Normal-incidence Data Research Articles

Spin‐Resolved PES and IPES Investigation of the Graphene/Ni(111) Interface

The chemically inert graphene/Ni(111) interface represents an intriguing system where a magnetic sample is protected by a single layer of carbon atoms, which passivates the surface. Despite the fact that it has been the subject matter of many researches, inverse photoemission (IPES) data are still missing as well as a comparison with spin‐resolved photoemission (SR‐PES) spectra. Here, for the sake of completeness, we present both in situ SR‐PES and SR‐IPES spectra. We observe that normal emission and normal incidence data are dominated by the Ni features while characteristic signals from carbon arise at higher angles. The spin polarization of Ni bands, investigated here along the ‐ direction of the surface Brillouin zone, is only slightly affected by the graphene overlayer in both filled and empty states.

Bottom-up Fabrication and Optical Characterization of Dense Films of Meta-Atoms Made of Core–Shell Plasmonic Nanoparticles

In an attempt to fabricate low index metamaterials by a bottom-up approach, meta-atoms constituted of silica-coated silver nanoparticles are assembled by a Langmuir-Schaefer technique into thin films of large area and well-controlled thickness. The silica shells ensure a constant distance between the silver cores, hence providing a constant coupling of the localized surface plasmon resonance (LSPR) of the nanoparticles in the assembled composite material. The optical response is studied by normal angle spectral reflectance measurements and by variable angle spectroscopic ellipsometry. The normal incidence data are described well in the framework of a single effective Lorentz oscillator model. The resonance of the assembled material is blue-shifted and shows no significant broadening with respect to the absorption band of the individual nanoparticles. The observation of these two effects is enabled by the core-shell structure of the meta-atoms that prevents aggregation of the metallic cores. The ellipsometry study confirms the general behavior and reveals the natural birefringence of the few-layer materials. The amplitude of the observed resonance is weaker than expected from the Maxwell-Garnett mixing rule. This well-characterized system may constitute a good model for numerical simulations.

Flemish Cap-Goban Spur conjugate margins: New evidence of asymmetry

We present the combined results of deep multichannel reflection and refraction seismic surveys across the Flemish Cap–Goban Spur conjugate margin pair (North Atlantic), which we use to infer rifting style and breakup. Profiles on both margins cross magnetic anomaly 34 and extend into oceanic crust, making it possible to observe the complete history from continental rifting through to the formation of initial oceanic crust. The deep multichannel seismic (MCS) reflection data have previously been used to support a model of symmetric pure shear extension followed by asymmetric breakup and a sharp continent-ocean boundary. Using both types of seismic data, our results indicate instead that asymmetric structures are formed during all stages of rifting, breakup, and complex transition to oceanic spreading. The differing nature of the two ocean-continent transition zones is particularly striking. For Flemish Cap, our reprocessed image of the MCS profile clearly shows tilted fault blocks beneath back-tilted sediment packages, consistent with a wide region of highly thinned continental crust inferred from wide-angle seismic data. In contrast, normal incidence and wide-angle seismic data for the Goban Spur transition zone indicate the presence of exhumed serpentinized mantle.

Surface electron band structure and very low-energy electron diffraction reflectivity for Al(111)

The two-dimensional 2D layer Green's function scattering method is used to calculate the energy of surface states and resonances at $\overline{\ensuremath{\Gamma}}$ for Al(111) for both below and above the vacuum level. The surface-barrier potential is represented by an empirical form. The above vacuum-level surface electron band structure for this surface has not been calculated before and it is important in understanding many surface phenomena. The geometric structure of the Al(111) surface is known from intensity analysis in low-energy electron diffraction at energies 60--450 eV. The details of the surface barrier for Al(111) were obtained from a match with the below vacuum-level experimental energy position of the first Rydberg surface resonance and the Shockley surface state at ${\mathbit{k}}_{\ensuremath{\parallel}}=0(\overline{\ensuremath{\Gamma}})$. The calculation was then extended to the above vacuum-level case for 0--27 eV with the inclusion of inelastic electron interactions. Tamm-type resonances at 6.9 eV and possibly also at 8.3 eV, a Shockley-type resonance at $14.0\ifmmode\pm\else\textpm\fi{}0.5\text{ }\text{eV}$ and a series of Rydberg (image) resonances near 24 eV all above vacuum level are found at ${\mathbit{k}}_{\ensuremath{\parallel}}=0$. The same 2D layer Green's function scattering method using the same input data was then used to calculate the intensity of the 00 beam for ${\mathbit{k}}_{\ensuremath{\parallel}}=0$ (normal incidence) in very low-energy electron diffraction (VLEED) from this surface in the energy range 0--65 eV. Features in the VLEED intensities are found due to the Shockley and Rydberg resonances. Experimental data from over 26 years ago found surface features near the energies found in this work. Beam intensities from low-energy electron microscope measurements at normal incidence and new data from other surface spectroscopies could provide experimental confirmation of the resonances predicted in this work.

