Function Of Azimuthal Angle Research Articles

Fourier Transform Reflection−Absorption IR Spectroscopy Study of Formate Adsorption on TiO2(110)

The adsorption of the formate species produced by dissociative adsorption of formic acid on TiO2(110) has been investigated using a combination of X-ray photoelectron spectroscopy (XPS) and Fourier transform reflection−absorption IR spectroscopy (FT-RAIRS). Adsorption at 300 K produces a formate coverage of 0.59 ML, in agreement with previous estimates and higher than that expected for a perfect (2 × 1) overlayer observed in low-energy electron diffraction (LEED). p-Polarized FT-RAIRS measurements carried out as a function of azimuthal angle (Φ) reveal the presence of two formate species through the coupling of both the tangential (It) and normal (In) components of the radiation to the symmetric (νsym(OCO)) and antisymmetric (νasym(OCO)) vibrational modes of the molecule. Species A has the OCO plane aligned in the 〈001〉 direction with C2v symmetry; νasym(OCO) = 1566 cm-1 is observed as an absorption band (coupled to It) at Φ = 90°, and νsym(OCO) = 1363 cm-1, observed as a transmission band (coupled to In). Species B has the OCO plane aligned in the 〈110〉 direction with Cs symmetry; νasym(OCO) = 1535 cm-1, observed as an absorption band (coupled to It) at Φ = 0°, and νsym(OCO) = 1393 cm-1, observed as a transmission band (coupled to In). Changes in the region of the νsym(OCO) at 1363 cm-1 for species A as a function of azimuthal angle (this band should be insensitive to Φ) are associated with an absorption band resulting from a coupling of νsym(OCO) of species B to It. This is a result of the reduction in symmetry from C2v to Cs and, as expected from the adsorption geometry, is observed at Φ = 0° and not at Φ = 90°. We estimate that the coverages of species A and species B at 300 K are 0.4 and 0.2 ML, respectively. Species A is the adsorbate responsible for the formation of the (2 × 1) ordered structure and adsorbed between the TiO2(110) oxygen rows. Species B we suggest is an adsorbed formate molecule with one of the oxygens incorporated in the TiO2(110) oxygen rows and is the species associated with the so-called disordered phase. It is clear from FT-RAIRS that the coverage of species B is increased by adsorbing formic acid at higher surface temperatures. We suggest that species B adsorbs at defects (oxygen vacancies) created through dissociative adsorption of formic acid to produce species A and the subsequent reaction of two hydroxyl groups to produce water. The existence and interdependence of the two adsorbed formate species provide an explanation for the complex activity observed for various catalytic reactions of formic acid over titania.

Nondestructive evaluation of textile-reinforced composites based on spacial dependence of ultrasonic scattering

The lateral symmetry of the fiber architecture of standard composite laminates, having unidirectional plies stacked in parallel planes, often allows the propagation of a normally incident compressional wave to be described as a one-dimensional problem. Simple transmission measurements of the amplitude and time of flight of pulsed ultrasound become simply related to the average sound velocity and attenuation of the composite. In recent years, composites have been developed using woven, braided, knitted, and stitched textiles. Propagation of ultrasound in these materials can no longer be adequately described by a one-dimensional model. Although simple transmission ultrasonic measurements can still be made, interpretation of the ultrasonic signals becomes ambiguous. It is believed that improved material characterization can be obtained by increasing the number of degrees of freedom in the measurement through the use of multiple element transducer arrays. In order to demonstrate the feasibility of using arrays to better characterize textile based composites, measurements were made of the angular variation of ultrasonic scattering from several composites. The incident transducer was aligned normal to the surface of the specimens, while a second transducer was used to measure the scattering as a function of azimuthal angle at various nonzero polar angles.

Anomalous diffraction effect on the surface core-level photoemission from Si(001)2 × 1-Cs surface

The angular dependence of the Si 2p photoelectron intensity has been measured for a Cs-saturatedSi(001)2 × 1 surface. A surface Si 2p component, which is attributed to the Si-dimer underneath the Cs overlayer, shows a large intensity variation as a function of azimuthal angle. This denies the asymmetrical Si-dimer model proposed recently [Y.-C. Chao, L.S.O. Johansson, R.I.G. Uhrberg, Phys. Rev. B 54 (1996) 5901] on the basis of photoemission intensity of the surface Si 2p component. The present result shows that the effect of photoelectron diffraction must be taken into account when discussing the origin of a surface component from a seemingly angle-integrated photoemission intensity of the surface component.

