Spectral Peculiarities Research Articles

Applicability of energy-theta mapping in resonant soft X-ray reflectometry to probe depth dependence of oxidation state and crystallographic environment in iron oxide multilayers

In the present paper we discuss applicability of the synchrotron method of resonant x-ray reflectometry to non-destructive depth profiling of multilayer heterostructures with low optical contrast between the sublayers. The two-dimensional mapping approach comprising evaluation and measurement of reflectance as a function of photon energy and grazing angle is shown to provide a convenient way to effectively utilize the enhancement of optical contrast between the sublayers that exists at the absorption edges of chemical elements due to particular spectral shape peculiarities related to oxidation state, crystallographic environment and magnetization. In the present study the evaluation of the resonant X-ray reflectivity maps is performed using a specially developed modeling and fitting software. Estimation of the photon energy / grazing angle combinations at which the reflectance is most sensitive to the minor changes in the physical properties of the sublayers is illustrated by the example of ferroic-on-semiconductor nanoscale heterostructures composed of nanoscale film of metallic iron oxidized from either top or bottom side. The subtle differences in resonant soft x-ray reflectance caused by variation of the oxide film thickness, oxidation state and crystallographic environment are discussed. The presented approach is applicable to a large class of heterostructures composed of sublayers that can be distinguished only by the differences in their near edge X-ray absorption fine structure.

Chemically peculiar stars on the pre-main sequence

Context. The chemically peculiar (CP) stars of the upper main sequence are defined by spectral peculiarities that indicate unusual elemental abundance patterns in the presence of diffusion in the calm, stellar atmospheres. Some of them have a stable local magnetic field of up to several kiloGauss. The pre-main-sequence evolution of these objects is still a mystery and contains many open questions. Aims. We identify CP stars on the pre-main sequence to determine possible mechanisms that lead to the occurrence of chemical peculiarities in the (very) early stages of stellar evolution. Methods. We identified likely pre-main-sequence stars by fitting the spectral energy distributions. The subsequent analysis using stellar spectra and photometric time series helped us to distinguish between CP and non-CP stars. Additionally, we compared our results to the literature to provide the best possible quality assessment. Results. Out of 45 candidates, about 70% seem to be true CP stars or CP candidates. Furthermore, 9 sources appear to be CP stars on the pre-main sequence, and all are magnetic. We finally report a possible CP2 star that is also a pre-main-sequence star and was not previously in the literature. Conclusions. The evolution of the peculiarities seems to be related to the (strong) magnetic fields in these CP2 stars.

AGE DYNAMICS OF VEGETATIVE AND NEURODYNAMIC FUNCTIONS AMONG CHILDREN AT THE AGE OF 5-7 DURING MENTAL LOADS

Purpose. To trace changing dynamics of vegetative and neurodynamic functions among children at the age of 5-7 during mental loads. The study usually starts for the senior pre-school and junior school age. Individual children’s features and work of organ systems ensure adaptive reactions (including those during intensive mental loads). Therefore, we should pay attention to age dynamics of statistical and spectral peculiarities of heart rhythm for pre- and post-load states of kids. Material and methods. 112 children at the age of 5-7 participated in the research. They come from senior pre-school and junior school institutions in Sumy, a Ukrainian city. The research determines functional activities and nervous power of children via the Diahnost-1M methodology. The Fazahraf methodology estimates values of the cardiovascular system and influence of the vegetative nervous system on the heart rhythm. The technique collected and analyzed electrical potentials of spectral peculiarities of regulatory functions and significant statistical values of the heart rhythm. Because of the stage-by-stage research approach and data heterogeneity, we conducted a multi-level mathematical assessment. Most data were not subject to normal distribution since there was a sharp asymmetry of low-stage discretization. Results. Children may have different features during and after mental loads. They can be defined via most spectral and statistical values of the varying heart rhythm. The heart rhythm reactions among children at the age of 5 are shifted to sympathicotonia. During mental loads, the dominance of sympathetic influence and instable control of regulatory systems was typical for the 6-year-old sample. Besides, we established a relation between nervous power and cardiovascular activities (r=0.42; р<0.05). In contrast to other age samples, the 7-year-old group showed a balanced cooperation of the sympathetic and parasympathetic nervous systems. Simultaneously, there was an obvious resistance to external stimuli during mental loads (r=-0.36; р<0.05). Conclusions. Children at the age of 7 are more adapted to solve mental tasks with a certain speed. Therefore, we should pay a great attention to age functioning of physiological systems. In this way, we may apply the person-centered approach within the study process to raise adaptive abilities of the children’s nervous system

Density Functional Study of Structural and Vibrational Properties of α-Moganite

α-moganite is a recently discovered polymorph of silica, commonly intergrown with quartz in natural microcrystalline silica samples. An important challenge is finding an effective method for estimating its amount in a sample under study, which is important for its applications, related to the technology of growth of dielectric layers, as well as for fundamental problems, related to the formation of both terrestrial and lunar mineral deposits and biogenic formation. One of these methods is vibrational spectroscopy, with the help of which the presence of a particular compound is determined by the presence of characteristic spectral lines. In this work, the search for such lines is carried out using density functional theory calculations and comparisons of the IR and Raman spectra of α-quartz and α-moganite. With the help of such calculations, the stability of the moganite structure has been proven for the first time, and its spectral characteristics have been determined over the entire range of vibrational frequencies. Several new spectral lines characteristic of α-moganite were discovered in the 65–85 cm−1 region. Moreover, the evolution of spectral peculiarities under hydrostatic pressure was studied.