The effect of intra-trappean heterogeneities on seismic data: A case study from the Deccan Traps

Hydrocarbon exploration interests have renewed the need for developing new sub basalt imaging techniques. One of the most important problems encountered today is seismic imaging below basalt. In recent years, this problem appears to have been overcome partly by using long offset seismic data. However near offset data are yet to be fully utilised due to the complex waveform caused by the surface as well as internal heterogeneity of the basalts. The near normal incidence data, which influence the sub-basalt imaging, are highly useful to understand the internal structure within a basalt layer. The use of converted waves for such targets has been proposed as an alternative in a rather homogeneous basalt layer. With a few synthetic modelling exercises here we highlight the practical difficulties in dealing with more realistic and heterogeneous basalt flow. Full waveform seismograms are computed to understand the effects of intra-trappean sediments on the seismic data. A case study from the Deccan Traps of India is presented in this paper. First, we discuss the effects of intercalated sediments on the overall seismic image. Later, the sonic log data from the field are used to compute the full wave-field response using the reflectivity method and compared with the field data. The feasibility of using mode converted waves (P to S and vice-versa at the top and bottom basalt interfaces) for sub-basalt imaging in Kutch region is discussed through a series of velocity-depth profiles. By comparing with the field data we demonstrate that the effects of multiple thin layering within the basalt can strongly deteriorate the image we seek to interpret and exploit.

Seismic stratigraphy of the southern Rockall Basin: a comparison between wide-angle seismic and normal incidence reflection data

A wide-angle reflection/refraction profile from the RAPIDS 3 (Rockall and Porcupine Irish Deep Seismic) experiment is compared to a coincident high-quality normal incidence reflection line, both of which traverse the southern part of the Rockall Basin on the Atlantic continental margin, offshore Ireland. The positions and geometries of the independently-derived layers show a generally close comparison between the two data sets. The normal incidence seismic reflection data show a greater degree of detail on seismic facies while the wide-angle seismic profile displays greater penetration of the entire sedimentary succession and of the crust. The wide-angle geometries and the normal incidence seismic stratigraphy provide complementary data that help constrain the geology and evolution of the lightly explored Rockall region. A seismic stratigraphy is presented, based on correlation between the wide-angle and normal incidence data, shallow seismic site surveys, shallow borehole cores and tectonostratigraphic studies. Within the central part of the Rockall Basin, the sedimentary layers are generally flat-lying and have a maximum thickness of 7 km. The succession is interpreted as Jurassic to Recent in age. Some of the layers are confined to the basin whilst others extend onto the Rockall and Porcupine highs. Marginal basins at the western (Conall Basin) and eastern (Macdara Basin) edges of the Rockall Basin contain up to 4 km of pre-Tertiary sedimentary rocks, some possibly as old as Permo-Triassic.

Seismic and gravity modelling of crustal structure in the Central Graben, North Sea. Observations along MONA LISA profile 3

High quality wide-angle and deep normal-incidence seismic data sets recorded during the MONA LISA (Marine and Onshore North Sea Acquisition for Lithospheric Seismic Analysis) experiment have allowed detailed crustal investigations in the area of the Central Graben and south-eastern North Sea to be conducted. An integrated velocity–density model based on a joint inversion of traveltimes and gravity data is presented for the crust and uppermost mantle along MONA LISA line 3, which crosses the Central Graben approximately perpendicular to its strike direction. This model shows pronounced crustal thinning and associated Moho uplift beneath the deepest part of the Central Graben. In the graben area, Moho is interpreted to be situated at 25km depth and the crystalline crust is about a factor of 2 thinner than in the surrounding crustal provinces, where Moho is interpreted to reach depths between 32 and 36km. Maximum thickness of the sedimentary column in the Central Graben is 10–11km. The mass deficit of the thick sediments is compensated by the Moho uplift. The modelled Moho coincides with the base of lower crustal reflectivity observed in the coincident normal-incidence seismic section. Good overall agreement between the integrated velocity–density model and the normal-incidence data is observed. The migrated deep seismic reflection section recorded to 26s traveltime contains deep west-ward and east-ward dipping events symmetric about the deepest part of the Central Graben. These dipping upper mantle reflections may be linked to extensional processes that formed the Central Graben. At least one of the upper mantle dipping reflectors may be a reactivated structure formed originally during Caledonian collision.

