Angular Dependence Of Intensity Research Articles

Small non-uniform basal crystal fields in HVPE free-standing GaN:Mg as evidenced by angular dependent and frequency-dependent EPR

We studied thin-film and free-standing Mg-doped GaN using multi-frequency electron paramagnetic resonance (EPR) at 3–3.5 K and 9.4–130 GHz. Free-standing samples exhibit a highly anisotropic intensity, varying by a factor of 20 from 0° to 60°. In contrast, the intensity of the thin-film samples is significantly more isotropic, varying by no more than 10% over the same range of angles. The angular dependent intensity can be modeled in both free-standing and thin-film samples similarly to the g-factor anisotropy reported for thin films, supporting the theoretical predictions that the hole is on a basal site around the Mg acceptor. In addition, frequency-dependent transmission EPR measurements reveal a distribution of in free-standing samples, indicating that the local basal crystal field is non-uniform.

Angular-Dependent Spontaneous Emission in Cholesteric Liquid-Crystal Films

Because of the presence of a selective reflection band, cholesteric liquid crystals strongly modify the emission of fluorescent guest molecules. While emission along the helical axis is well understood, the emission properties at oblique angles with respect to the cholesteric helix have so far not been explored in detail. We present a systematic study of the angular-dependent spontaneous emission properties of dye-doped cholesteric films, including a full Stokes analysis of the emitted light. Variation of the cholesteric pitch allows us to probe the effect of the photonic band structure on spontaneous emission in a wide angular range. We find exceptional angular-dependent intensity and polarization variations of the emitted light and give experimental proof for the splitting of the first-order reflection band as well as its overlap with the second-order band at large detection angles.

Three- and Two-Photon NIR-to-Vis (Yb,Er) Upconversion from ALD/MLD-Fabricated Molecular Hybrid Thin Films.

We report blue, green, and red upconversion emissions with strongly angular-dependent intensities for a new type of hybrid (Y,Yb,Er)–pyrazine thin films realized using the atomic/molecular layer deposition thin-film fabrication technology. The luminescence emissions in our amorphous (Y,Yb,Er)–pyrazine thin films of a controllable nanothickness originate from three- and two-photon NIR-to-vis excitation processes. In addition to shielding the lanthanide ions from nonradiative de-excitation, the network of interconnected organic molecules serves as an excellent matrix for the Yb3+-to-Er3+ excitation energy transfer. This suggests a new approach to achieve efficient upconverting molecular materials with the potential to be used for next-generation medical diagnostics, waveguides, and surface-sensitive detectors.

Formation of the angular dependence of intensity of the light scattered on the optically dense atomic ensemble

The probe light scattering on the cold optically dense atomic ensemble is studied theoretically in the paper. In order to describe multiple scattering of light in the context of quantum electrodynamics, the Konstantinov–Perel–Keldysh diagram technique has been used. This technique allows to rewrite the considered case of light scattering in terms of diagram series where each term describes incoherent scattering of certain order. Decoding these terms allowed us to obtain the explicit analytical expression for the cross-section of multiple incoherent scattering for the case of stationary two-level atoms (transition Jg = 0 → Je = 1). The numerical analysis of this expression carried out by applying the Monte-Carlo simulation made it possible to find an influence of different orders of scattering on forming the angular dependence of scattered light intensity.

Polarized Raman scattering of large crystalline domains in VO2 films on sapphire

Polarized Raman scattering measurements were performed on (011)-oriented, over 10μm-size VO2 grains on an Al2O3 (001) substrate. Angular intensity dependence of Raman active modes allowed to observe 17 modes among the 18 expected and to demonstrate the single crystalline character of the large domains. Analyses of intensity evolution have been used to correctly assign phonon modes and to calculate the Raman tensor elements. In addition, a Raman Ag mode related to the VV stretching vibration was found to have a different behavior versus angle rotation from other Ag modes, which supports the previous theoretical study and photoexcitation measurements.

