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 ($\text{3}\text{4}$√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.     

The adsorption of acetylene on W(100) 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 $\text{2.3}\text{eV}\text{molecule}\text{(53}\text{kcal mole}{}^{\mathrm{?1}}\text{)}$ 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.     

Low temperature XPS and AES studies of O2 adsorption on Al(100)

Adsorption of O₂ on Al(100) at 80 K was studied by means of Auger and XP spectroscopies. At low surface coverages Al_xO_y oxides were formed with x : y ratios from 3 : 1 to 1 : 1, depending on surface coverage. At higher coverages or heating to room temperature the oxide layer transformed to the familiar Al₂O₃.     

ESDIAD studies of fluorine and chlorine adsorption at Si(100)

Electron stimulated desorption ion angular distribution (ESDIAD) patterns are reported for fluorine and chlorine adsorbed at the silicon (100) surface. Analytical and numerical methods of establishing the degree of angular compression imposed by the sample bias frequently employed in ESDIAD measurements, are described. Measurements of the two halogen adsorbates contrast strongly, particularly with regard to their response to annealing in the temperature range 300 to 1000 K. In the case of F⁺ desorption, a minimum in ion intensity occurs along the surface normal. Maxima in the ion production are observed at a polar angle of 30°, and in registry with the principal crystal axes as reported by other workers. Cl⁺ ion angular distributions however, indicate a mixture of normal and off-normal emission for adsorption at ≤ 300 K. This pattern transforms, irreversibly, to a single maximum directed along the surface normal in response to annealing to 650 K. The transformation is linked to the thermal depopulation of one adsorption state, leaving a higher binding energy state intact.     

AES and LEED studies of xenon adsorption on (100)NaCl

The thermal and electro impact behaviour of NO adsorbed on Pt(111) and Pt(110) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb non-dissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N₂ and O₂ in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(111) and Pd(111), and in particular, the unusual desorption kinetics of N₂ production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(111) and Pt(110), and the adsorption kinetics of NO on the (110) surface have been examined.     

A very low energy electron reflection study of hydrogen adsorption on W(100) and W(110) surfaces

Experimental results of the elastic backscattering of electrons with energies between 0 and 20 eV from the surface systems H/W(100) and H/W(110) are reported and interpreted on the basis of the Darwin model. On W(100) the adatom distance from the reconstructed substrate is found to be different from that on the relaxed surface. Large vertical displacements of the substrate atoms during reconstruction can be excluded. On W(110) the results indicate two different binding states which are occupied sequentially and which differ significantly in their distances from the surface. Work function change data are also reported for both systems.     

AES quantitative interpretation of the first stages of Pd2Si formation during Pd adsorption on Si(111) at room temperature

The quantitative analysis of the evolution of Pd and Si N(E) Auger peak amplitudes has been carried out as a function of Pd concentration on a Si(111) surface. Modelization of these evolutions has allowed us to conclude that the formation of the Pd₂Si silicide occurs from the beginning of Pd adsorption even at room temperature.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Evidence for two oxygen chemisorption sites on the Cr(100) surface at room temperature

The interaction of oxygen with the clean Cr (100) surface has been investigated by work function measurements, angle resolved photoemission spectroscopy and low energy electron diffraction. At low coverages (≈0.15 monolayer (ML)) the adsorbed oxygen is characterized by a work function decrease, a feature at 6.8 eV binding energy in UPS and a weak diffuse C(2 × 2) diffraction pattern. This form of oxygen labeled form a is quite stable at temperatures below 500°C. Upon further exposure at room temperature (⪆0.20 ML) a second form of oxygen labeled form b and characterized by a work function increase and peaks at 3.8 and 5.9 eV at normal photo-emission is identified. It is proposed that oxygen in forms a and b is chemisorbed in the fourfold hollow and on-top sites respectively.     

Benzene adsorption on nickel (100) and (111) faces studied by leed and high resolution electron energy loss spectroscopy

Studies of benzene (C₆H₆ and C₆D₆) adsorption have been performed by high resolution electron energy loss spectroscopy (HRELS) and LEED experiments on nickel (100) and (111) single crystal faces at room temperature. Chemisorption induces ordered structures, c(4 × 4) on Ni(100) and (2√3 × 2√3)R30° on Ni(111), and typical energy loss spectra with 4 loss peaks accurately identified with the strongest infrared vibration bands of the gazeous molecules. Benzene chemisorption preserves the aromatic character of the molecule and involves respectively 8 nickel surface atoms on the (100) face and 12 on the (111) face by adsorbed molecule. The interaction takes place via the π electrons of the ring. Significant shifts of the CHτ bending and CH stretching vibrations show a weakening of the CH bonds due to the formation of the chemisorption bond and a coupling of H atoms with the nickel substrate.     