Simultaneous velocity and interface tomography of normal-incidence and wide-aperture seismic traveltime data

A tomographic inversion technique that inverts traveltimes to obtain a model of the subsurface in terms of velocities and interfaces is presented. It uses a combination of refraction, wide-angle reflection and normal-incidence data, it simultaneously inverts for velocities and interface depths, and it is able to quantify the errors and trade-offs in the final model. The technique uses an iterative linearized approach to the non-linear traveltime inversion problem. The subsurface is represented as a set of layers separated by interfaces, across which the velocity may be discontinuous. Within each layer the velocity varies in two dimensions and has a continuous first derivative. Rays are traced in this medium using a technique based on ray perturbation theory, and two-point ray tracing is avoided by interpolating the traveltimes to the receivers from a roughly equidistant fan of rays. The calculated traveltimes are inverted by simultaneously minimizing the misfit between the data and calculated traveltimes, and the roughness of the model. This ‘smoothing regularization’ stabilizes the solution of the inverse problem. In practice, the first iterations are performed with a high level of smoothing. As the inversion proceeds, the level of smoothing is gradually reduced until the traveltime residual is at the estimated level of noise in the data. At this point, a minimum-feature solution is obtained, which should contain only those features discernible over the noise. The technique is tested on a synthetic data set, demonstrating its accuracy and stability and also illustrating the desirability of including a large number of different ray types in an inversion.

Reflectivity characteristics of the crust from modelling refraction/wide-angle and normal-incidence reflection data on BABEL Line 1

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.

Westline: a deep near-normal incidence reflection profile across the Rockall Trough

Abstract Westline is a 450 km long, near-normal incidence, deep seismic reflection profile crossing the Rockall Trough. The profile was shot perpendicular to the edges of the trough in order to image its conjugate margins. An interpretation of the brute stack of the normal incidence data, partly constrained by existing near-normal incidence and wide angle-data, reveals that the trough is asymmetric, with a broad faulted margin on its western edge and a steep faulted margin where it meets the Irish Shelf. A sill complex extends the whole width of the trough within the 5 km thick sedimentary infill.

Combined vertical‐incidence and wide‐angle seismic study of a gas hydrate zone, Blake Ridge

Wide‐angle ocean bottom seismic data and single‐channel seismic reflection data were collected in June 1992 over an area where gas hydrates are thought to be extensive on Blake Ridge, offshore South Carolina. Wide‐angle reflections were observed on four Woods Hole Oceanographic Institution ocean bottom hydrophones at offsets up to 15 km. Results from traveltime inversion show that the bottom simulating reflector (BSR) that marks the base of the hydrate stability field lies 400–500 m below the seafloor and is overlain by a 200‐ to 300‐m‐thick layer of average velocity 1.9 km s−1. There is no evidence for significant lateral velocity variation associated with lateral changes in BSR character. Ray‐tracing calculations show that the observation of the BSR out to large offsets (7 km) constrains the maximum vertical velocity gradient to about 0.5 s−1. Amplitude‐versus‐offset (AVO) analysis was performed for ranges up to about 6 km and incidence angles of more than 70°. The high amplitude of the BSR in the normal incidence data can be explained by two models, both of which include a strong negative impedance contrast: the “hydrate wedge” model and the free gas layer model. Synthetic seismogram calculation for the wedge model, using the reflectivity method, shows a strong increase in the BSR amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge. This amplitude increase is clearly not observed in the data, which shows only a relatively modest increase in amplitude with offset. This observation agrees better with the calculated AVO for the gas layer model. The lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and/or that the base of the gas layer is a gradational boundary.

