B.J. Hopkins

The diffusion of carbon to and from W(100)

Abstract The equilibrium segregation in the system W(100) plus two monolayers of total carbon content has been studied in the range 1600 to 2073 K. The energy of segregation is −56(±2) kcal mole −1 . The kinetics of carbon segregation at 1350 and 1420 K were observed. From a semi-empirical treatment an upper limit to the activation energy for volume diffision is deduced to be 59 (±8) kcal mole −1 .

Auger and electron energy loss spectra of adsorbed species: α and β states of carbon monoxide and nitrogen on W(100)

Abstract Auger and electron energy loss spectra have been recorded for both α and β chemisorbed states of carbon monoxide and nitrogen on W(100). The α state spectra have been analysed with reference to results for the gas phase molecules, those for the β states have been analysed following a quasi-atomic final state model. The results illustrate both the potential and the limitations of AES and ELS in surface chemical analysis.

The adsorption of hydrogen on (110), (100), (111), and (113) oriented tungsten single crystal surfaces

Abstract Work function/coverage curves have been determined for the (100), (113), (111) and (110) orientations of large single crystals of tungsten during the adsorption of hydrogen. The conditions were controlled with care and the purity of the hydrogen gas monitored with a mass spectrometer. The first three orientations all showed an increase in work function at completion of the adsorption with surface potentials of − 0.54, − 0.43, and − 0.30V, respectively. The (110) orientation alone showed an overall decrease in work function with a surface potential of + 0.14V. A qualitative interpretation of these results is attempted in terms of the electronegativity differences between the hydrogen and the various orientations of the tungsten making use of the empirical Gordy-Thomas relation. Measurements were also made on polycrystalline tungsten foil; at complete coverage this had a surface potential of − 0.63V.

Work function changes due to the adsorption of chlorine, bromine and iodine on tungsten single crystal surfaces

Abstract The adsorption of chlorine, bromine and iodine onto large oriented tungsten single crystals has been studied using the Kelvin contact potential difference technique. Work function changes have been determined for the (110), (100) and (111) orientations as a function of the number of molecules incident on each surface. The surface potentials for atomic adsorption were, for chlorine, +0.26, −0.58 and −1.03 V respectively, for bromine + 0.32, −0.80 and −0.88V and for iodine +0,78, +0.18 and +0.13 V. In some cases adsorption at higher coverages revealed the presence of weakly bound phases which invariably caused a decrease in the surface potential. A qualitative interpretation of the results for atomic adsorption is discussed in terms of a simple dipole model in which three contributions to the surface potential are considered. These arise from charge transfer between the adsorbate and the tungsten surface, polarization of the adsorbed halogen in the metals external field and the smoothing of the electronic charge distribution in the adsorbed film. With such a simple model it is possible to explain qualitatively the variation in magnitude of the dipole and its reversal between different crystal planes for the same adsorbate.

Simultaneous LEED and RHEED studies of the growth of zirconium on the tungsten (100) surface

Abstract Zirconium was evaporated under ultra high vacuum conditions onto an atomically clean W(100) surface in a combined low energy electron diffraction and reflection high energy electron diffraction apparatus (LEED/RHEED). Both diffraction patterns were used to follow and interpret the two- and three-dimensional growth of zirconium up to an estimated mean coverage of about fifty monolayers. Deposition onto a substrate at room temperature gave rise to a 1 × 1 pseudomorphic layer of zirconium with an improving order up to 600° K. Higher temperature annealing of very thin deposits led to a new structure W(100)( 4 3 ×2) −Zr , at roughly three-quarter monolayer coverage. Annealing of thicker films gave a return of the 1 × 1 pseudomorphic growth followed by the development of islands of an abnormal fcc phase of zirconium. The formation of these islands and their removal, at a lower temperature than the pseudomorphic layer, could be readily followed by RHEED although no changes in the LEED pattern were observable.

The role of impurities in the stability of ZnO surfaces

Abstract Both “as-grown” and “real” etched prism and (000 1 ) oxygen surfaces have been studied by LEED and Auger electron spectroscopy. Heat treatment up to 800 K was sufficient to remove impurities other than calcium on all surfaces and potassium on the polar “real” surface. These could only be removed by ion bombardment. The Ca was associated with a (3 × 1) superstructure on the prism surface and a ( 3 × 3 ) on the polar surface. On the “as-grown” polar surface it was also possible to see (3 × 3) structure associated with reduced amounts of Ca. The especially strong binding of the electropositive elements on the negative oxygen polar surface is due to charge transfer, i.e. impurity stabilisation, this in turn can lead to chemical shifts in some of the Zn Auger transitions and to changes in the oxygen peak shape.

