean tungsten surface was studied by direct and simultaneous measurements of the reaction rate and the amounts and behaviour of adsorbed species during the course of the reaction in the temperature range 773–1473 K under an ammonia pressure of 10–6–10–3 Pa. The order of the reaction rate with respect to ammonia pressure changed with temperature from first order to fractional at 1473 and 773 K, respectively. No hydrogen was adsorbed on the surface in any form above 973 K and an increase in the hydrogen partial pressure (PH2) during the reaction had no effect on either the reaction rate or on the amount of surface nitrogen (θN). Under higher ammonia pressures, up to 100 Pa, thick surface nitride layers were formed during the decomposition, which decomposed at the same rate in vacuo as in the steady-state NH3 decomposition, provided the uptake of nitrogen was the same. It was also found that the rate of nitrogen desorption from the surface depended only on θN, being faster with increasing θN, whereas the rate of nitrogen uptake from ammonia decreased with increasing θN and increased with ammonia pressure (PNH3). On the basis of these data, it was concluded that the overall reaction proceeds through a reaction mechanism of “dynamic balance” between two consecutive ratedetermining steps; the supply and consumption of surface nitrogen.

Coadsorption of Thorium and Hydrogen on a Tungsten Surface

The coadsorption of thorium and hydrogen on a tungsten surface has been investigated with a field emission microscope (FEM). When the coverage of thorium (θTh)is less than half a monolayer, the work function of the surface decreases with θTh owing to the formation of surface complexes such as WH-Th+ by the adsorption of hydrogen on the Th-covered tungsten surface. The maximum decrease in work function is found to be 0.23 eV at θTh=θH=0.5, when the number of the surface complex, WH-Th+, is largest. When θTh is larger than half a monolayer, the change in the work function (Δφ) decreases; it becomes zero at θTh=0.64. Thus, when θH=0.5, the curve for Δφ vs. θTh is symmetrical about the point θTh=0.5. When the coverage of hydrogen is larger than 0.5, Δφ also decreases owing to the increase in the number of the surface complex, Th+H-, on the coadsorption surface. The sticking probability of hydrogen on a tungsten surface covered with Th is found to be 0.025 to 0.05, half that on a clean tungsten surface. When the coadsorbed surface is heated to 900∼1000 K, only the hydrogen is desorbed, with an activation energy of 2.36 eV, comparable to that on a clean tungsten surface.

Isothermal ramped field-desorption of benzene from tungsten

The binding of molecular benzene on clean (field-evaporated) tungsten surfaces at 78 K has been investigated as a function of electric field strength using a new ramped field-desorption technique. The resulting spectra, analogous to total pressure thermal desorption spectra (which do not mass analyze the desorbing species), show a physisorbed structure below 0.36 V/Å which increases continuously in area with increasing benzene coverage, a peak at 0.36 V/Å which grows until a saturation coverage is reached (and is thought to represent a physically adsorbed transition layer between chemisorbed and multiple layer physisorbed benzene at the surface), and a series of poorly resolved chemisorbed peaks (between ?0.38 and 1.13 V/Å) whose detailed shape depends on the local morphology of the surface region being examined. By recording the image of the desorbing species over the narrow field ranges which define these distinct features, the crystallographic behavior of the physisorbed and chemisorbed layers on the surface has been visually determined with angstrom resolution.

The interaction of helium metastable atoms with a clean and co covered polycrystalline tungsten surface

An investigation has been made of secondary electron ejection resulting from the impact of helium metastable atoms on a clean and CO covered polycrystalline tungsten surface respectively. The ejected election energy distribution from the clean tungsten surface is found to differ from that resulting from the neutralization of slow incident ions. In the present case the distribution is narrow and peaked at approximately 2 eV. When the tungsten surface is exposed to CO it is found that the total yield of secondary electrons increases, but that the energy distribution remains essentially the same. The increase in the total yield following CO exposure is shown to be associated with bonding of the CO in the α-state. A tentative explanation for the de-excitation mechanism is proposed, based on Auger de-excitation of the helium metastable atom.

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 × 10 14 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 × 10 19, 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.

