R-factor Analysis Research Articles

Structural Analysis of Chemisorbed Benzene and Pyrazine on Silicon Surfaces by using Photoelectron Diffraction

We have studied the chemisorption of benzene and pyrazine on Si(001)-2×1 at room temperature using photoelectron diffraction (PED). For benzene, comparison between the experimental and simulated PED patterns with R-factor analysis leads to the conclusion that a di-σ-bonded configuration is mainly formed at the saturation coverage. For pyrazine, the PED results show that the double dimer bridging configuration is formed, in which pyrazine molecule forms a bridging between the dimer rows with the two Si-N bonds.

Adsorption model determination of N 2O/Ag(1 1 0) by theoretical studies of near-edge X-ray absorption fine structure

The nitrogen 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of the N 2O adsorbed on Ag(1 1 0) have been studied by the multiple-scattering cluster (MSC) and self-consistent field (SCF) DV-Xα methods. Two adsorption models, in which the N 2O molecule attached to the Ag substrate through the central nitrogen (N C) atom and the terminal nitrogen (N T) atom, respectively, have been checked up thoroughly. The MSC calculation and the R-factor analysis show that the N 2O molecule is attached to the Ag substrate through the terminal nitrogen atom with the adsorption height h = 3.4 ± 0.1 Å. In the overlayer the N 2O molecules arrange themselves into a tilted chain due to the interaction between the cations and the anions in the molecules. The physical cause of the resonances in the NEXAFS spectra mentioned above has been discussed by the DV-Xα method, which confirms the MSC calculations.

Directional Auger electron spectroscopy and single-scattering cluster calculations study of the Ni(111)-Pb system

The crystalline structure of ultrathin Pb layers deposited on the Ni111 surface in ultrahigh vacuum was investigated with the use of directional Auger peak electron spectroscopy DAES. Experimental DAES data recorded for the primary electron beam energies in the range 1.2– 2.2 keV were compared with theoretical profiles obtained with the use of single-scattering cluster calculations for clean and covered Ni111. Similar intensity maxima reflecting the crystalline structure of the Ni111 sample were observed on experimental and theoretical profiles. Results obtained after deposition of 4 ML of Pb on Ni111 at the substrate temperature T = 150 K exhibit intensity maxima corresponding to two mutually rotated Pb111 domains. The populations of two possible Pb domains were determined by an R-factor analysis. The growth of one domain was found to be preferred, which can be rationalized by the miscut of the Ni111 sample and the resulting step orientation.

Investigations of Pb/Ni(1 1 1) using incident beam electron diffraction

The crystalline structure of ultrathin Pb layers deposited on the Ni(1 1 1) surface in ultrahigh vacuum was investigated with the use of directional elastic peak electron spectroscopy (DEPES). Experimental DEPES data recorded for the primary electron beam energies in the range 1.0–1.9 keV were compared with theoretical profiles obtained with the use of single scattering cluster (SSC) calculations for clean and covered Ni(1 1 1). Similar intensity maxima reflecting the crystalline structure of the Ni(1 1 1) sample were observed on experimental and theoretical profiles. Results obtained after deposition of 10 ML of Pb on Ni(1 1 1) at the substrate temperature T = 150 K exhibit intensity maxima corresponding to two mutually rotated Pb(1 1 1) domains. The populations of two possible Pb domains were determined by an R-factor analysis. The growth of one domain was found to be preferred, which can be rationalised by the miscut of the Ni(1 1 1) sample and the resulting step orientation.

Structural analysis of the SiO2/Si100 interface by means of photoelectron diffraction.

The local environment of Si atoms at the interface between a thermally grown SiO2 film and Si(100) was studied by angle-scanned photoelectron diffraction. Experimental photoelectron diffraction patterns for each Si oxidation state were obtained from the results of least squares fitting on Si 2p core-level spectra. A comparison of the diffraction patterns with multiple-scattering calculations including an R-factor analysis was performed. An excellent agreement between experimental and simulated data was achieved within the proposed bridge-bonded interface model [Phys. Rev. Lett. 84, 4393 (2000)]].