Comparison of the Surface and Subsurface Oxygen Reactivity and Dynamics with CO Adsorbed on Rh(111)

The translational and angular intensity dependence of CO2 produced from the reaction of CO with an initial oxygen coverage of 0.5 ML ≤ ΘO ≤ 1.6 ML on Rh(111) at a surface temperature of 450 K was measured. This range of coverages goes from only surface absorbed O to both adsorbed and selvedge absorbed O. The measured CO2 translational energy is independent of ΘO and is larger than steady-state experiments [Colonell et al. J. Chem. Phys. 1995, 103, 6677], which were done at lower O coverages, but follows the trend that higher ΘO leads to faster CO2. The angular dependence of the intensity is well-described by a sharply peaked function of the form I = a cosn(θ) + (1 – a) cos(θ), with a = 0.6 and n = 8.6. Again, this was consistent with the previous steady-state results. These observations point to a similar reaction geometry regardless of ΘO; the measured dynamical parameters are independent of whether or not some of the O is subsurface and suggest that absorbed oxygen in the selvedge acts only as a reservo...

Precise determination of phonon constants in lead-free monoclinic (K0.5Na0.5)NbO3 single crystals

A polarized Raman analysis of ferroelectric (K0.5Na0.5)NbO3 (KNN) single crystals is presented. The Raman modes of KNN single crystals are assigned to the monoclinic symmetry. Angular-dependent intensities of A′, A″, and mixed A′ + A″ phonons have been theoretically calculated and compared with the experimental data, allowing the precise determination of the Raman tensor coefficients for (non-leaking) modes in single-domain monoclinic KNN. This study is the basis for non-destructive assessments of domain distribution by Raman spectroscopy in KNN-based lead-free ferroelectrics.

Characterization of ultra-shallow aluminum implants in silicon by grazing incidence and grazing emission X-ray fluorescence spectroscopy

In this work two synchrotron radiation-based depth-sensitive X-ray fluorescence techniques, grazing incidence X-ray fluorescence (GIXRF) and grazing emission X-ray fluorescence (GEXRF), are compared and their potential for non-destructive depth-profiling applications is investigated. The depth-profiling capabilities of the two methods are illustrated for five aluminum-implanted silicon wafers all having the same implantation dose of 1016 atoms per cm2 but with different implantation energies ranging from 1 keV up to 50 keV. The work was motivated by the ongoing downscaling effort of the microelectronics industry and the resulting need for more sensitive methods for the impurity and dopant depth-profile control. The principles of GIXRF and GEXRF, both based on the refraction of X-rays at the sample surface to enhance the surface-to-bulk ratio of the detected fluorescence signal, are explained. The complementary experimental setups employed at the Physikalisch-Technische Bundesanstalt (PTB) for GIXRF and the University of Fribourg for GEXRF are presented in detail. In particular, for each technique it is shown how the dopant depth profile can be derived from the angular intensity dependence of the Al Kα fluorescence line. The results are compared to theoretical predictions and, for two samples, crosschecked with values obtained from secondary ion mass spectroscopy (SIMS) measurements. A good agreement between the different approaches is found proving that the GIXRF and GEXRF methods can be efficiently employed to extract the dopant depth distribution of ion-implanted samples with good accuracy and over a wide range of implantation energies.

Anisotropy in structural and physical properties in tetrathiafulvalene derivatives-based zone-cast layers as seen by Raman spectroscopy, UV-visible spectroscopy, and field effect measurements