The identification of sea swell using polarimetric radars: A new full wave approach

The statistical characteristics and the orientation of a remotely generated swell superimposed upon a local wind generated rough sea surface are determined from computer simulated polarimetric data. The full wave approach is used to determine the incoherent backscatter Mueller matrix elements and the incoherent backscatter Graves power matrix elements as functions of azimuth angle, the colatitude (angle measured from the vertical axis), and rough sea surface (with swell) parameters. The relationships between the incoherent backscatter Mueller matrix as well as the incoherent backscatter Graves power matrix and the statistical characteristics of the swell and its orientation are presented here. On making suitable selections of the backscatter angle and polarization of the Mueller matrix elements or Graves power matrix elements, it is possible to determine reliably the orientation and statistical characteristics of remotely generated swells. Traditionally only the scattering cross sections are utilized in remote sensing.

The alignment of a surface species determined by FT-RAIRS: rhodium gem-dicarbonyl on TiO 2(110)

p-polarised and s-polarised FT-RAIRS measurements have been carried out as a function of azimuthal angle θ of the incident radiation on the rhodium gem-dicarbonyl species Rh(CO) 2 adsorbed on TiO 2(110). The intensity of ν sym(CO) in the p-polarised spectra is independent of θ. The intensity of ν asym(CO), however, exhibits a dependence on θ in both the s-polarised and p-polarised spectra. When coupled to the tangential component of the p-polarised field, ν asym(CO) exhibits a absorption band with maximum intensity at θ = 90°, corresponding to alignment of the field in the 〈110〉 azimuth. The s-polarised field must be polarised in the 〈110〉 direction for ν asym(CO) to be observed as a transmission band, corresponding to an incidence angle θ = 0°. The results indicate a preferential alignment of the RhCO bonds along the 〈110〉 azimuth. There is evidence that a minority species of misaligned Rh(CO) 2 species is also present.

Optical determination of growth variants in ordered GaInP

Measurements on the dependence of the ellipsometric parameters as a function of azimuthal angle allow the determination of the dielectric tensor including its orientation with respect to the sample surface. This is done in ordered GaInP at the fixed energy of 3.3 eV where the anisotropy manifests at largest. In such material this method allows to discriminate between the presence of singly or doubly ordered variants and, in the former case, allows to establish which one of the two subvariants ([ 1 11] or [1 1 1]) is grown.

Theoretical and experimental study of natural convection heat transfer from isothermal hemisphere

The simplified analytical solution and experimental study of laminar free convection heat transfer from an isothermal hemisphere in unlimited space have been presented. The solution is based on adaptation of the methods used for inclined isothermal plates. In the proposed solution the control surface of the hemisphere was considered as a small inclined surface. Inclination of this surface was not a constant one but it was a function of azimuth angle. The result of theoretical consideration is presented in the relation of Nusselt and Rayleigh numbers: Nu = 0.533 × Ra 1 4 . The comparison of theoretical solutions with experimental results presented in this paper and results of other authors shows good agreement. Copyright © 1996 Elsevier Science Ltd.

Surface core-level shift photoelectron diffraction from [formula omitted

We have studied the arsenic-terminated Si(111)-(1 × 1) surface with low energy photoelectron diffraction. On this ideally terminated surface, the Si 2p core-level consists of a bulk component and a single well separated surface component shifted to 0.75 eV higher binding energy compared to the bulk component. We have measured the intensities of the surface and bulk components of the Si 2p core-level, and the single component As 3d core-level, as functions of azimuthal angle. The recorded azimuthal scans for the Si 2p surface component showed extremely large intensity variations. For a photon energy of 130 eV and emission angle of 45° for example, the intensity of this component was found to vary by 400 percent upon an azimuthal rotation of the sample by 45°. The experimental data were compared to model calculations of the photoelectron diffraction utilizing a fully convergent multiple scattering formalism. The best fit to the data was consistently found in all the azimuthal scans for an outward relaxation of the As atoms from the Si bulk positions by 0.2 Å. This is in good agreement with earlier structural determinations of this surface.

The [formula omitted] surface studied by angle scanned low energy photoelectron diffraction

Photoelectron diffraction experiments from the As Si(111) -(1 × 1) surface are reported in which the surface and bulk components of the Si 2p core-level were measured as functions of azimuthal angle. The Si 2p surface component shows large intensity variations of up to a factor of 4. Comparisons to fully convergent multiple scattering calculations show a high sensitivity to surface atomic geometry and indicate that the As overlayer is relaxed toward the vacuum by 0.2 Å compared to the bulk Si lattice spacing. The measurements also show sensitivity to the vertical position of the underlying Si layer with a best fit found for an unrelaxed Si layer. A single scattering treatment predicts rather well the large diffraction effects observed for the Si 2p surface peak and indicates that they are due primarily to large angle scattering from the three nearest neighbor As atoms. The strong intensity variations observed show that diffraction effects can introduce large errors into surface or adsorbate coverage determinations based on core-level intensities and the single parameter electron escape depth model if these effects are not specifically taken into consideration.

