Face Of Tungsten Research Articles

A combined LEED/RHEED Auger study of oxygen adsorption on the W(112) surface

Abstract LEED, RHEED and Auger spectroscopy have been used to study the adsorption of oxygen on to a clean and carbon contaminated (112) face of tungsten. At room temperature all the features reported previously were observed together with a p(1 × 4) surface structure which appeared at an exposure of about 1. 4L just before the formation of the p(1 × 2). Previously a p(1 × 4) structure has been reported only after heating to 2000K. RHEED showed this p(1 × 4) structure clearly; using LEED, the structure was difficult to distinguish. This appears to confirm suspicions that in some situations involving gas adsorption, RHEED has a greater sensitivity than LEED. Possibly most of these situations involve, as does the present p(1 × 4) structure, monolayer islands where the differing coherence widths of the RHEED and LEED beams account for the differing sensitivities. Carbon on the (112) surface also appears to exist as thin islands, either of the previously reported c(6 × 4) structure, or in smaller amounts, on a surface showing (1 × 1) symmetry. Removal of all carbon by heat treatment alone was found to be impossible in a reasonable time and heating in oxygen was necessary. Oxygen adsorption on a carbon contaminated surface did not give rise to any new structures but rather a reduction in the visibility/formation of the clean surface/oxygen structures.

Some polarisation effects in photoemission from a cubic crystal

The energy distribution spectra of photoelectrons emitted normal to the (100) face of tungsten were investigated with respect to their polarisation dependence. The results are compatible with the assumption that surface photoemission vanishes if the exciting light is polarised parallel to the emitting surface. This is demonstrated for the examples of emission from a high density of states at the surface, a surface resonance, and an adsorbate induced level. The measurements also show that emission following direct optical excitation in the bulk is polarisation sensitive, supporting the idea of an anisotropic excitation along a star of k-vectors in the bulk. Some indication is also given of emission in ‘secondary cones’.

Adsorption of oxygen at tungsten single crystal: the (110) face (Electron stimulated desorption)

The interaction of oxygen gas with the (110) face of tungsten single crystal has been investigated by the techniques of electron stimulated desorption combined with surface mass spectrometry. At 300K two states with desorption efficiencies of 6*10-7 and 5*10-6 ion/electron for production of O+ ions are observed in the adsorption scheme. The former state forms rapidly on introducing gas to the clean surface and is essentially complete at an exposure of 0.3*10-6L. The two states are identified with different temperature regimes of around 350 and 750K for the removal of ion current and with total desorption cross sections of respectively 2*10-17 and 5*10-19 cm2. Measurements of the energy distribution of desorbed ions reveal a difference of around 1.2 eV between the values of most probable ion energy from the two states.

Numerical data and experimental proof of the unified theory of electron emission (Christov)

In order to check the unified theory of electron emission (Christov), the temperature dependence of the electron emission from the (013), (001) and (112) single crystal faces of tungsten is measured between 300 °K and 1500 °K at relatively low fields yielding low current densities (10 −1 to 10 −7 A/cm 2). Experiments are performed in a field electron microscope using a probe hole technique with an electron multiplier. The experimental results are in a fairly good agreement with Christov's theory. Tables of numerical data of Christov's formulae are presented. Using these tables, typical results of the theory are described.

Helium Diffraction from W(112)

Helium scattering from the (112) face of tungsten has been shown by Stickney to,exhibit diffraction when the incident beam is in the (11̄0) direction, i.e. perpendicular to the ridge-trough structure of the surface. This study reports a more complete study of helium diffranction from this surface. The positions of the (1̄0) and (10) beams are shown to follow the Bragg relation for incident angles from 20° to 70° and the relative intensities of the diffraction beams to the specular beam are in agreement with simple diffraction theory. Debye-Waller analysis of the thermal attenuation of the specular scattering leads to unrealistically high values of the surface Debye temperature while electron scattering from the same surface gives Debye temperatures of 1/2 to 2/3 the bulk. Neon scattering in the (11̄0) direction gives rainbow scattering while argon is scattered in a single pseudo-specular lobe. Attempts to deduce interaction energies from measurements of selective helium adsorption will also be described.