We have studied anisotropy of thin layers of amphiphilic tetrathiafulvalene derivatives (TTF-4SCn, with n=12, 18, and 22) obtained by zone-casting technique. All the films show optical anisotropy, as seen by polarized optical microscopy and polarized UV-visible spectroscopy. By using polarized Raman spectroscopy an angular dependence of intensity of different vibrational modes in respect to the zone-casting direction was determined. It was found that intensities of the modes related to central and ring C=C vibrations in the TTF core depend very strongly on the angle between the zone-cast direction and polarization plane of incident laser light. Comparison of the deduced orientation of the molecules in one of the films (TTF-4SC18) with its crystal structure shows that the polarized Raman spectroscopy can be useful for controlling orientation of molecules in thin films (e.g., for online monitoring). Organic field effect transistors (OFETs), with channels oriented in parallel and perpendicularly to the zone-casting direction, were built using the oriented TTF-4SCn films. In all cases a strong anisotropy of the charge carrier mobility (μ) was found; the best results were obtained for OFETs with TTF-4SC18, for which μ∥=0.25 cm2/V s, ON/OFF>105, and μ∥/μ⊥≈170.

Angular distribution of photoelectrons with regard to non-dipole effects in photoionization and elastic electron scattering in solids

The angular distribution of photoelectrons from a semi-infinite solid sample is considered with regard to non-dipole transitions in photoionization with polarized and unpolarized radiation and elastic scattering. Non-dipole transitions are taken into account up to the octupole approximation. Calculations were carried out based on transport theory and Monte Carlo methods. Good agreement between two independent approaches is obtained. All angular parameters, necessary to calculate the angular dependence of intensities of photoelectrons for all elements Z < 100 in a wide range of ionizing radiation energies, are calculated in the relativistic approximation.

Iridescent ultraviolet signal in the orange sulphur butterfly (Colias eurytheme): spatial, temporal and spectral properties

Many of nature’s most striking animal colours are iridescent, exhibiting a high degree of spectral purity and strong angular dependence of intensity and hue. Although a growing number of studies have detailed the intricate mechanisms responsible for producing iridescent colours, few attempts have been made to describe their dynamic appearance in ecologically and behaviourally realistic contexts. We suggest that the optical properties unique to iridescent structural colours are important for understanding how they function as signals during behavioural interactions. Using males of the orange sulphur butterfly, Colias eurytheme, which exhibit an iridescent ultraviolet (UV) reflectance on their dorsal wing surfaces, we develop a holistic framework for inferring the appearance of this signal to conspecifics under field conditions that incorporate data on their spectral sensitivity. We show that, during flight, the UV signal is brightest within a wing beat cycle when viewed from directly above the male. Spectral properties of the signal under natural lighting indicate that male wing colour should be readily perceived and distinguished from that of females and from the dark green visual background of UV-absorbing vegetation. Finally, our analyses permit predictions regarding how signal senders and receivers should orientate themselves for maximal transmission and reception of this ultraviolet iridescent signal. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 349–364.

DDA simulations of light scattering by small irregular particles with various structure

We present DDA investigations of light scattering by irregular particles whose size is comparable with wavelength. We consider four types of randomly irregular particles: strongly damaged spheres, rough-surface spheres, pocked spheres, and agglomerated debris particles. Each type of particle is generated with a well defined algorithm producing an ensemble of stochastically different particles that have a common origin. The different types of irregular particles produce different angular dependencies of intensity and linear polarization degree. Transformation of phase curves of intensity and polarization with changing size parameter for irregular particles tends to be more monotonic, unlike spheres. We find that the magnitude of the negative polarization branch (NPB) tends to shrink as particle absorption increases; whereas, the maximal value and position of the positive polarization branch tends to increase. The most frequently observed shape of the negative polarization at small phase angles is asymmetric with a shift of the minimum position towards the angle of polarization sign inversion. All types of considered irregular particles reveal such asymmetry at x<10. Symmetric negative polarization branches occur seldom. The necessary conditions for their appearance are a relatively large size parameter x⩾10 to 12 and low absorption.