Determination of Ag domain populations onCu(111)using directional Auger and directional elastic peak electron spectroscopies

The Auger signal and the intensity of elastically backscattered electrons were measured as a function of the incidence angle of the primary electron beam to find the crystalline structure of ultrathin Ag layers on the $\mathrm{Cu}(111)$ surface. Experimental profiles were compared with theoretical data obtained with the use of single scattering cluster (SSC) calculations for clean and covered $\mathrm{Cu}(111)$. Auger scans for Ag MNN transition and elastic peak profiles exhibit intensity maxima corresponding to two mutually rotated Ag(111) domains. Different domain populations were found by an R-factor analysis, which can be rationalized by the miscut of the $\mathrm{Cu}(111)$ sample and the resulting step orientation.

Validation of the Scale of Allergic Rhinitis

Allergic symptoms have been linked to behavioural and affective disorders. The Scale of Allergic Rhinitis (SAR) was constructed to address the need for a validated homogeneous self-report instrument to assess allergic rhinitis (AR). Reliability and validity were found by testing 199 university students. Cronbach's alpha was 0.95, p < 0.001, indicating excellent reliability. An R-Factor Analysis rotated to a Varimax criterion indicated four factors: ‘Eyes’, ‘Nose’, ‘Throat’, and ‘Ears’, with ‘Eyes’ contributing the majority of the variance. These results matched the hypothesized construct of AR. In addition, a statistically significant difference was found in the predicted direction with the clinical AR group scoring significantly higher on the SAR than the control group. In total, these results indicate good evidence of construct validity. Thus, the SAR may be useful in medicine and psychology, particularly for researching the relationship between AR symptoms and other medical or psychological conditions.

Evaluation of on-line high-performance size-exclusion chromatography, differential refractometry, and multi-angle laser light scattering analysis for the monitoring of the oligomeric state of human immunodeficiency virus vaccine protein antigen

Chiron has developed a novel mutant form of the human immunodeficiency virus (HIV) envelop protein, o-gp140, that is currently entering Human Phase 1 clinical trials for testing as a prophylactic HIV vaccine. The o-gp140 protein is oligomeric and the quaternary structure is thought to play an important role in its activity as an antigen. As o-gp140 proceeds through the clinical trial process and prior to marketing approval, analytical methods that are able to demonstrate manufacturing consistency with respect to degree of oligomerization will need to be developed and validated. On-line high-performance size-exclusion chromatography, differential refractometry, and multi-angle laser light scattering analysis (HPSEC–RI–MALLS), a method commonly used to obtain the molar mass of macromolecules based on the Rayleigh–Gans–Debye approximation, was evaluated for this purpose. The results obtained demonstrated intra- and inter-day precisions to be 0.9 and 3.6% R.S.D., respectively. Accuracy was found to be equal to, or better than, 11% when comparing the known molar masses of test proteins to that of the molar masses determined by the method. Additionally, the method compared favorably to orthogonal native polyacrylamide gel electrophoresis and ultracentrifugation analyses. R-factor analysis was used to demonstrate that HPSEC–RI–MALLS is capable of discriminating compositional differences between o-gp140 test lots. Based on the data presented, HPSEC–RI–MALLS may be a suitable manufacturing control method.

Using multiple methods to understand agency values and objectives: Lessons for public lands management

The Government Performance and Results Act of 1993 (GPRA) directs agencies to determine their objectives based on what stakeholders value. Identifying, measuring and analyzing values and objectives is a key challenge for public land management agencies. Using R and Q factor analysis we assess the values and objectives within the Leadership Team (LT) of the United States Forest Service (USFS). R-factor analysis demonstrates strong support among the LT for a more preservation-oriented view to manage public lands. Q-factor analysis reveals divergent perspectives among the LT and how the aggregated preservation orientation found with the R-factor analysis plays out according to individual perspectives. The findings illustrate the challenges inherent in identifying what values and objectives exist, how they should be analyzed, and the implications for choosing one set of values and objectives over another in public lands management to fulfill the mandates under GPRA.

Structural Study of Benzene Adsorbed on Si(001) Surface by Photoelectron Diffraction

The structure of benzene adsorbed on a Si(001)(2 × 1) surface up to saturation at room temperature (RT) has been studied by C 1s photoelectron diffraction (PED). Experimental C 1s PED patterns were measured for two regimes of photoelectron kinetic energies at ~ 100 eV and ~ 1000 eV. Simulation of PED patterns was made for three possible structural models predicted so far. Comparison of actual PED patterns and R-factor analysis leads to the conclusion that the so-called standard butterfly (SB) configuration of benzene is present and stable at the saturation coverage at RT. The present PED patterns were not very sensitive to geometrical parameters of absorbed benzene except for the bond length between the topmost carbons in the SB configuration.