Development of the Rockall Trough and the northwest European continental margin

During the Cretaceous and early Tertiary a series of sedimentary basins opened by rifting on the northwest margin of Europe. None of these developed into a fully-fledged ocean until the northern North Atlantic opened at 55 Ma between NW Europe and Greenland. The subsidence, magmatism, and thermal history of these basins and of the continental margin depended critically on the amount and style of stretching and on the temperature of the underlying asthenospheric mantle. Small variations in the mantle temperature can have dramatic effects on both the magmatism produced during rifting and on the subsidence of the basin. An increase of as little as 10% in the mantle temperature can cause uplift above sea-level rather than the more common subsidence when continental crust undergoes large amounts of extension. The initiation of the Iceland mantle plume at about 62 Ma caused abnormally elevated mantle temperatures across a 2000 km diameter region and had a dramatic effect on the subsidence and rifting history of sedimentary basins west of Britain. Their thermal history can be documented from the subsidence and magmatic record in the sedimentary basins and the continental margin. Using normal incidence and wide-angle seismic data and drilling results from the continental margin and from basins of NW Europe, their subsidence and thermal history can be modelled as a function of the amount of stretching and the temperature of the underlying mantle. The Rockall Trough itself is underlain by 5 km of sediments intruded by Tertiary volcanics. The basement imaged on seismic . . .

Mapping of quaternary deposits and bedrock topography using high-resolution seismic reflection techniques

Marine normal-incidence reflection and P- and S-wave refraction/wide-angle reflection data from the BABEL project, line 1, in the Bothnian Sea have been used for investigating the nature of the reflectivity in the area. The data are available along the same line, and therefore the same piece of crust is probed by two seismic methods. We use this favourable circumstance for model calculations concerning both data sets by analysing the waveforms of reflections from normal-incidence traveltimes at approximately 4, 12 and 16 s TWT. Strong sub-horizontal reflections are observed on the normal-incidence reflection section at about 4 s TWT. These reflections have been modelled using the reflectivity method (1-D modelling) utilising the water bottom reflection as an estimate of the source wavelet and show a positive polarity. Thickness and P-wave velocity consistent with these reflections are 50 m and 7 km/s, respectively, with a host rock velocity of 6 km/s. From the polarity, velocity and thickness, these reflections are attributed to dolerite sills in the upper crust. Strong lower-crustal reflections are observed on the wide-angle and normal-incidence reflection sections. However, the corresponding S-wave reflections from the lower crust are considerably weaker. The implied high positive impedance contrast for the P-wave lower-crustal reflection and the weak S-wave reflectivity suggests the presence of a highly mafic sheet at about 40 km depth. The wide-angle P- and S-wave sections show strong reflections at the Moho. Coincident normal incidence data show only weak Moho reflectivity indicating a gradient in velocity at the crust/mantle boundary. Dipping events onlap the strong sub-horizontal lower crustal and the P-wave Moho reflections on the wide-angle data. These are explained by dipping structures above the lower crustal reflector and below the Moho, with the latter also consisting of a gradient in velocity.

Frequency dependence of sound backscattering from live individual zooplankton

A broadband laboratory chirp sonar was used to investigate the frequency characteristics of the backscattering by two individual live decapod shrimps (Palaemonetes vulgaris). The measurements illustrate that for fixed shape and orientation, the backscattering amplitude varies dramatically with frequency with as low as 30-dB-deep nulls at certain frequencies (30 dB corresponds to a factor of 1000 in an echo integration value). The overall echo level and frequency characteristics vary from ping to ping as the animal changes shape and orientation. The fluid bent cylinder model [T. K. Stanton (1989), J. Acousl. Soc. Am. 86:691–705] describes the single-realization normal incidence data reasonably well, especially the frequencies at which the peaks and nulls occur. Our theoretical analysis only involved physical parameters that were either directly measured or taken from the literature where physical parameters involving similar animals are published. As expected, once the data are averaged across a number of pings (as one would do in a field measurement), the nulls are mostly “filled in” and the backscatter versus frequency curve becomes smoother with variability of the order 5–8 dB (factor of 3–6 in echo integration value). These data are useful for predicting volume scattering strengths observed in the field by taking averages with respect to shape and orientation.