Quantitative Measurements of Polymer Chain-End Edge Dislocation Strain Fields by High Resolution Electron Microscopy

The strain fields around chain-end edge dislocations in poly(diacetylene) crystals were analyzed by high resolution electron microscopy (HREM). Experimental measurements of the tilt of the polymer chain axis as a function of azimuthal angle ? at a constant radius r from the dislocation core were compared to theoretical predictions. The shear deformation was localized in parabolic regions parallel to the Burger's vector b near the chain end. For an edge dislocation in the poly(diacetylene) 1,6-di(N-carbazolyl)-2,4-hexadiyne (DCHD) with a Burger's vector of b = 3a/2 [100] (2.4 nm), we found a tilt distortion of ±6° at 12 nm from the core. A parameter W was introduced to describe the anisotropy of the compliance matrix with respect to the chain direction. A parameter of W = 3.5 was needed to fit the measured tilt deformation with anisotropic linear elastic dislocation theory. We also found that the theory of distortions near dislocations in columnar liquid crystals could closely predict our experimental observations. A value of 0.8 nm (the interchain spacing) for the characteristic length, λ 3 , was the best fit for the columnar liquid crystal solution. This analysis reveals similarities between the elasticity of anisotropic crystals and liquid crystals.

Non-linear optical spectroscopy of interfaces: adsorption sites of a self-assembled [formula omitted

We have used infrared-visible sum-frequency generation (SFG) to probe spectroscopically the interfacial structure of a self-assembled monolayer of alkyl thiol on a Au(111) surface. The SFG spectra of the CH 3 group, measured as a function of azimuthal angle, is not consistent with sulfur atoms being situated at sites of a single type, e.g. hollow or bridge, but is consistent with a mixed arrangement.

STRUCTURAL ANALYSIS OF THE INITIAL INTERACTION BETWEEN OXYGEN AND A CLEAN Si(100) BY TIME OF FLIGHT AND RECOILING SPECTROSCOPY

The structure of both Si (100)in situ thermally annealed Si (100) at 1373 K and Si (100) exposed to dry oxygen at room temperature in the range 5 to 1700 Langmuirs, have been studied using time of flight and recoiling spectroscopy (TOF-SARS). Structural analyses were performed by recording the intensities of recoiled oxygen and silicon ions as a function of azimuthal angle. The results show that (1) the clean annealed Si (001) is dimerized, (2) oxygen chemisorbs at the surface at the Si-unsaturated bond, which are left unoccupied after the (2×1) reconstruction, and (3) at 100 L and above the Si surface is reconstructed into a (1×1) structure with the oxygen binding to the broken Si-Si dimer bonds.

Interfacial atomic structure of a self-assembled alkyl thiol monolayer/Au(111): A sum-frequency generation study.

Infrared-visible sum-frequency generation (SFG) is used to spectroscopically probe the interfacial structure of a self-assembled monolayer of alkyl thiol on a Au(111) surface. The SFG spectra of the ${\mathrm{CH}}_{3}$ group measured as a function of azimuthal angle indicates that sulfur atoms cannot be situated at sites of a single type, e.g., hollow or bridge, but must be in a mixed arrangement. SFG is also used to study the reconstruction of the gold substrate by alkyl thiol.

Design of an acoustical bench

An acoustical bench was designed to facilitate ultrasonic target strength experiments requiring monostatic geometry for frequencies up to 100 kHz. The design of an acoustical bench is analogous to that of an optical bench. Its purpose is to facilitate accurate positioning in setting-up and running acoustical experiments. As an example of its use, the acoustical bench will enable the researcher to move precisely the source and receiver and obtain backscattered target strength as a function of azimuthal angle. The acoustical bench is designed to accommodate a variety of target sizes and shapes. The distance of the source and receiver from the target can be easily adjusted to provide both geometric near and far field conditions for most targets. To reduce errors due to diffraction effects, the source and receiver are mounted flush on a planar baffle.