The adsorption of acetylene on W(110) at room temperature: An electron spectroscopic study

The adsorption of acetylene on W(100) at room temperature has been studied by AES, ELS, thermal desorption, mass spectrometry, work function and LEED in one vacuum chamber. AES line profile analysis shows that there are at least two adsorption processes occurring at room temperature. Further, it is possible to explain all the AES results by assuming non-sequential adsorption into just two states, denoted by α and β. This picture was substantiated and embellished by comparison with other standard surface techniques. The α-state comprises either a C2H2 unit with an activation energy for desorption of 2.3 eVmolecule (53 kcal mole−1) or CH units bounded through the carbon of the β-state. Saturation coverage for the α-state is 3 × 1014 molecules cm−2. The β-state is dissociative at low acetylene exposures and comparison between a carbon covered surface and the β-state suggest the latter to be dissociative up to saturation. There also appears to be ca. 1014 hydrogen atoms cm−2 on W(100) on room temperature acetylene saturation, the carbon content of the β-state being 9 × 1014 atoms cm−2. The residual C⋯C bond from the molecule in the β-state remains unknown. No sign of ordering in the adsorbed species was detected, save the possibility of (1 × 1) in the β-state. Acetylene adsorption at 580 K showed hydrogen from the β-state to block acetylene adsorption by 15% at saturation. A two-site adsorption model for the β-state is proposed to explain the results. The α-state is bonded through the carbon of the β-state and it is speculated that the former adsorbs onto “β” domains where there is a critical minimum size for the latter.

Decomposition of ethylene on W(100) studied by Auger spectroscopy

Abstract The interaction of ethylene with a W(100) surface was studied in the temperature range 80–500 K by monitoring changes in the carbon Auger peak shape. On adsorption at 80 K decomposition to acetylene occurred at low coverage followed by non-dissociative adsorption. Heating the adsorbate to 300 K resulted in further decomposition to acetylene. Final decomposition to chemisorbed carbon atoms was detectable above 300 K and rapid above 400 K.

Chemisorption on Ta (100)

Abstract The role of absorption into Ta(100) during the chemisorption of oxygen, carbon monoxide, carbon dioxide, nitric oxide, nitrogen and ethylene has been studied using primarily LEED and Auger spectroscopy. Only nitrogen formed a well ordered surface at 300 K, all the other gases required heat treatment to produce a range of surface structures. The main feature observed is that dissociation of oxide gases takes place at the surface with solution of the nitrogen or carbon into the crystal with increasing temperature. The structures then observed are due to oxygen ordering on the surface in arrangements requiring successively less oxygen as the temperature increases to 2000 K. No oxygen desorption took place at any stage and again solution into the bulk appears to take place. At 2800 K the crystal cleans and TaO is observed to evaporate.

A LEED and AES study of the adsorption of bromine on W(100) at room temperature

Bromine gas adsorbs atomically on W(100) at room temperature to a saturation concentration of θ = 0.88 relative to the surface tungsten atom density (1019 m−2). Below θ ∼ 0.4, a c(2 × 2) overlayer is formed. Beyond this a (34√2 × √2)R45° structure is preferred and this saturates at θ = 0.67. Higher surface bromine concentrations result in hexagonal variable compression structures on W(100). The sequence begins w structures on W(100). The sequence begins with a c(4 × 2) coincidence mesh which at higher coverages is compressed in one 〈0,1〉 substrate direction. At certain compressions the overlayer achieves p(5 × 2), c(6 × 2), p(7 × 2) coincident configurations and perhaps c(8 × 2) at saturation. This would correspond to θ = 0.875 and is the closest coincidence structure to a perfect hcp overlayer. Bromine prefers a rectangular overlayer geometry on W(100) and compression into an hexagonal array greatly reduces the overlayer stability. The nn repulsions incurred limit room temperature adsorption as the overlayer compresses to perfect hep. Halogen behaviour on W(100) is compared with that on Fe(100). Most differences can be explained in terms of geometrical and bond strength differences but chlorine on W(100) appears to be an exception to this rule.