The termination and multilayer relaxation at the Co{10 [formula omitted]0} surface

Utilising a large normal-incidence data base the surface termination and multilayer relaxation parameters have been determined for a Co{10 1 0}-(1×1) surface utilising fully dynamic LEED calculations and a multi- R factor analysis. The surface is shown to be uniquely terminated by a short interlayer spacing (0.68 Å). An oscillatory relaxation occurs at the cobalt surface with a first layer contraction of −6.5±2% and a small second layer expansion of 1.0±2%, with subsequent layer spacings equal to the bulk values within the precision of the analysis. The high level of agreement of theory and experiment (Rp=0.26) indicates the absence of stacking faults in the selvedge region. A pair potential calculation demonstrates a significantly lower surface energy for the short outermost interlayer termination. In contrast to fcc {110} surface no surface reconstruction has been detected for this surface, either clean or in the presence of alkali metal.

A re-examination of multilayer relaxation of Ag(110) by LEED structural analysis

Although the existence of an oscillatory relaxation of the topmost two interlayer spacings of Ag(110) is well established by both low energy electron diffraction (LEED) and Rutherford backscattering spectroscopy (RBS) the magnitude of the first layer contraction and second layer expansion is still a matter of considerable dispute. In an attempt to clarify the situation we have performed an independent LEED study utilising a large normal incidence data set and 7 different reliability factors ( R-factors) to judge the level of theory-experiment agreement. The distance dz 12 between the first and second layers is smaller than the bulk interlayer distance by 7 ± 2% and the distance dz 23 between second and third layers is expanded by 1 ± 2%. Furthermore, a contraction of 2 ± 2% of the third interlayer spacing ( dz 34) has been extracted, with subsequent interlayer spacings equal to the bulk value within the precision of this analysis. These results are compared with theoretical embedded atom method and tight binding predictions of changes of up to the top four interlayer spacings. These results confirm and extend the surface geometry determined in an earlier LEED study of Davis and Noonan. Finally, the multilayer relaxations obtained by LEED are compared with recent RBS studies and possible reasons for discrepancies discussed.

LEED intensities of TiSe 2 including off-normal electron incidence

LEED spectra from the (001) surface of ITTiSe 2 are measured and calculated for various diffracted beams including off-normal electron incidence. Normal incidence data confirm former work concerning the potential model and the surface relaxation. Slight deviation from normal incidence causes pronounced variations in the calculated spectra yielding a better understanding of the experimental results. The effect of distinct off-normal incidence was examined for the (00) beam.

Unoccupied electronic states of single-crystal graphite by angle-resolved ultraviolet inverse photoemission.

The unoccupied electronic states of a single crystal of natural graphite have been measured by angle-resolved inverse photoemission, with tunable photon energies in the range 14--26 eV. The normal-incidence data suggest that the interlayer band minimum probably lies at \ensuremath{\sim}5 eV above the Fermi level. An image-potential state is observed at 3.9 eV above the Fermi level along \ensuremath{\Gamma}A, existing in the gap below the interlayer state. The absence of turbostratic disorder has allowed unam- biguous determination of the distinct conduction-band dispersions in the two principal symmetry directions, \ensuremath{\Gamma}ALM and \ensuremath{\Gamma}AHK, up to 15 eV above the Fermi level.

The PUMA experiment west of Lewis, U.K.

In August-September 1984 a 165 km long, reversed, wide-angle, seismic line was shot along part of the BIRPS WINCH deep seismic reflection profile west of Lewis in the Outer Hebrides. The experiment was recorded using a new type of sea-bottom receiver, the PUMA (Pull-Up Multichannel Array). This consists of an 1100 m long array of hydrophones connected to a sea-bottom recording package. Airgun and explosive shots recorded by the PUMA provide a densely sampled record section, allowing even low amplitude arrivals to be traced across the section. An initial, 1-D interpretation of the data reveals a crustal thickness of 27 km, and confirms that a band of reflectors seen at around 8.6 s TWT on the normal incidence data, and interpreted as the ‘reflection Moho’, coincide with the Moho determined by wide-angle reflections and refractions.

Photoelastic stress separation along lines of symmetry

By utilizing the compatibility and equilibrium field equations a differential relationship for the variation of the separated stress components along a line of symmetry is derived. This is in terms of only normal-incidence isochromatic data and does not use any isoclinic data. Its application to two problems using experimental data is demonstrated.