Angular Distribution Patterns Research Articles

Variable laser profiles and polarization states in the wave-mixing scheme for the generation of THz radiation in collisional-magnetized clustered-plasma

The four-wave mixing (FWM) mechanism for the generation of Terahertz (THz) radiation in magnetized-collisional clustered-plasma is studied. Clusters can trap the incident laser fields, leading to efficient energy transfer to plasma electrons and the creation of a strong nonlinear current. The size and density of clusters can be tuned to control the emission of THz radiation. The smaller clusters can bring about higher intensity THz radiation, while larger clusters can contribute to the broader THz frequency range. The THz radiation angular distribution patterns in the forward direction have been assessed, and the impact of plasma interaction length, cluster radius, and plasma electrons collision frequency on the generated THz wave patterns has been investigated. The FWM analysis highlights the importance of plasma clusters as well as magnetic fields in the efficient generation of THz emission. The model also takes into consideration how an induced electron current density initiated by DC external magnetic field can boost radiation power without changing the directivity diagram.

Field emission angular distribution from single molecules

We demonstrate that the field emission from a single molecule at the apex of a metallic tip exhibits a well-defined angular distribution pattern (field emission angular distribution, FAD) with a spherical harmonics shape, regardless of the species of the molecule. From carefully controlled simultaneous measurements of the emission pattern, the emission current and emission energy, we deduced the formation mechanism of the FAD pattern in which the electron from the metallic tip resonantly tunnels into vacuum via a molecule-induced state situated at the Fermi level of the tip. The characteristic FAD patterns were consistent with the Fourier transform of the superatom molecular orbitals (SAMOs). Thus, the present method is unique for real-time imaging of the SAMOs, or the low-lying Rydberg states, of single molecules.

Analysing far-red SIF directional anisotropy of three structurally contrasting forest canopies towards improved GPP estimation

Solar-induced chlorophyll fluorescence (SIF) is used to estimate terrestrial gross primary production (GPP) due to its physiological link to photosynthesis. However, strong angular variations in satellite-observed SIF (SIFobs), especially for forest canopies with a high structural heterogeneity, hampers robust estimation of GPP. Here, we use directional SIFobs datasets from OCO-2 and TROPOMI satellite sensors and the 3D discrete anisotropic radiative transfer model to investigate the directional properties of far-red SIFobs and their relationships with two GPP products obtained from MODIS and FluxCom datasets for three European forest types with contrasting canopy structures, i.e., boreal, temperate and Mediterranean forests. We found bowl-like angular distribution patterns of SIFobs in both observed and simulated boreal and temperate forests, but no generic distribution pattern of SIFobs was found for the Mediterranean forest. In our GPP estimation of boreal forest, oblique SIFobs performed better than the widely used nadir SIFobs. However, we did not find an optimal view angle suitable for all three forest types. Further, the angular dependencies and forest structure impacts present in SIFobs were found to still propagate into the estimation of total emitted SIF (SIFtotal), which generally regarded as free of these confounding effects. Finally, our results demonstrate that two commonly used variables, i.e., SIFobs with a constant viewing angle and the approximated SIFtotal, both fail to produce a robust relationship with GPP for the Mediterranean forest. These findings highlight the importance of forest canopy structural heterogeneity, including forest floor, on interpretation of directional satellite SIF observations, even when corrected and expressed as a total canopy SIF emission.

Finite-time distributed localization and multi-orbit circumnavigation with bearing-only measurements

This paper investigates the localization and circumnavigation problem of a stationary target by multiple agents with multi-orbit within finite time. The agents can only obtain their own positions and the bearing information of the target. They are required to travel along their prescribed trajectories in three-dimensional (3-D) space and circumnavigate the target. First, a novel finite-time distributed positional estimator, using bearing-only measurements, is developed for agents to localize the target. Afterward, with the estimate of the target position, a distributed control law is designed to steer the agents to circumnavigate the target on different orbits with any preconcerted angular spacing distribution pattern within finite time. Simultaneously, the corresponding stability and convergence analyses are presented. Finally, simulation results are provided to demonstrate the effectiveness of the theoretical results.

Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography.

Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.

On the correlation of angular distributions of keV ions and trajectory-dependent electronic excitations in transmission channelling geometry

We use energy discrimination of keV ions transmitted through a thin, single-crystalline silicon membrane to correlate specific angular distribution patterns formed in channelling geometry with trajectory-dependent electronic energy loss. The integral energy and intensity distribution of transmitted ions can thus be dissected into on one side axially channelled projectiles travelling along rather straight trajectories and on the other side dechannelled projectiles predominantly experiencing blocking. Angular distributions of transmitted ions are further simulated with two different Monte-Carlo codes.

Spin and polarization analysis of the Zcs state

A polarization analysis is performed for the recent observation of $Z_{cs}$ exotic state in the $e^+e^-$ annihilation experiment with motivation for measuring its spin quantum number in the future. Starting with the unpolarized electron and positron beam, the polarization transfer to the $Z_{cs}$ state and its decay angular distribution patterns are investigated. Some observables are suggested for determination of the spin parity quantum numbers. An ensemble of Monte-Carlo events are used to show some moment distributions special for manifestation of the different $Z_{cs}$ spin scenarios.

Radial and Angular Electron Ejection Patterns of Two-Electron Quantum Dots in THz Fields

In this work, we develop and apply an ab-initio method to calculate the joint radial- and- angular electron distributions following the interaction of two-electron spherical quantum dots (QD) with intense terahertz pulses of subpicosecond duration. By applying the method to two QDs of different size, we could investigate two particular ionization mechanisms: the direct and the sequential two-photon double ionization. According to our results, the two ionization mechanisms show some similarity in the angular distribution patterns, whereas the corresponding radial distributions are distinctly different, associated with their joint kinetic energy spectrum. We also discuss the time-evolution of the ionization process in the context of the different nature of the interaction of the QD with the external radiation and the electron–electron correlation interactions.

Mapping nanometer and micrometer‐scale structures at graphite surface by photoelectron diffraction

A series of C 1s photoelectron intensity angular distribution (PIAD) patterns from a crystalline graphite surface were mapped in two dimensions at intervals of 20 μm. Two kinds of PIAD patterns rotated by 30° from each other, corresponding to the two domains from the twinned crystal, were found. The standard deviation of the diffraction contrast was evaluated for each PIAD pattern from its intensity histogram. Surprisingly, we found that odd‐number‐ (most probably single‐) atom‐height steps on the flat terrace region and their local atomic arrangement can be identified by two‐dimensionally mapping the standard deviation values. Furthermore, we analyzed the photoelectron diffraction patterns and noticed that the normal direction of the sample surface at each point was slightly inclined differently. By connecting all the inclination angle data, the micrometer‐scale corrugation of the sample surface was successfully visualized. Scanning photoelectron diffraction microscopy has been shown to be useful for the nanometer‐ and micrometer‐scale structural imaging.

Directional emission of rhodamine 6G on top of a silver grating.

We have observed directional spontaneous emission of rhodamine 6G dye deposited on top of a silver grating and found that its angular distribution patterns were very different in TE and TM polarizations. The latter was related to the dispersion curves determined based on the polarized reflection spectra measured at multiple incidence angles. The most intriguing finding of this Letter was a resonance, which was coupled with TE-polarized light and determined the characteristic double-crescent patterns in the TE-polarized spontaneous emission. This observation, as well as nearly similar resonance observed in TM polarization, was tentatively explained in terms of leaky waveguide modes supported by a film of dye-doped polymer.