Thermodynamics and kinetics of Xe monolayer adsorption on Cu(100) by LEED and AES

The thermodynamical properties of the adsorption of the first layer of Xe on a Cu(100) surface have been studied by LEED and AES. We show, from the adsorption isotherms, that the adsorbed xenon undergoes a first order 2D gas ? 2D solid phase transition. The presence of impurities and heterogeneities on the surface alters the form of the isotherm in the prestep region, but does not change the phase transition pressure. The value of the latent heat of transformation is 6.2 kcal/mole. The kinetics of adsorption and desorption have been studied using AES. We determined that both the adsorption and desorption are zero order processes. The sticking coefficient has been found to be practically equal to the unity and the value of the activation energy for the desorption is about 6 kcal/mole.     

Theoretical analysis of ARUPS of a Si(100) surface at room temperature

ARUPS spectra of a Si(100) surface with disorder perpendicular to the dimer row are studied by the tight-binding method for the ensemble of surface buckled dimers. We assume order along the row. Results of ARUPS experiment on the surface at room temperature agree qualitatively with calculated results for disordered structure in which the distance to the next buckled dimer along the row and the height of the buckled dimer change alternately along the row. Peaks of two branches observed in a recent experiment at room temperature appear distinctly in calculated results as long as the short range order among the rows is not weak. The intensity of the two branches is very small compared to the other for conventional structure of the surface.     

The reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature

Ultraviolet photoemission spectroscopy using hv = 21.2 eV and filtered 40.8 eV radiation as well as temperature programmed thermal desorption spectroscopy are used to investigate the chemical reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature. Striking crystallographic effects and several coexisting phases are observed and found to be coverage and temperature dependent. A methodology is described and used to predict the relative energy levels for a variety of adsorbed hydrocarbon fragments on Ni surfaces. Such levels together with the thermal desorption spectra are used to identify the observed species. In particular, CH and CCH species are isolated on Ni(100) and Ni(110) surfaces, respectively, via low temperature adsorption and subsequent pulsed sample warming experiments. The room temperature adsorption phases are deduced using these ionization levels together with those of chemisorbcd acetylene, atomic hydrogen and carbon. At room temperature on Ni(100), H, C, CH and C₂H₂ species form together below 2 L exposure while CH species form thereafter, up to a saturation exposure of ~10 L. On Ni(110), H and CCH species form below 1.5 L exposure followed by the formation of CH₂ and likely CH species. The relative stabilities of these species at elevated temperatures is: C₂H₂ < CCH ? CH < CH₂. A model for the bonding of acetylene and its reaction to form CCH species on Ni(110) is proposed.     

Angle-resolved AES study on initial oxidation of Ni (100) surface

The Ni-M_2,3VV Auger electron angular distributions have been measured from oxygen-adsorbed Ni (100) surfaces and from cleaved NiO (100) clean surfaces. Significant variations in the angular profiles have been observedin the second reaction stage of initial oxidation process. Present results support the island growth model of NiO layers. It is also shown that the new information on the ratio of domain area of NiO to that of c (2 × 2)-O on Ni (100) surface can be obtained by angle-resolved AES method.     

A molecular dynamics study of Lennard-Jones physisorption on W(100)

The physisorption of Xe on W(100) was modeled by Lennard-Jones pair-wise interaction potentials and the dynamics of coverages ranging from one to four adlayers obtained by molecular dynamics simulation. At 115 K, the first two layers were well-ordered and each adsorbed with c(2 × 2) symmetry. Further adsorption produced a surface similar to that of a distorted Xe(100) face. In accord with the work of Broughton and Woodcock [1], the top layers of the three- and four-adlayer coverages were “rough” and had liquid-like diffusion coefficients. The potential energies of all layers other than the first were similar, thus corroborating one of the postulates of BET theory [2]. Generally, the effect of adsorbing a layer was to reduce the entropy of all those beneath.