Polarimetric thermal emission from periodic water surfaces

In this paper, experimental results and theoretical calculations are presented to study the polarimetric emission from water surfaces with directional features. It is observed that the measured Stokes parameters of corrugated fiberglass‐covered water surfaces are functions of azimuth angles and agree very well with theoretical calculations. The theory, after being verified by the experimental data, was then used to calculate the Stokes parameters of periodic surfaces without fiberglass surface layer and with rms height of the order of wind‐generated water ripples. The magnitudes of the azimuthal variation of the calculated emissivities at horizontal and vertical polarizations corresponding to the first two Stokes parameters are found to be comparable to the values measured by airborne radiometer and SSM/I. In addition, the third Stokes parameter not shown in the literature is seen to have approximately twice the magnitude of the azimuth variation of either Th or Tυ. The results of this paper indicate that passive polarimetry is a potential tool in the remote sensing of ocean wind vector.

Chemisorption of Self Assembled Alkyl Thiol on Au(111)

ABSTRACTWe have used infrared-visible sum-frequency generation (SFG) to spectroscopically probe the interfacial structure of a self assembled monolayer (SAM) of alkyl thiol on a Au(111) surface. The SFG spectra of the CH3 group measured as a function of azimuthal angle indicates that sulfur atoms cannot be situated at sites of a single type, e.g., hollow or bridge, but must be in a mixed arrangement. SFG is also used to study the reconstruction of the gold substrate by alkyl thiol.

Rotation-like behavior as a function of projectile energy and target mass

The azimuthal distributions produced in the intermediate energy40Ar-induced reactions are calculated via the nuclear transport theory. The rotational collective motion is observed from the mid-rapidity particles emission. In addition to the investigation on particle emission in the coordinate space, the momentum distributions of emitted particles as a function of azimuthal angle are also discussed. The azimuthal distributions are fitted by performing Legendre polynomial expansion to second order. By incorporating the fluctuation between the estimated and true reaction plane into our calculations, quantitative agreements with the data are obtained for40Ar+27Al reactions below 85 MeV/nucleon. It is found that the rotation-like behavior becomes stronger with the increasing of the impact parameter and weaker with the increasing of the projectile energy. For40Ar+27Al system at about 85 MeV/nucleon the rotation-like behavior almost vanishes and the mid-rapidity azimuthal distribution tends to be isotropic. For 35 MeV/nucleon40Ar-induced reactions the rotation-like behavior becomes weaker and the out-of-plane enhancement of particle emission appears with the increasing of target mass. The possible origins of these target mass dependent azimuthal distributions at mid-rapidity are discussed.

Vegetation-Limited Ground-to-Air Surveillance

It is possible to estimate the limitations on ground-to-air visibility around a point using digital terrain models in which terrain height is given on a rectangular grid and the vegetation class within defined areas is specified. The screening effect of the vegetation can be estimated better by using the inherent autocorrelation in elevation angle data given as a function of azimuth angle about a fixed point. This type of model is useful when the proposed number of observation sites is too large to warrant surveying them, or the precise actual locations cannot be ascertained in advance of the requirement, as is often the case in simulation studies involving mobile observers. The distribution of elevation angle may be non-normal. A procedure is described here to facilitate the use of autoregressive equations which require normally distributed variables.

Vegetation-Limited Ground-to-Air Surveillance

It is possible to estimate the limitations on ground-to-air visibility around a point using digital terrain models in which terrain height is given on a rectangular grid and the vegetation class within defined areas is specified. The screening effect of the vegetation can be estimated better by using the inherent autocorrelation in elevation angle data given as a function of azimuth angle about a fixed point. This type of model is useful when the proposed number of observation sites is too large to warrant surveying them, or the precise actual locations cannot be ascertained in advance of the requirement, as is often the case in simulation studies involving mobile observers. The distribution of elevation angle may be non-normal. A procedure is described here to facilitate the use of autoregressive equations which require normally distributed variables.

An experimental study of microwave scattering from rain- and wind-roughened seas

This paper investigates radar cross-section (RCS) characteristics of rain- and wind-roughened sea-surfaces. We conducted experiments in laboratory wind-wave tanks using artificial rain. The study includes light rain rates, light wind speeds, and combinations of these. A 36 Ghz scatterometer was operated at 30 deg incidence angle and with vertical polarization. RCS data were obtained not only with the scatterometer pointing up-wind but also as a function of azimuthal angle. We use a scatterometer rain and wind model SRWM-1, which relates the total average RCS in storms to the sum of the average RCS due to rain plus the average RCS due to wind. Implications of the study for operational monitoring of wind in rainy oceanic areas by satellite-borne instruments is discussed.