A simulation study on few parameters of Cherenkov photons in extensive air showers of different primaries incident at various zenith angles over a high altitude observation level

We have studied the distribution patterns of lateral density, arrival time and angular position of Cherenkov photons generated in Extensive Air Showers (EASs) initiated by γ-ray, proton and iron primaries incident with various energies and at various zenith angles. This study is the extension of our earlier work [1] to cover a wide energy range of ground based γ-ray astronomy with a wide range of zenith angles (≤40°) of primary particles, as well as the extension to study the angular distribution patterns of Cherenkov photons in EASs. This type of study is important for distinguishing the γ-ray initiated showers from the hadronic showers in the ground based γ-ray astronomy, where Atmospheric Cherenkov Technique (ACT) is being used. Importantly, such study gives an insight on the nature of γ-ray and hadronic showers in general. In this work, the CORSIKA 6.990 simulation code is used for generation of EASs. Similarly to the case of Ref. [1], this study also revealed that, the lateral density and arrival time distributions of Cherenkov photons vary almost in accordance with the functions: ρch(r)=ρ0e−βr and tch(r)=t0eΓ/rλ respectively by taking different values of the parameters of functions for the type, energy and zenith angle of the primary particle. The distribution of Cherenkov photon’s angular positions with respect to shower axis shows distinctive features depending on the primary type, its energy and the zenith angle. As a whole this distribution pattern for the iron primary is noticeably different from those for γ-ray and proton primaries. The value of the angular position at which the maximum number of Cherenkov photons are concentrated, increases with increase in energy of vertically incident primary, but for inclined primary it lies within a small value (≤1°) for almost all energies and primary types. No significant difference in the results obtained by using the high energy hadronic interaction models, viz., QGSJETII and EPOS has been observed.

Hybrid centrifugal spreading model to study the fertiliser spatial distribution and its assessment using the transverse coefficient of variation

Studying centrifugal spreading by carrying out field or in-door experiments using fertiliser collection trays is tedious and labour intensive. This is particularly true when several implementation methods need to be compared, numerous replications are required or fertiliser sample characterisation is required. To circumvent cumbersome experiments, an alternative approach consists in performing in silico studies. In order to reach this objective, a hybrid centrifugal spreading model is designed by combining theoretical fertiliser motion equations with statistical information. The use of experimental measurements to characterise fertiliser properties, outlet velocity, angular mass flow distribution and spread pattern deposition, ensure a realistic calibration of the model. Based on this model, static spread patterns and transverse distributions are computed for a virtual twin-disc spreader. The number of fertiliser granules used to compute a spread pattern is deduced from the target application rate while the granule properties and their motion parameters are randomly selected from pre-established statistical distributions. This Monte Carlo process reproduces the random variability of fertiliser spread pattern depositions. Using this model, simulations demonstrate the mean and standard deviation of CV value decrease with the application rate. The CV mean value also decreases with the collection tray surface, while the standard deviation decreases with the collection tray length. Mathematical relationships are deduced from simulation results to express the mean and standard deviation of the CV as functions of the application rate and collection tray surface or length. The simulation model is also used to compare spreader test methods and study the influence of some fertiliser particles properties on the transverse distribution.

Free-form thin lens design with light scattering surfaces for practical LED down light illumination

The free-form optical quasilens surface technology was utilized to develop and design a solid transparent plastic optical lens for the LED down light with the narrow angular light distribution requirement in the LED lighting applications. In order to successfully complete the mission, the precise mid-field angular distribution model of the LED light source was established and built. And also the optical scattering surface property of the Harvey BSDF scattering model was designed, measured, and established. Then, the optical simulation for the entire optical system was performed to develop and design this solid transparent plastic optical lens system. Finally, the goals of 40 deg angular light distribution pattern defined at full width half maximum with glare reduced in the areas of interest and the optical performance of nearly 82% light energy transmission optics were achieved for the LED down light illumination.