Sb/Si(1 1 0) 2×3—a photoelectron diffraction study

The Sb/Si(110) 2×3 surface has been investigated by means of X-ray photoelectron diffraction (XPD). Low kinetic energies of the photoelectrons have been chosen to increase the surface sensitivity of the method. Simulation calculations have been carried out in order to test previously proposed structure models. The simulated diffraction patterns were compared with the experimental results by means of a R-factor analysis. The comparison indicates that at least part of the Sb atoms adsorbed on the surface form Sb trimers.

Orientation of chiral heptahelicene C30H18 on copper surfaces: An x-ray photoelectron diffraction study

The orientation and the intramolecular relaxation due to adsorption of the chiral phenanthrene-derivative heptahelicene, C30H18, on Cu(111) and Cu(332) surfaces have been investigated by means of angle-scanned full-hemispherical x-ray photoelectron diffraction. Although the C 1s diffraction patterns of the adsorbed submonolayer coverage helicene films exhibit scattering anisotropies of less than two percent, a detailed analysis involving simple molecular mechanics calculations of the atomic coordinates, photoelectron diffraction single-scattering cluster calculations and an R-factor analysis permits the determination of the helicene molecular orientation. On Cu(111), the molecules are found to bind to the substrate surface via their terminal phenanthrene group oriented parallel to the surface plane, while on Cu(332) the three terminal C-6 rings are oriented parallel to the (111) terrace plane. Six azimuthal molecular orientations are found to coexist on Cu(111), on Cu(332), however, the step–molecule interaction leads to a unique azimuthal alignment of the heptahelicene molecules. The heptahelicene on Cu(332) system thus represents a chiral surface with single-phase orientational order.

An X-ray photoelectron diffraction structural characterization of an epitaxial MnO ultrathin film on Pt(1 1 1)

An epitaxial ultrathin MnO/Pt(1 1 1) film (13 Å thick) has been grown by reactive UHV deposition of Mn 2(CO) 10 on Pt(1 1 1) at 200°C in the presence of water. Angle scanned X-ray photoelectron diffraction (XPD) and LEED have been used to structurally characterize the film. The observed 1×1 LEED pattern is in accord with the bulk lattice parameter of a MnO(1 1 1) surface (3.14 Å) and demonstrates that the film is ordered in the long range. Full hemispherical O1s and Mn2p XPD plots have been obtained and analysed on the basis of multiple scattering calculations. The XPD data confirm that the MnO(1 1 1) surface (as a single domain) is exposed and a best fit procedure based on a R-factor analysis provides a direct evidence for a relaxation of the outermost double layer, whose values are similar to those found in other similar systems (CoO and FeO).

Local atomic environment of Si suboxides at the SiO2/Si(111) interface determined by angle-scanned photoelectron diffraction.

Local environments of Si suboxides at the interface between a thermally grown SiO2 film and Si(111) were studied by angle-scanned photoelectron diffraction. Si 2p core-level spectra containing chemically shifted components were recorded. The components were deconvoluted by least squares fitting and assigned to different Si oxidation states. The obtained diffraction patterns of the various suboxides exhibit different features. Comparison of these patterns with multiple scattering calculations including a multipole R-factor analysis shows that a simple chemical abrupt interface model describes well the environment of the suboxides and indicates ordered SiO2 close to the interface.

Structure and properties of stacking faulted A15 tungsten thin films

A combination of energy-filtered electron diffraction, x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, field-emission scanning electron microscopy, and x- ray diffraction are used to establish that oxygen impurities incorporated in the tungsten films prepared by magnetron sputtering play a dominant role in the formation of the stacking faulted A15 W structure. Energy-filtered electron diffraction data collected from A15 films were Fourier transformed to a reduced density function (RDF), which is compared to theoretical calculations based on several possible structural models. By using a reliability R-factor analysis the A15 W structure has been determined to be a mixed phase consisting of ordered and stacking faulted W3W structures. The effect of oxygen in stabilizing the stacking faulted A15 structure was also elucidated by in situ anneal and discussed on the basis of structural and thermodynamic stability.