Unique spectral manifestations around the D3 line observed in the region close to the seismic source of a large solar flare

ABSTRACT The main goal of our research is to search for super-strong magnetic fields in active processes on the Sun. Our method is based on Stokes V spectro-polarimetry in a wide spectral range from −14 Å to +29 Å relative to the D3 He i line. The object of the study is the area of a seismic source in the large solar flare on 2003 October 28 of the X17.2/4B class. The novelty of our study: we found characteristic spectral manifestations – secondary Stokes V peaks – far from D3 line, at distances of several angstroms. These secondary peaks have the following features: (a) their amplitudes reach 4 per cent – almost an order of magnitude more than previously detected in other flares, (b) in the general picture, the sign of the circular polarization changes when passing through the D3 centre, and (c) narrower spectral peculiarities were found at distances of +4.0, +6.3, and +8.5 Å, where the polarization sign also changes sharply. This unusual feature likely results from significant factors in the area of the seismic source, including substantial descending plasma velocities which exceed 400 km s−1 and, perhaps, super-strong magnetic fields.

Exploring the Extremes: Characterizing a New Population of Old and Cold Brown Dwarfs

Mapping out the populations of thick disk and halo brown dwarfs is important for understanding the metallicity dependence of low-temperature atmospheres and the substellar mass function. Recently, a new population of cold and metal-poor brown dwarfs has been discovered, with T eff ≲ 1400 K and metallicity ≲−1 dex. This population includes what may be the first known “extreme T-type subdwarfs” and possibly the first Y-type subdwarf, WISEA J153429.75−104303.3. We have conducted a Gemini YJHK/Ks photometric follow-up campaign targeting potentially metal-poor T and Y dwarfs, utilizing the GNIRS and Flamingos-2 instruments. We present 14 near-infrared photometric detections of eight unique targets: six T subdwarf candidates, one moderately metal-poor Y dwarf candidate, and one Y subdwarf candidate. We have obtained the first-ever ground-based detection of the highly anomalous object WISEA J153429.75−104303.3. The F110W − J color of WISEA J153429.75−104303.3 is significantly bluer than that of other late T and Y dwarfs, indicating that WISEA J153429.75−104303.3 has an unusual spectrum in the 0.9–1.4 μm wavelength range which encompasses the J-band peak. Our J-band detection of WISEA J153429.75−104303.3 and corresponding model comparisons suggest a subsolar metallicity and temperature of 400–550 K for this object. JWST spectroscopic follow-up at near-infrared and mid-infrared wavelengths would allow us to better understand the spectral peculiarities of WISEA J153429.75−104303.3, assess its physical properties, and conclusively determine whether or not it is the first Y-type subdwarf.

A spectral classification system for hydrogen-deficient carbon stars

ABSTRACTStellar spectral classification has been highly useful in the study of stars. While there is a currently accepted spectral classification system for carbon stars, the subset of hydrogen-deficient carbon (HdC) stars has not been well described by such a system, due predominantly to their rarity and their variability. Here we present the first system for the classification of HdCs based on their spectra, which is made wholly on their observable appearance. We use a combination of dimensionality reduction and clustering algorithms with human classification to create such a system with eight total classes corresponding to temperature, and an additional second axis corresponding to the carbon molecular band strength. We classify over half of the known sample of HdC stars using this, and roughly calibrate the temperatures of each class using their colours. Additionally, we express trends in the occurrence of certain spectral peculiarities such as the presence of hydrogen and lithium lines. We also present three previously unpublished spectra, report the discovery of two new Galactic dustless HdC stars, and additionally discuss one especially unique star that appears to border between the hottest HdCs and the coolest extreme helium stars.

Evaluation of the Ability of SLSTR (Sentinel-3B) and MODIS (Terra) Images to Detect Burned Areas Using Spatial-Temporal Attributes and SVM Classification

Forest fires are considered one of the major dangers and environmental issues across the world. In the Cerrado biome (Brazilian savannas), forest fires have several consequences, including increased temperature, decreased rainfall, genetic depletion of natural species, and increased risk of respiratory diseases. This study presents a methodology that uses data from the Sea and Land Surface Temperature Radiometer (SLSTR) sensor of the Sentinel-3B satellite and the Moderate Resolution Imaging Spectroradiometer (MODIS) of the Terra satellite to analyze the thematic accuracy of burned area maps and their sensitivity under different spectral resolutions in a large area of 32,000 km2 in the Cerrado biome from 2019 to 2021. The methodology used training and the Support Vector Machine (SVM) classifier. To analyze the spectral peculiarities of each orbital platform, the Transformed Divergence (TD) index separability statistic was used. The results showed that for both sensors, the near-infrared (NIR) band has an essential role in the detection of the burned areas, presenting high separability. Overall, it was possible to observe that the spectral mixing problems, registration date, and the spatial resolution of 500 m were the main factors that led to commission errors ranging between 15% and 72% and omission errors between 51% and 86% for both sensors. This study showed the importance of multispectral sensors for monitoring forest fires. It was found, however, that the spectral resolution and burning date may gradually interfere with the detection process.