Effect of atmospheric refraction on radiative transfer in visible and near‐infrared band: Model development, validation, and applications

AbstractRefraction is an important factor influencing radiative transfer since it can modify the propagation trajectory and polarization states of lights; therefore, it is necessary to quantitively evaluate the effect of atmospheric refraction on radiative transfer process. To this end, a new atmospheric radiative transfer model including refraction process is proposed. The model accuracy is validated against benchmark results, literature results, and well‐tested radiative transfer models such as discrete coordinate method and RT3/PolRadtran. The impact of atmospheric refraction on both polarized radiance and fluxes is discussed for pure Rayleigh scattering atmosphere, atmosphere with aerosol, and cloud. The results show that atmospheric refraction has a significant influence on both the radiance and polarization states of diffuse light, where the relative change of the radiance of reflected light and transmitted light due to refraction can achieve 6.3% and 7.4% for Rayleigh scattering atmosphere, 7.2% and 7.8% for atmosphere with aerosol, and 6.2% and 6.8% for cloudy atmosphere, respectively. The relative change of the degree of polarization ranges from near zero in the horizon to 9.5% near neutral points. The angular distribution pattern of the relative change of the radiance for atmosphere with aerosol and cloud is very similar to that for pure Rayleigh scattering case, where its magnitude decreases gradually with the increasing of zenith angle for reflected light; but for transmitted light, the variation characteristics is opposite. The impact of refraction is gradually enhanced with the increasing of solar zenith angles and the optical depth of aerosol and cloud. As the wavelength of incident light increases, the impact declines rapidly for Rayleigh scattering medium. The relative change of the fluxes due to refraction is most notable for Middle Latitude Winter profile (about 8.2043% and 7.3225% for the transmitted and reflected light, respectively, at 0.35 µm). With increasing the optical depth of aerosol, the influence of refraction on the fluxes is gradually enhanced. For cloudy atmosphere, the relative changes of the fluxes due to refraction are not very sensitive to the variation of cloud optical depth and effective radius of cloud drops.

Novel optical lens design with a light scattering freeform inner surface for LED down light illumination.

Using the precise mid field angular distribution model of the LED light source and the optical scattering surface property of the Harvey BSDF scattering model, we perform optical simulation to develop and design a novel solid plastic optical lens with freeform inner surface to achieve the optical performance of 89.89% light energy transmission optics for the sake of energy saving reason. The 70 degrees angular light distribution pattern defined at full width half maximum (FWHM) light energy level is performed and the optical utilization factor (OUF) of 51.8% is obtained within -45 degrees and 45 degrees range in the areas of interest with glare reduced for the down light illumination.

Measurement of leakage neutron spectra for Tungsten with D-T neutrons and validation of evaluated nuclear data

Integral neutronics experiments have been investigated at Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS) in order to validate evaluated nuclear data related to the design of Chinese Initiative Accelerator Driven Systems (CIADS). In the present paper, the accuracy of evaluated nuclear data for Tungsten has been examined by comparing measured leakage neutron spectra with calculated ones. Leakage neutron spectra from the irradiation of D-T neutrons on Tungsten slab sample were experimentally measured at 60° and 120° by using a time-of-flight method. Theoretical calculations are carried out by Monte Carlo neutron transport code MCNP-4C with evaluated nuclear data of the ADS-2.0, ENDF/B-VII.0, ENDF/B-VII.1, JENDL-4.0 and CENDL-3.1 libraries. From the comparisons, it is found that the calculations with ADS-2.0 and ENDF/B-VII.1 give good agreements with the experiments in the whole energy regions at 60°, while a large discrepancy is observed at 120° in the elastic scattering peak, caused by a slight difference in the oscillation pattern of the elastic angular distribution at angles larger than 20°. However, the calculated spectra using data from ENDF/B-VII.0, JENDL-4.0 and CENDL-3.1 libraries showed larger discrepancies with the measured ones, especially around 8.5–13.5MeV. Further studies are presented for these disagreements.