The adsorption and decomposition of formaldehyde by tungsten

A hot, clean tungsten surface was highly effective in decomposing formaldehyde vapour to carbon monoxide and hydrogen. At temperatures above 1300 K about 30% of the impinging molecules decomposed. At lower temperatures the reactivity was reduced, probably as the result of inhibition by the adsorbed carbon monoxide. Pre-adsorbed oxygen affected the reactivity in an unusual way. At low coverages of oxygen (θ < 0.4) and intermediate filament temperatures ( T < 1000 K) no effect was observed, whereas under more extreme conditions (θ > 0.5 or T > 1200 K) a nearly linear decline in activity was recorded. These results are attributed to a similarity between the inhibiting effects of adsorbed oxygen and carbon monoxide.

Energy accommodation and reactivity of O2 on tungsten

We have determined by direct molecular beam velocity measurements that translational energy accommodation of O2 molecules scattered from a hot polycrystalline tungsten target is inefficient at high surface temperatures. Translational energy accommodation is inefficient whether the surface is clean or covered with oxygen to a varying extent, even though in the latter case the scattering is diffuse. On a clean tungsten surface the scattering of the O2 was approximately specular and the reaction probability of O2 was constant and greater than 90% over the temperature range 1000K to 2800 K. It was shown by simultaneous helium scattering that atomic surface roughness of an oxygen chemilayer, rather than trapping, is a major cause of the observed diffuse scattering of oxygen. At the lowest surface temperature of 1000 K, with an oxygen chemilayer present, the velocity of the most probable number density of the scattered O2 was lower than in the incoming beam or than that expected for complete equilibration with the surface.

The scattering of ions from a clean solid surface in the 1 to 10 eV range

We have proposed a surface model which consists of partially screened spherical caps oscillating in a direction normal to the undisturbed surface to correlate measurements of the trapping probability of sodium and potassium ions on clean (110) tungsten surfaces at energies of 1 to 15 eV and incidence of 10° to 70° from the normal to the surface. The trapping probability may be represented as a relatively simple function of angle, mass ratio, surface temperature and beam energy to ionization potential ratio, with good agreement with experimental data. The model depends on some assumptions about elevation and azimuthal screening of lattice sites which cannot fully be justified from first principles. Further analysis of the data, particularly with respect to scattered particle distribution and additional data for the scattering of lithium and cesium might help refine those assumptions.

Measurement of the surface density of states by field ionization

It is shown that recent measurements of field ion energy distributions from clean tungsten surfaces probe the density of metal states in the vicinity of the surface. We find j( ω) = (2 π/ kh) Σ m | ∫ d 3 rψ m ( r) γ z | 2 δ( ω− ϵ m ), where j(ω) is the ion current a ω, ψ m and ϵ m are electronic metal eigenfunctions and eigenvalues in the presence of the external electric field used in field ionization and γ( z) is a function which is large near the noble gas atom. An explicit expression for γ( z) is given in the text. It is estimated that tungsten metal states with values of k ‖ at least as large as 0.5 Å −1 make an appreciable contribution to j(ω) where k ‖ is the electron momentum parallel to the surface.

A RHEED study of the epitaxial growth of antimony on tungsten (112)

Antimony has been evaporated under ultra high vacuum conditions onto an atomically clean tungsten (112) surface at 300K. The structural changes occurring, from submonolayer up to 60 mean monolayers coverage, have been monitored using in situ reflection high energy electron diffraction. Deposition yields first two distinct submonolayer structures at coverages of theta =0.5 and theta =0.67; the conversion from one to the other is continuous. A (1M1) pseudomorphic layer then develops which exists for a maximum of three equivalent monolayers. Further deposition then caused the growth of two types of long, thin three dimensional islands of antimony, one similar to bulk antimony, the other an abnormal fcc phase with the same lattice constant. Both types of islands grew with their pseudocubic (100) face parallel to the (112) substrate and the relaxation from perfect fourfold symmetry was detected by the RHEED.

A RHEED study of the growth of antimony on tungsten (110)

Antimony has been evaporated under ultrahigh vacuum conditions on to an atomically clean tungsten (110) surface at 300K. The structural changes occurring from submonolayer up to 60 mean monolayers coverage, have been monitored using reflection high-energy electron diffraction. Deposition under these conditions yielded a series of submonolayer structures followed by a spontaneous change of symmetry at a coverage of theta approximately=2 from two-fold to four-fold symmetry. Further deposition caused the growth of two types of long, thin three-dimensional islands of antimony, one similar to bulk antimony, the other an abnormal FCC phase with the same lattice constant. Both types of islands grew with their pseudocubic (100) face parallel to the (110) substrate and the relaxation from perfect four-fold symmetry was detected by the RHEED. Subsequent electron microscopic examination of the thicker films confirmed the structural results deduced from the RHEED experiments.