A comprehensive model to describe light scattering properties of cometary dust

We present a model of cometary dust that could provide the correct behaviour of the main observational characteristics in the visual spectral regime. The model reproduces the right shape in angular dependencies of intensity and polarisation as well as their spectral gradients (colours) including the change of colour with scattering angle. The dust particles are described as aggregates of a large number of constituent spheres of radius equal to ∼0.1μm. We use a refractive index that is characterized by high values of its real and imaginary parts both increasing with wavelength. This refractive index reproduces the comet-Halley element content of dust derived from in-situ measurements but the values are typical, in general for carbonaceous materials. The assumed composition is also in a good agreement with laboratory studies of interplanetary dust particles as well as with the current understanding of the material evolution in comets, although further studies of comet dust properties should include a comparison with thermal observations.

Structural changes and aggregation of human influenza virus.

The pH-induced change in the structure and aggregation state of the PR-8 and X-31 strains of intact human influenza virus has been studied in vitro. Reducing the pH from 7.4 to 5.0 produces a large increase in the intensity of light scattered to low angles. A modest increase in the polydispersity parameter from cumulants fits to the dynamic light scattering correlograms accompanies the increase, as does a change in how that parameter varies with scattering angle. These trends imply that the virus particles are not uniform, even at pH 7.4, and tend to aggregate as pH is reduced. The scattering profiles (angular dependence of intensity) never match those of isolated, spherical particles of uniform size, but the deviations from that simple model remain modest at pH 7.4. At pH 5.0, scattering profiles calculated for aggregates of uniformly sized spheres come much closer to matching the experimental data than those computed for isolated particles. Although these observations indicate that acid-induced aggregation develops over a period of minutes to hours after acidification, a nearly instantaneous increase in hydrodynamic size is the first response of intact virus particles to lower pH.

X-ray specular reflection under conditions of extremely asymmetric noncoplanar diffraction from a bicrystal

The angular dependence of the intensities of X-ray specular reflection has been rigorously analyzed under conditions of noncoplanar grazing Bragg diffraction in a crystal coated with a crystalline film (bicrystal). It is shown that the anomalous angular dependence of the specular-reflection intensity is extremely sensitive to the thickness (from fractions of a nanometer up to several nanometers), deformation, and the amorphization degree of the crystalline films. The optimum conditions for recording intensities are attained at grazing angles equal to 1.5–4.0 of the critical angle of the total external reflection.

Polarization properties, high-order Raman spectra, and frequency asymmetry between Stokes and anti-Stokes scattering of Raman modes in a graphite whisker

The Raman spectra of a new type of graphite whiskers have been measured in the range of $150--7800{\mathrm{cm}}^{\ensuremath{-}1}.$ The intensity of the overtone $(2D)$ located at $\ensuremath{\sim}2700{\mathrm{cm}}^{\ensuremath{-}1}$ is found to be about 10 times stronger than that of the C-C stretching mode (G) at $1582{\mathrm{cm}}^{\ensuremath{-}1}.$ Because of the peculiar enhancement of the $2D$ mode, high-order Raman bands up to fifth order at $\ensuremath{\sim}7500{\mathrm{cm}}^{\ensuremath{-}1}$ have been observed. Polarized micro-Raman spectroscopy has been performed on an individual graphite whisker, and angular-dependent intensity measurements of all Raman modes in the $\mathrm{VV}$ and $\mathrm{HV}$ geometries are in agreement with the theoretical calculated results. Laser-energy-dependent dispersion effects and the frequency discrepancy of Raman modes between their Stokes and anti-Stokes lines in graphite whiskers are also carefully investigated. The energy dispersion of the D mode and G mode is very similar to that of highly oriented pyrolytic graphite (HOPG). In contrast to the Raman spectra of HOPG and other graphite materials, two laser-energy-dependent Raman lines are revealed in the low-frequency region of the Raman spectra of graphite whiskers, which are believed to be the resonantly enhanced phonons in the transverse-acoustic and longitudinal-acoustic phonon branches. Moreover, the obvious energy dispersion of the ${D}^{\ensuremath{'}}$ mode at \ensuremath{\sim}1620 ${\mathrm{cm}}^{\ensuremath{-}1}$ is observed in graphite whiskers. The results clearly reveal how strongly the peak parameters of Raman modes of graphite materials are dependent on their structural geometry. The Stokes and anti-Stokes scattering experiments show that the frequency discrepancy between the Stokes and anti-Stokes sides of a Raman mode in graphite materials is equal to the frequency value covered by the one-phonon energy of this Raman mode in its frequency versus laser energy curve, which is the product of the one-phonon energy of this mode $(E{\ensuremath{\omega}}_{s})$ and the value of its laser-energy dispersions $(\ensuremath{\partial}E{\ensuremath{\omega}}_{s}/\ensuremath{\partial}{\ensuremath{\varepsilon}}_{L}).$