Coadsorption of caesium and barium on faces of tungsten and niobium crystals: Yu M Konoplev et al, Fiz Tverd Tela, 14 (2), 1972, 326–333 ( in Russian)

Coadsorption of caesium and barium on faces of tungsten and niobium crystals: Yu M Konoplev et al, Fiz Tverd Tela, 14 (2), 1972, 326–333 ( in Russian)

915. Structure of lead films on the (100) tungsten face: D A Gorodetskiy and A A Yasko, Fiz Tverd Tela, 14 (3), 1972, 746–748 ( in Russian)

915. Structure of lead films on the (100) tungsten face: D A Gorodetskiy and A A Yasko, Fiz Tverd Tela, 14 (3), 1972, 746–748 ( in Russian)

Chemisorption of H2 on W(211)

Adsorption, desorption, and work function data for hydrogen adsorption and mixed adsorption of deuterium and ethylene show that multiple binding states exist for hydrogen on the (211) face of tungsten. The desorption spectrum consists of two peaks: β1 with peak maximum at 330°K and β2 with peak maximum at 675°K. Both follow second order desorption kinetics with activation energies of 16 kcal/mole and 35 kcal/mole, respectively. The rate of adsorption into the β1 state is proportional to the fraction of unoccupied β1 sites with a sticking probability of 0.05. Until the β2 sites are nearly filled, adsorption into this state is independent of coverage and occurs with a sticking probability of 0.57. For both states the work function change is linear with coverage, but with a maximum increase of 0.61 eV for the β2 state and a maximum decrease of 0.27 eV for the β1 state. Consideration of the topography of the (211) surface leads to the conclusion that the hydrogen sites on this face are associated with the close packed rows of tungsten atoms and with the troughs between the rows. With these two possible sites both the adsorption kinetics and the results for mixed adsorption of deuterium and ethylene are shown to be consistent with the assignment of the β1 state to the rows and the β2 state to the troughs.

Catalytic decomposition of ammonia on tungsten (100), (110), and (111) crystal faces

The rates of catalytic decomposition of ammonia on (100), (110), and (111) single crystal faces of tungsten were measured over the temperature range 800–970 °K for ammonia pressures ranging from (0.5–100)× 10−3torr and for nitrogen and hydrogen pressures varying from (0–50) × 10−3torr. In all cases the rate of decomposition was of the form, rate=A+BPNH3(2/3), and was independent of nitrogen and hydrogen partial pressures. The constants A and B varied substantially with crystal face; the values of B for the (111), (100), and (110) faces were in approximate ratio 8.4:1.55:1 and these substantially established the decomposition rate for PNH3>5 × 10−3torr. Rate forms for NH3 and ND3 were compared on the (111) face at 860 °K, with results ANH3=AND3,BNH3≈ 1.47 BND3. The observed rate form is derived from a model involving nearly complete surface coverage by the species WN and small surface coverages by species W2N, W2N3H2, and WNH. The A term in the rate law is generated by the reaction, 2WN→W2N+(1/2)N2. This process was advocated as rate limiting in ammonia decomposition by Matsushita and Hansen and their rate law for this process obtained on polycrystalline tungsten furnishes an estimate of the A parameter in the same order of magnitude as that observed. The BPNH32/3 term results from a steady state balance of W2N3H2 and WNH decomposition reactions. Peng and Dawson have advocated decomposition of W2N3H as rate limiting in the decomposition of ammonia, and have also suggested WN dissociation may be rate limiting at low pressures, W2N3H dissociation at higher pressures. The present model therefore resembles that of Peng and Dawson in concept but not in detail.

Directional photoemission from tungsten

Photoemission energy distribution spectra have been scanned for photoelectrons emitted normal to two different crystal faces of tungsten, namely (100) and (110). The energy distributions and the derivative spectra show large differences between emission from the two faces. Whereas strong surface state emission is observed from the (100) face, no surface states are associated with the (110) face. The spectra show features characteristic of direct and also nondirect transitions, but these features appear very different for the two faces. Using existing band structure calculations, the spectral structure is assigned to electronic transitions between bands along the Δ axis for the (100) face and the Σ axis for the (110) face. The experiments show that directional photoemission spectra taken normal to a crystal surface make it possible to investigate electronic energy bands along symmetry directions in the Brillouin zone.

Structure and work function of antimony films on (100) face of tungsten: D A Gorodetskiy and A A Yasko,Fiz Tverd Tela, 13 (11), 1971, 3462–3464 ( in Russian)

Structure and work function of antimony films on (100) face of tungsten: D A Gorodetskiy and A A Yasko,Fiz Tverd Tela, 13 (11), 1971, 3462–3464 ( in Russian)

Influence of a non-uniform electric field on the change of equilibrium coverage of ytterbium and neodymium on the (111) face of tungsten: Ts S Marinova and I B Glebova,Tiz Tverd Tela, 13 (11), 1971, 3445– 3446 ( in Russian)

Influence of a non-uniform electric field on the change of equilibrium coverage of ytterbium and neodymium on the (111) face of tungsten: Ts S Marinova and I B Glebova,Tiz Tverd Tela, 13 (11), 1971, 3445– 3446 ( in Russian)

Nucleation studies of gallium arsenide on (111) tungsten

This paper is concerned with the nucleation of gallium arsenide on the (111) face of tungsten. After a brief introduction to the more relevant theories of nucleation, a description is given of the vapour phase growth equipment. The growth is analysed in terms of the variation of saturation nucleation density with reciprocal temperature and time, and results are given for the variation of average nucleus diameter as a function of temperature and time. Experimentally determined values of activation energies are given and the saturation density of nuclei is determined. These values are shown to be quantitatively inexplicable by present nucleation theories.