Kinetics and atomic structure of O adsorption on W(110) from time- and state-resolved photoelectron spectroscopy and full-solid-angle photoelectron diffraction

We have studied the kinetics of the low-pressure adsorption of oxygen on W(110) via time- and chemical-state-resolved photoelectron spectroscopy (PS) and diffraction (PD). Using high-brightness third-generation synchrotron radiation from the Advanced Light Source, together with a new photoelectron spectrometer/diffractometer system, we are able to resolve four distinct chemical states in W 4f spectra (clean surface, bulk, W bound to two O atoms=O2, and W bound to three O atoms=O3) and to measure such spectra in about 20 s each so as to follow the kinetics of oxygen adsorption at 3×10 −9 Torr from the clean surface to near saturation. The time-dependent transformations from one state of the surface W atoms to another have been determined at three temperatures of 298, 360, and 593 K. We also find that, for this adsorption pressure on our surface, no long-range-ordered structures are observable in LEED, even though the previously observed ordered structures of p(2×1), p(2×2), and (1×1)×12 are formed at higher pressures of approximately 10 −6 Torr. The room-temperature state-resolved PS data are modelled using a simple Monte Carlo approach which assumes no mobility after molecular dissociation, and these calculations are found to describe the experimental data very well. Combining experiment and theory also permits deriving the sticking coefficient as a function of time, yielding results which agree with prior work. Full-solid-angle PD patterns have also been determined at the end of 298 K oxygen exposure for the O2 and O3 W atoms, and these have been analyzed using multiple scattering theory and R-factor analysis. The final local pseudo-threefold hollow geometries for oxygen are found to be very similar to those for a saturated one-monolayer structure of O on W(110) (the (1×1)×12 structure), including a lateral shift of O away from the position corresponding to three equal bond distances, but with some contraction of the OW vertical separation in going from O2 to O3 suggested. This study indicates considerable potential of such time- and state-resolved PS and diffraction for investigating surface reaction kinetics and structure, particularly for the large number of systems that do not exhibit long-range order and in view of future instrumentation improvements that should lead to much shorter data accumulation times and/or higher ambient pressures of measurement.

Structure determination for saturated (1×1) oxygen on W(110) from full solid angle photoelectron diffraction with chemical-state resolution

The local atomic geometry of oxygen adsorbed in a stable one monolayer (1×1) structure on W(110) has been determined for the first time from full solid angle site-resolved photoelectron diffraction (PD). This structure also exhibits an additional (1×12) periodicity, but PD allows determination of the local structure within this periodicity. The data were obtained with a new beamline and experimental system at the Advanced Light Source. W 4f7/2 spectra excited at kinetic energies of approximately 140eV were resolved into bulk and ‘oxide’ components, with the latter corresponding to the first layer of tungsten atoms bound directly to three oxygen atoms each. Analyzing these data with multiple scattering theory and R-factor analysis shows that the oxygen atoms sit in pseudo-threefold hollow sites at a vertical distance of 0.91±0.05Å above the first tungsten layer, and with a lateral position shifted by 0.1Å along [11̄0] or [1̄10] directions and away from that yielding three equal bond distances. Future applications of this method to systems with large unit cells which make beam-in/beam-out diffraction analyses difficult and/or with a lack of long range order are thus suggested.

X-ray photoelectron diffraction and Auger electron diffraction fromTiO2(100)

Full-hemispherical x-ray photoelectron $(\mathrm{Ti}2p/\mathrm{O}1s)$ and x-ray stimulated Auger electron $(\mathrm{Ti}{L}_{3}{M}_{23}{M}_{23}/\mathrm{O}\mathrm{}\mathrm{KVV})$ intensity distributions have been measured from ${\mathrm{TiO}}_{2}(100)1\ifmmode\times\else\texttimes\fi{}1$ at relatively high-angular resolution $(\ifmmode\pm\else\textpm\fi{}1.8\ifmmode^\circ\else\textdegree\fi{}).$ The results are compared with theoretical calculations using a multipole R-factor analysis. Multiple scattering up to fifth order and a slab thickness of $\ensuremath{\sim}16$ \AA{} are needed to obtain optimum agreement with experimental photoelectron distributions. We also investigate the contribution of the final state wave function in the Auger-electron diffraction patterns and show that it is possible to determine the symmetry of the final state angular momenta for oxides such as ${\mathrm{TiO}}_{2}.$ Both the x-ray photoelectron diffraction and the x-ray stimulated Auger intensity distributions are found to be insensitive to details of the surface structure.