Effects of autoionizing states on two-photon double ionization of the H2 molecule

Treating the effects of autoionizing intermediate states on two-photon double ionization (DI) of the H2 molecule using time-dependent laser pulses is a significant computational challenge. Relatively long exposure times are critical to understanding the dynamics. Using the fixed-nuclei approximation, we demonstrate how the doubly excited states enhance the angle-integrated generalized cross sections in H2, and how they affect the angular distribution pattern of the ejected electrons. As the energy approaches the threshold for sequential DI, there is a sharp rise in the cross section due to virtual sequential ionization.

Interference in the angular distribution of photoelectrons in superimposed XUV and optical laser fields

The angular distribution of photoelectrons ejected during the ionization of Ne atoms by extreme ultraviolet (XUV) free-electron laser radiation in the presence of an intense near infrared (NIR) dressing field was investigated experimentally and theoretically. A highly nonlinear process with absorption and emission of more than ten NIR photons results in the formation of numerous sidebands. The amplitude of the sidebands varies strongly with the emission angle and the angular distribution pattern reveals clear signatures of interferences between the different angular momenta for the outgoing electron in the multi-photon process. As a specific feature, the central photoelectron line is characterized at the highest NIR fields by an angular distribution, which is peaked perpendicularly to both the XUV and NIR polarization directions. Experimental results are reproduced by a theoretical model based on the strong field approximation.

Is the Registry Between Adjacent Graphene Layers Grown on C-Face SiC Different Compared to That on Si-Face SiC

Graphene grown on C-face SiC substrates using two procedures, high and low growth temperature and different ambients, was investigated using Low Energy Electron Microscopy (LEEM), X-ray Photo Electron Electron Microscopy (XPEEM), selected area Low Energy Electron Diffraction (μ-LEED) and selected area Photo Electron Spectroscopy (μ-PES). Both types of samples showed formation of μm-sized grains of graphene. The sharp (1 × 1) μ-LEED pattern and six Dirac cones observed in constant energy photoelectron angular distribution patterns from a grain showed that adjacent layers are not rotated relative to each other, but that adjacent grains in general have different azimuthal orientations. Diffraction spots from the SiC substrate appeared in μ-LEED patterns collected at higher energies, showing that the rotation angle between graphene and SiC varied. C 1s spectra collected did not show any hint of a carbon interface layer. A hydrogen treatment applied was found to have a detrimental effect on the graphene quality for both types of samples, since the graphene domain/grain size was drastically reduced. From hydrogen treated samples, μ-LEED showed at first a clear (1 × 1) pattern, but within minutes, a pattern containing strong superstructure spots, indicating the presence of twisted graphene layers. The LEED electron beam was found to induce local desorption of hydrogen. Heating a hydrogenated C-face graphene sample did not restore the quality of the original as-grown sample.

Na Induced Changes in the Electronic Band Structure of Graphene Grown on C-Face SiC

Studies of the effects induced on the electron band structure after Na deposition, and subsequent heating, on a C-face 2 MLs graphene sample are reported. Na deposition shifts the Dirac point downwards from the Fermi level by about 0.5 eV due to electron doping. After heating at temperatures from around 120℃ to 300℃,thep-band appears considerably broadened. Collected Si 2p and Na 2p spectra then indicate Na intercalation in between the graphene layers and at the graphene SiC interface. The broadening is therefore interpreted to arise from the presence of two slightly shifted, but not clearly resolved,p-bands. Constant energy photoelectron distribution patterns, E(kx,ky);s, extracted from the clean 2MLs graphene C-face sample look very similar to earlier calculated distribution patterns for monolayer, but not Bernal stacked bilayer, graphene. After Na deposition the patterns extracted at energies below the Dirac point appear very similar so the doping had no pronounced effect on the shape or intensity distribution. At energies above the Dirac point the extracted angular distribution patterns show the flipped, “mirrored”, intensity distribution predicted for monolayer graphene at these energies. An additional weaker outer band is also discernable at energies above the Dirac point, which presumably is induced by the deposited Na.