Atomic and Molecular Diffraction and Scattering from a Tungsten Carbide Surface Characterized by LEED

The scattering of collimated beams of helium, deuterium, neon, and argon are reported from two tungsten carbide surfaces which form on the (110) face of tungsten. On a (3× 5) surface both helium and deuterium give sharp, well-defined diffraction peaks corresponding to the (2̄, 0), (1, 0), (0, 0), (1, 0), (2, 0) directions along the 5 spacing. No diffraction is observed on the (1× 1) carbide surface or the clean metal surface which is also (1× 1). Fractions scattered in the specular ray, (0, 0), as high as 55% have been observed from the carbide surfaces. The scattering of neon is in the inelastic regime and argon is trapping dominated. Analysis of the scattering in combination with the LEED suggests a dense carbide, perhaps WC, with a Debye temperature of 1200°K or higher. This diffractive scattering is the first reported on a surface other than alkali halides and some comments on the sort of periodic potential which is likely to give rise to diffractive scattering of atoms are given.

Adsorption and Flash Decomposition of Ethylene on Tungsten Single-Crystal Faces

The interaction of ethylene with the (100), (110), (111), and (211) faces of tungsten was studied by means of thermal desorption spectroscopy. Ethylene decomposition yields a single broad hydrogen peak at ∼300 K on W(111), two poorly resolved hydrogen peaks at ∼300 and ∼400 K on W(211), hydrogen peaks at ∼300 and ∼450 K with an area ratio of 1:2 on W(110), and a single decomposition peak at ∼275 K on W(100). Assuming di-σ-bonded ethylene, these results are consistently interpreted in terms of the limitations on ethylene adsorption imposed by the structure of the surface; and the stability of adsorbed ethylene is interpreted from consideration of the structure of the resulting ethylene-surface complex. The absence of two equal peaks on all except the (211) face eliminates the possibility of a two-step decomposition suggested by previous work.

18. Structure of antimony films on the (110) tungsten face: D A Gorodetskiy and A A Yasko, Fiz Tverd Tela, 13 (5), 1971, 1298–1303 ( in Russian).

18. Structure of antimony films on the (110) tungsten face: D A Gorodetskiy and A A Yasko, Fiz Tverd Tela, 13 (5), 1971, 1298–1303 ( in Russian).

3. Electron states of adsorbed atoms of caesium, lithium and barium on (110) tungsten face: E V Klimenko and A G Naumovets, Fiz Tverd Tela, 13 (1), 1971, 33–40 ( in Russian).

3. Electron states of adsorbed atoms of caesium, lithium and barium on (110) tungsten face: E V Klimenko and A G Naumovets, Fiz Tverd Tela, 13 (1), 1971, 33–40 ( in Russian).

4. Intensity of scattered beams in the diffraction of slow electrons on the (110) face of tungsten covered with sodium: A G Fedorus, Fiz Tverd Tela, 13 (1), 1971, 3–9 ( in Russian).

4. Intensity of scattered beams in the diffraction of slow electrons on the (110) face of tungsten covered with sodium: A G Fedorus, Fiz Tverd Tela, 13 (1), 1971, 3–9 ( in Russian).

Investigation of chromium films on (110) face of tungsten: B V Bondarenko and V I Makhov, Fiz Metal Metalloved, 31 (2), 1971, 442–444 ( in Russian).

Investigation of chromium films on (110) face of tungsten: B V Bondarenko and V I Makhov, Fiz Metal Metalloved, 31 (2), 1971, 442–444 ( in Russian).

The chemisorption of several diatomic gases onto a W(210) plane and a polycrystalline tungsten surface

Plots of the sticking probabilities of CO and H 2 on the (210) face of tungsten against the fraction of the surface covered were found to be sigmoid. This shape has already been observed for at least one other system (15). It is explained by suggesting that adsorption on neighboring positions linearly increased the chemisorption activation energy experienced by a molecule in the mobile precursor state. The desorption spectra of CO on the W (210) plane contained an α and a β peak and an analysis of the spectra produced by CO on polycrystalline tungsten demonstrated that the β 3 peak desorbed by a second order reaction. The change in the work function when CO adsorbed on the W(210) face showed that the surface filled in two stages. The surface moment was constant during each stage. Hydrogen initially increased the work function of the W(210) plane. When the fraction of the surface covered reached 0.75, the work function was at a maximum and then decreased somewhat.