Symmetric Hollow Research Articles

Ordered and disordered oxygen and sulfur on Ni(100)

The adsorption structures of oxygen and sulfur are compared for both ordered and disordered phases as determined by means of LEED and DLEED, respectively. Emphasis is on the comparison of the different chemical species as well as on the coverage dependence in each case. In the ordered phases both oxygen and sulfur adsorb in fourfold symmetric hollow sites, whereby in the disordered phases, i.e., for low coverages, mixtures with off-center sites seem to exist. The adsorption heights are coverage dependent obviously due to adatom-adatom interactions. Both species induce a substantial expansion of the first substrate layer distance which is larger for oxygen than for sulfur but increases linearly with coverage in both cases. Also, there is an adsorbate-induced buckling in the second substrate layer which is weakened with increasing coverage.

Local environment of nitrogen in a surface nitride: A low-energy electron diffraction study of Cr(100)-(1 x 1)N.

We present a quantitative low-energy electron diffraction study of the surface nitride produced by nitrogen segregation to the surface of the (100) face of chromium. Emphasis is put on the local environment of N atoms, in reference to surfaces of bulk nitrides, this study being one step in a longer-term investigation of the role of vacancies at substoichiometric nitride and/or carbide surfaces. The nitrogen atoms are found to occupy symmetric fourfold hollow sites. The nitrogen atoms are 0.221\ifmmode\pm\else\textpm\fi{}0.016 A\r{} above the chromium first layer and have four metal neighbors in the top layer with a 2.045\ifmmode\pm\else\textpm\fi{}0.002 A\r{} bond length and a fifth metal neighbor in the second layer at a very similar distance, 2.015\ifmmode\pm\else\textpm\fi{}0.018 A\r{}. This environment is made possible by having a very large expansion (24.8%) of the topmost interlayer spacing of chromium and shows great similarities with the surface structures of : (i) surface nitrides like Ni(100)-P4g-(2\ifmmode\times\else\texttimes\fi{}2)-N (which reconstructs with a large lateral displacement of the surface metal atoms), Fe(100)-c(2\ifmmode\times\else\texttimes\fi{}2)-N (which does not reconstruct because the metal substrate is bcc like chromium, leaving room for the N atoms to set up the maximum coordination), and (ii) the surface of a bulk nitride ${\mathrm{VN}}_{0.89}$(100) (as obtained from preliminary results).

Structure determination of c(4x2)Cs/Rh(100) by LEED

The adsorption structure of Cs on Rh(100) in the c(4 × 2) phase at 120 K is analyzed by means of LEED intensities. Four integer order and three fractional order spot intensity spectra were measured using a TV camera operated under computer control. Intensity calculations for various adsorption sites were performed both full dynamically and within the quasidynamical approximation, and were compared to the experiment using the Pendry R-factor. The best fit results for fourfold symmetric hollow site adsorption with a Cs-Rh layer distance of 2.87 ± 0.07 A ̊ which corresponds to a Cs radius of 2.10 ± 0.06 A ̊ . The substrate layer configuration remains unrelaxed within the limits of error. The quality of the theory-experiment fit corresponds to a R-factor of R = 0.35 which, however, can be reduced to R = 0.31 by assumption of an energy dependent inner potential.

Surface structure analysis of oxidized Fe(100) by low energy ion scattering

The surface structures of differently prepared oxide layers on Fe(100) have been analysed using low energy ion scattering with time-of-flight analysis. The intensity of scattered 5 keV Ne + ions and neutrals was measured as a function of elevation and azimuth of the incoming beam at a constant scattering angle, producing photograms of clean as well as oxygen covered Fe(100) surfaces. The formation of the Fe(100) p(1 × 1)-O surface structure at full monolayer coverage was confirmed by measuring azimuthal distributions of reflected Ne + ions and neutrals at specular reflection and a scattering angle of 45 °. The oxygen atoms are found to be located in the fourfold symmetrical hollows of the substrate surface. From the O − recoil intensity as a function of the elevation angle of incidence at specular reflection it was calculated that the oxygen atoms are located at a distance of 0.56 ± 0.05 Å above the first substrate layer of iron atoms. A disordered oxide layer grown on Fe(100) at room temperature reconstructs at elevated temperatures leaving a well-ordered monolayer of oxygen on top of the surface. The surface structure of this oxygen monolayer appears to be identical with respect to the surface properties found for a single monolayer of oxygen on Fe(100).

Investigation of the Cu(111) (√3 × √3)R30°-Cl structure using sexafs and photoelectron diffraction

SEXAFS measurements above the Cl K-edge reveal that the Cu-Cl distance is the Cu(111) (√3 × √3)R30°-Cl structure is 2.39 ± 0.02 Å, with adsorption in three-fold symmetric hollow sites. These measurements are not, however, able to distinguish the two inequivalent hollow sites on this surface, even with the aid of multi-shell simulations. Photoelectron diffraction data for the Cl1s state, taken at kinetic energies between 100 and 400 eV are interpreted with the aid of single scattering calculations and shown to resolve this question, demonstrating that the Cl atoms adopt the same sites as would be occupied by the next fcc metal atom layer.