Molecular geometry modifications upon adsorption for N2O: N and O K-edge NEXAFS

The interaction of a monolayer and submonolayer N2O with the Cu(100) and Ag(110) surfaces has been studied using near-edge X-ray absorption fine structure spectroscopy. From a detailed analysis of the angular dependent intensity of the molecular resonances we derive a substrate and coverage dependent bent of the N2O molecules upon adsorption. From the comparison of the π* resonance intensity from the terminal Nt, and central Nc nitrogen atoms we conclude that the N2O molecules couple with the Cu surface through the terminal Nt atom in contrast to the Ag surface where the Nc atom is involved.

Light Scattering by Aggregate Particles Comparable in Size to Wavelength: Application to Cometary Dust

Many naturally occurring particles (including, most likely, cometary dust) have an aggregate structure. We study the scattering properties of polydisperse independent aggregate particles (clusters) comparable in size to visible wavelengths. The sizes of the monomers constituting a cluster play a significant role in forming the angular dependences of intensity and linear polarization of the scattered light. Irregularly structured aggregates composed of a moderate number of spheres (<50) with size parameters 1.3–1.65 exhibit properties typical of cometary dust particles: a slight increase in backscattering intensity, a negative polarization at small phase angles, an inversion phase angle close to the observed one, an increase in brightness, and a linear polarization with increasing wavelength. In this case, the imaginary part of the refractive index for particles can increase with decreasing wavelength in the visible spectral range, which is typical of silicates with an admixture of iron or organic material. The spectral dependence of extinction efficiency for aggregates is less steep than that for equivalent spherical particles, and its maximum is shifted to larger size parameters. Therefore, when analyzing extinction measurements, the scatterer shape must be taken into account to avoid underestimation of the scattering-particle sizes.

A systematic study of light scattering by aggregate particles using the microwave analog technique: Angular and wavelength dependence of intensity and polarization

Microwave analog experiments [e.g., Gustafson, 1996a, 1999] allow us to simulate light‐scattering properties of particles of almost any combination of composition, size, and shape. We have taken advantage of this ability and the broadband nature of the University of Florida microwave facility in a systematic study of light scattering by aggregates. We present angular dependencies of intensity and polarization in the scattering angle range 0°–165°, and in the wavelength interval 2.7–4 mm, which simulates 0.430–0.660 μm in the visual. The data are for a variety of sizes, shapes, and compositions of the constituent particles, as well as the number of particles in an aggregate and their packing. Wavelength dependencies are shown to be powerful diagnostics for the interpretation of comet dust observations and we urge observers to report color and polarimetric color. As an example, we estimate the size of constituent particles in cometary grains (0.1–1 μm). We also show that the grains evolve with the distance from the nucleus and that the reason apparently is the loss of some dark matter. Techniques and results reported here are general enough to apply to a broad range of natural and artificial aggregates.

Photoelectron elastic scattering effects in XPS

The photoelectron elastic scattering effects in XPS are reviewed. Using the transport theory approach and Monte-Carlo calculations as well experimental data, the influence of elastic scattering is demonstrated for the escape probability of the photoelectrons from solids, the intensities from bulk and surface systems, the angular intensity dependence including dipole and quadrupole transitions, attenuation lengths from various definitions, mean escape depths, the path-length distribution of photoelectrons in solids, the overlayer thickness determination, in-depth profiling and photoelectron diffractions patterns from single crystals.