XPD and STM investigation of hexagonal boron nitride on Ni(111)

Monolayers of hexagonal boron nitride have been grown by the reaction of benzene-like borazine (BN) 3H 6 with Ni(111) at 1050 K. The resulting layers are analyzed by means of Si Kα excited N 1s and B 1s X-ray photoelectron diffraction (XPD) and scanning tunneling microscopy (STM). STM shows large terraces without defects and resolves two different atomic species that are commensurate with Ni(111). From XPD it is found that the system discriminates fcc from hcp adsorption sites, that the h-BN layer is corrugated and that nitrogen terminates the surface. The B–N corrugation is determined quantitatively by R-factor analysis between single scattering cluster calculations and experiment. The results are discussed in connection with an existing LEED analysis on the same system [Y. Gamou, M. Terai, A. Nagashima, C. Oshima, Sci. Rep. RITU A 44 (1997) 211].

X-ray photoelectron diffraction study of thin Cu films grown on clean Ru(0001) and O-precovered Ru(0001)

We have studied the epitaxial growth modes and near-surface interlayer relaxation of thin Cu films on Ru(0001) using X-ray photoelectron diffraction (XPD), measuring experimental Cu 2p 3/2 ( E kin=556 eV) and Ru 3d ( E kin=1206 eV) intensities over one-third of the nearly full 2 π solid angle above the surface for Cu coverages from submonolayer up to 40 monolayers. Reference Cu 2p 3/2 data for a clean Cu(111) surface have also been obtained from Naumović et al. and in our laboratory. These data have been compared to single scattering cluster (SSC) and more accurate multiple scattering cluster (MSC) calculations via a sum of five R-factors to derive precise structural information. MSC calculations are found to give a more accurate description for layers of ≥4 ML thickness, and comparisons of experiment and theory are also improved by allowing more accurately for the effective degree of angular averaging involved. Calculations for thicker layers are also found to converge by ∼5 ML. Our analysis indicates that the first Cu layer grows pseudomorphically on Ru(0001), in agreement with prior studies. An R-factor analysis comparing MSC and SSC calculations to experimental results further indicates that the Cu–Ru interlayer spacing at 1 monolayer (ML) is about 2.15 Å, in excellent agreement with prior low-energy ion scattering (LEIS) and low-energy electron diffraction (LEED) experimental studies, as well as with prior linearized augmented plane-wave (LAPW) calculations. At higher coverages, comparison of our data to SSC and MSC calculations for various atomic clusters indicates that the short-range structure is fcc Cu(111)-like, but with significant interlayer contraction which persists up to ≥5 ML coverage. Prior STM work by Behm et al. has shown a series of misfit dislocation structures in the top layer of the Cu film at higher coverages from 2 to 4 ML. Our data indicate that these misfit dislocation structures thread to the Cu/Ru interface rather than occurring only in the top Cu layer or layers. An R-factor comparison of the more accurate MSC calculations to experiment also indicates that the ratio of the Cu–Cu interlayer distance ( d ⊥) to the Cu–Cu in-plane nearest-neighbor distance ( d ‖), d ⊥/ d ‖=0.729±0.034 at 2 ML, and reaches 0.777±0.020 by 25 ML. For reference, the bulk value is d ⊥/ d ‖=0.816, and the analysis of experimental data for Cu(111) yields 0.801±0.035, in good agreement with this value and prior LEED studies. This analysis shows that there is significant interlayer contraction for very thin Cu layers, and that it persists (at least in the top few layers, to which XPD is the most sensitive) for longer than would be expected on the basis of a prior theoretical analysis using the 2D Frenkel–Kontorova model by Hamilton and Foiles, as used to estimate d ⊥/ d ‖ via either a constant atomic-volume assumption or elasticity theory. In addition, the Cu overlayer grows in two possible orientations rotated by 180° on the Ru(0001) surface, with a preference towards one of the two possible orientations at certain coverages. Finally, we have investigated the effect of oxygen preadsorbed on the Ru(0001) surface on the growth of the Cu overlayer. For this case, we find that all of the oxygen floats on top of the Cu in a highly disordered configuration, and that the oxygen promotes multilayer or island growth relative to growth on the clean Ru surface up to at least 3 ML coverage, rather than acting as a surfactant promoting smoother growth.