Oxygen chemisorption on Cr(110) I. Dissociative adsorption

The initial stages of oxygen chemisorption on Cr(110) at 300 K have been investigated using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Dissociative chemisorption occurs with near unit sticking probability, leading to an ordered p(4 × 2)O overlayer at θo ⋍18. A model with atomic oxygen located in the two-fold symmetric hollow sites is proposed. The previously reported c(3 × 1)O overlayer was not observed in this study. A disordered oxygen adlayer is found as θo increases. ESDIAD and HREELS data suggest that chemisorbed oxygen atoms occupy inequivalent local binding sites for θ > 0.25; subsurface oxygen is not found under these mild oxidation conditions.

Oxygen chemisorption on Cr(110): I. Dissociative adsorption

The initial stages of oxygen chemisorption on Cr(110) at 300 K have been investigated using high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Dissociative chemisorption occurs with near unit sticking probability, leading to an ordered p(4 × 2)O overlayer at θ o ⋍ 1 8 . A model with atomic oxygen located in the two-fold symmetric hollow sites is proposed. The previously reported c(3 × 1)O overlayer was not observed in this study. A disordered oxygen adlayer is found as θ o increases. ESDIAD and HREELS data suggest that chemisorbed oxygen atoms occupy inequivalent local binding sites for θ > 0.25; subsurface oxygen is not found under these mild oxidation conditions.

CO chemisorption on Cr(110): Evidence for a precursor to dissociation

High resolution electron energy loss spectroscopy (EELS), electron stimulated desorption ion angular distributions (ESDIAD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES) have been combined to study CO chemisorption on the Cr(110) surface. At 120 K, two molecular binding modes are sequentially populated. The first, α1CO, is a novel binding mode for a clean metal surface in which the molecules are bound with the C–O axis nearly parallel to the Cr(110) surface. These α1CO molecules exhibit a νCO of 1150–1330 cm−1, no detectable ion emission in ESDIAD, and form a c(4×2) α1CO overlayer at saturation (θCO≈0.25). A model is proposed in which the α1CO are π bonded to the Cr(110) surface at twofold symmetric surface hollow sites. Annealing experiments demonstrate that the α1 binding mode is the precursor to dissociation on Cr(110). At higher CO coverages, 0.25<θCO<0.35, the α1CO overlayer is disordered by the addition of the second binding mode, α2CO. Three new vibrations, at 495, 1865, and 1975 cm−1, are observed by EELS while O+ ion emission along the [110] direction is seen in ESDIAD. These α2CO results are indicative of terminally bound molecules adsorbed at atop and bridge sites and aligned approximately perpendicular to the surface. Annealing studies of CO and O/CO adlayers provide evidence for a surface reaction in addition to direct dissociation above 170 K. In contrast, adsorption at 300 K leads to direct dissociative chemisorption with no detectable intermediate molecular adsorbate. The implications of these results to catalytic reactions of CO are considered and comparisons to CO adsorption on clean and ‘‘promoted’’ transition metals are made.

Hollow cathode etching with CF4 gas mixtures

Earlier papers have shown that high etch rates and controlled profiles can be obtained with the rf hollow cathode configuration. The etch selectivity of SiO2 with respect to Si was of the order 5 to 1 with a CF4 etch gas, and this paper investigates the effect of H2 and O2 gas additions on the etch selectivity in a symmetrical hollow cathode system. CF4/O2 mixtures cause the SiO2/Si etch rate ratio to vary between 0.75 and 1.5. CF4/H2 mixtures have to date yielded a SiO2/Si etch ratio of 16. The etched angle in CF4/H2 mixtures is controlled as in CF4 hollow cathode etching, and it is shown that this control is caused by the competitive coating process.

A low-energy electron diffraction intensity analysis of Cu(001)c(2×2)-Cl

A LEED intensity analysis of the Cu(001)c(2*2)-Cl structure finds the Cl atoms in the fourfold symmetrical hollow sites with a Cl-Cu interlayer spacing of 1.60+or-0.03 AA and a slightly expanded Cu-Cu first interlayer spacing of 1.85+or-0.03 AA (bulk value 1.807 AA). The calculated Cu-Cl bond length is 2.41+or-0.02 AA, in fair agreement with the value 2.37+or-0.02 AA determined with SEXAFS by Citrin and co-workers (1978, 1982). The effect of the Cl scattering potential on the LEED intensities, and hence on the structural parameters, is quantitatively analysed with r-factor calculations. The results obtained in this work speak in favour of the same overlayer model found by an earlier LEED analysis of the closely related Ag(001)c(2*2)-Cl structure but contradicted by photoemission studies, and thus help resolve the discrepancy between LEED and photoemission about the atomic geometry of this system in favour of the LEED structure.