Field-Ion Spectroscopy of Electronic States at Clean and Adsorbate-Covered Tungsten Surfaces

Field-Ion Spectroscopy of Electronic States at Clean and Adsorbate-Covered Tungsten Surfaces

Measurement of Thermal Accommodation Coefficients of Noble Gases on Tungsten with Different Surface Structures Between 300 and 370 K

Thermal accommodation coefficients (a.c.) of noble gases on clean tungsten surfaces were measured using UHV-technique (all-metal-valves, titanium-sublimations-pumps, sensitive mass-spectrometers). In the low pressure range (10-3–10-1 Pa) a relative (related to Xe/gas covered W) and the absolut “hot-wire”-method were compared. The latter method gave only insignificantly lower values for tungsten/noble gases. In the temperature range 300 K∼370 K, measurements were made with all noble gases on tungsten, and the results are compared with eailier measurements. A remarkable influence of the surface structure on the a.c. value (roughness, texture of the wires and foils) was found.

Measurements of Mean Residence Time of Potassium on Clean Tungsten Surfaces by Means of Molecular Beams

Mean residence time(τ)of potassium on atomically clean and gas-covered tungsten surfaces has been studied. Beam pulses shorter than those of previously reported measurements were used to improve the time resolution. Taking into consideration the shape of the gate function, which has seldom been considered, the following formulas were obtained: G(p)/F(p)=k+1/Kτ+ p+(1+1/β')=θp+1/β G(p)=a∫∞0ep1g(1)dt. F(p)=∫∞0ep1f(t)dt. where p is a parameter of the Laplace transfbrmation, g(t)and f(t)are the gate function and the intensity function of potassium ions, respectively. The quantity a can be determined from the experiment. K is an equilibrium constantbetween the transient adatoms and adions M(s)↔M+(s)+c. where s represents the surface. The quantity β, the ionization coefficient, is considered to be constant under the same surface condition and temperature. In order to make a comparison with the published values, the relationτ=βθ was introduced. From the Arrhenius type expressions of the from τ=τ0 exp(l/kT)andτ0+ exp(l+/kT), ignoring the temperature change of β, the desorption energies(l+) were found to be 2.51 eV for the clean tungsten surface and 1.63 eV for the strongly gas covered tungsten surface. Considering β, the desorption energies(l)were fbund to be 2.30 eV for the clean tungsten surface and 1.85 eV for the strongly gas-covered tungsten surface. The corresponding pre-exponential factors(τ0)were estimated to be 3.24×10-14 sec and 2.2×10-12 sec, respectively.

Desorption kinetics of alkaline earth ad-particles on tungsten with variation of the oxygen surface coverage

Utilizing the phenomenon of surface ionization, the mean residence time τ of alkaline earth ad-particles on a polycrystalline tungsten surface has been investigated as functions of oxygen surface coverage and temperature with a pulsed beam method. From the temperature dependency of τ the values of the desorption energy E M 0 of alkaline earth atoms and the corresponding pre-exponential factor 1/P 0 M 0 have been determined at different values of oxygen coverage. In the special case of a clean tungsten surface the values of E M 0 obtained for Ca, Sr, and Ba were 3.3, 3.7, and 4.1 eV, respectively; the corresponding values of 1/P 0 M 0 were 4 × 10 −12, 5 × 10 −13, and 8 × 10 −13 s. A substantial increase of E M 0 with increasing oxygen surface coverage was observed for all alkaline earth elements, whereas the logarithm of 1/P 0 M 0 decreased simultaneously resulting in a linear dependency between E M 0 and log(1/P 0 M 0 ) (“linear logarithmic compensation effect”).

A field emission measurement of the influence of adsorption on surface self-diffusion

A field emission technique is described to measure the influence of a definite adsorption layer on surface self-diffusion. The rate determining activation energy for this diffusion is determined in a field electron microscope by measuring time and temperature of the transformation of a build-up form into an annealed form of an emitter crystal. If a clean tungsten surface is covered with one tenth or less of a monolayer of carbon, the surface self-diffusion remains unchanged. However, coverage of one half to one monolayer of carbon leads to an increase of the surface self-diffusion energy from 3.1 to 8.7 eV. If oxygen is adsorbed on clean tungsten an increase of the surface self-diffusion energy from 3.1 to 4.2 eV is found. These results imply some earlier self-diffusion data on other metals may be influenced by undetected adsorption layers.