LEED structure analysis of A √3 × √3 -30° overlayer of iodine on Ag{111}

New preparation methods, new LEED intensity data and a new analysis on the classic much studied adsorbed system Ag{111}√3 × √3-30°-I suggest that this system can have several structural arrangements: in fact, we found a somewhat different structure than that found in 1973 by Forstmann, Berndt and Büttner. We were unable to produce a stable structure satisfactory for LEED analysis by the method used in the 1973 study. The earlier model with I in the three-fold symmetrical hollows with no Ag atom below does not fit our experiments satisfactorily, but models in which I atoms occupy both types of three-fold hollows fit better.

Atomic structure of Cu{0 0 1} c(2 × 2)-Cl

A LEED intensity analysis of Cu{0 0 1} c(2 × 2)-Cl has given an excellent fit of theory to experiment with Cl in the fourfold symmetrical hollows in a simple overlayer structure with a Cu-Cl bond length of 2.38 Å. This structure is consistent with an analysis of angle-resolved photoemission measurements made in the same experiments and published elsewhere. These results help confirm the LEED analysis of the analogous system Ag{0 0 1} c(2 × 2)-Cl which gave the same structure, but had been rejected by an independent photoemission analysis.

LEED analysis of the Co{001}c(2 × 2)O structure

A LEED intensity analysis of the c(2 × 2) structure obtained upon adsorption of oxygen gas on Co {00l} is reported. Three structural models have been tested on the basis of a total of 13 LEED spectra for three angles of incidence and one azimuth. The correct model has oxygen atoms chemisorbed in the four-fold symmetrical hollows formed by four adjacent substrate atoms. Atomic arrangement and metal-oxygen distances are analogous to those found in Ni{001}c(2 × 2)O.

LEED structure analysis of a p (2×2) chemisorbed layer of CO on Ti(0001)

Exposure of a clean Ti(0001) surface to CO gas gives rise to the formation of first a p (2×2) and then a 1×1 LEED pattern. An intensity analysis of the p (2×2) phase has been carried out using six integral-order and four fractional-order spectra. Several models involving the CO molecule with axis normal to the surface have been tested with negative results. Fair agreement between observed and calculated intensities has been obtained for models involving C and O atoms (both 1/4 monolayer) in the same plane parallel to the surface and either both in the ’’fcc’’ or both in the ’’hcp’’ threefold symmetrical hollows. The former model is better than the latter, but for neither is the agreement wholly satisfactory. Experience with the N/Ti system suggests that the ’’correct’’ model may involve formation of an underlayer. This model, however, cannot be tested with our computer programs at the present time.

Early stages of oxidation of the Fe{001} surface: Atomic structure of the first monolayer

The reaction of a clean Fe{001} surface with oxygen gas was monitored with low-energy-electron diffraction (LEED) and Auger-electron spectroscopy. The formation of an Fe{001} 1 \ifmmode\times\else\texttimes\fi{} 1-O structure at full monolayer coverage was confirmed, whereas the Fe{001} $c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}\mathrm{O}$ structure reported by three other groups could not be reproduced. A LEED structure analysis of the Fe{001} 1 \ifmmode\times\else\texttimes\fi{} 1-O structure revealed that the oxygen atoms are located deep inside the fourfold symmetrical hollows of the substrate surface, with the first substrate interlayer spacing 7.5% expanded with respect to the bulk. The effective hard-sphere radius of the oxygen atom in this structure is 0.78 \AA{}. The results of the structure analysis are consistent with the results of photoemission experiments reported by others.

Atomic arrangement in thec(2×2)structure of Cl on Ag(100)

A LEED (low-energy electron diffraction) structure analysis is described of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure obtained by reacting a clean Ag(100) surface with Cl. Two of the structural models considered in the analysis produce encouraging agreement between theory and experiment for some LEED beams: the overlayer model with Cl adsorbed on the fourfold symmetrical hollows of the Ag(100) surface in the $c(2\ifmmode\times\else\texttimes\fi{}2)$ arrangement and the mixed-layer model with non-coplanar Ag and Cl atoms. The full analysis of 21 beams at three different angles of incidence and one azimuth, however, leads to rejection of the mixed-layer model and adoption of the simple overlayer model. Refinement of the structure with regard to several nonstructural parameters reveals an interference between the choice of the potential for the adsorbed Cl and the value of the distance ${d}_{z}^{S}$ between overlayer and first substrate layer. For the proposed structure, ${d}_{z}^{S}=1.72$ \AA{}, corresponding to a hardsphere Cl radius of 1.23 \AA{}, but range of acceptable ${d}_{z}^{S}$ values extends from 1.57 to 1.78 \AA{}, depending on the Cl potential used in the intensity calculations.