Luminescence spectral peculiarities of polymethine dye, bonded covalently to polyurethane matrix

Luminescence spectral properties of the two symmetrical indocyanine dyes with different substituents near nitrogen atoms have been investigated in the polyurethane matrices. The analysis of the IR-spectra, measured for the product of a reaction between indolenine salt, being a terminal heterocyclic fragment in one of these dye molecules, and phenyl isocyanate, and this salt and isocyanate as single bulk compounds has shown possibility of covalent bonding between this indocyanine and polyurethane. Such covalent bonding of the dye with the polymer matrix results in improvement of its luminescence spectral properties, compared to the dye-polymer solutions.

Energy and daylighting performance of building integrated spirooxazine photochromic films

The study of chromogenic materials and systems is particularly promising for innovative, transparent building envelopes, with thermo-optical properties adaptable to surrounding environmental conditions. This work spots light on the multiple effects of photochromic glazing on the energy consumption of buildings and on the impact that such technologies would have on the visual comfort of occupants. To our knowledge, this is the first work dealing with building integration of spirooxazine-based photochromic films. This experimental and theoretical work aims helping to fill this gap by reporting the results of a study concerning the spirooxazine photochromic molecules, integrated in transparent matrices of polymethylmethacrylate. This work discloses the potential of specific photochromic materials, especially as a function of spectral peculiarities. The figures of merit of this technology have been studied by assuming its integration in an ideal multi-storey office building, and by studying the effects in terms of energy consumption and visual comfort and proposing a comparison with several static commercial glazing technologies. The photochromic glazing demonstrated to offer significant reduction of energy use for cooling, compared to a clear glass (with yearly saving of 4079 kWh, on the Southern facade), and for artificial lighting (with a saving of 3711 kWh), if compared to the commercial solar control glazing. Furthermore, the dynamic behaviour of the photochromic glazing represented the best choice in terms of visual comfort.

Spectral Peculiarities of Products of Special Matrices

Spectral Peculiarities of Products of Special Matrices

Tuneable interplay of plasmonic and molecular excitations in self-assembled silver - fullerene nanocomposites

Elaboration of functional hybrid nanostructures integrating noble metal nanoparticles (NPs) is an attractive topic in nanoplasmonics prompting unique technologies. Here we report on remarkable plasmonic properties of the self-assembled AgxC60 nanocomposite (NC) films, consisting of dense 3D-assembly of small Ag NPs and supporting C60 medium. Systematic study of optical absorption spectra allowed us to disclose the spectral peculiarities of plasmonic and molecular excitations in the AgxC60 NC films in the broad range of Ag concentrations, 0<x < 30. Analysis of the plasmonic modes evaluated in this study designates intriguing features of the Ag NP plasmonic coupling, which are found to be rather different in the NC films with x<10 and x>15. We found that increase of x in the range of 0<x < 10 results in strong enhancement of the electric dipole plasmon mode, whereas optical spectra of the NC films with x > 15 demonstrate pronounced efficiency of tunnelling charge transfer plasmons. Careful analysis of the detected excitations revealed evident conformity in the x-induced alterations of the plasmonic and excitonic modes as hallmarks of plasmon–exciton interactions. The detected plasmonic effects demonstrate great potential of this metal-organic system in molecular sensing, photocatalysis and optoelectronics.

Magnetic chemically peculiar stars investigated by the Solar Mass Ejection Imager

ABSTRACT Since the discovery of the spectral peculiarities of their prototype α2 Canum Venaticorum in 1897, the so-called ACV variables, which are comprised of several groups of chemically peculiar stars of the upper main sequence, have been the target of numerous photometric and spectroscopic studies. Especially for the brighter ACV variables, continuous observations over about a century are available, which are important to study long-term effects such as period changes or magnetic cycles in these objects. This work presents an analysis of 165 Ap/CP2 and He-weak/CP4 stars using light curves obtained by the Solar Mass Ejection Imager (SMEI) between the years 2003 and 2011. These data fill an important gap in observations for bright ACV variables between the Hipparcos and TESS satellite missions. Using specifically tailored data treatment and period search approaches, we find variability in the accuracy limit of the employed data in 84 objects. The derived periods are in excellent agreement with the literature; for one star, the here presented solution represents the first published period. We discuss the apparently constant stars and the corresponding level of non-variability. From an investigation of our target star sample in the Hertzsprung–Russell diagram, we deduce ages between 100 Myr and 1 Gyr for the majority of our sample stars. Our results support that the variable CP2/4 stars are in a more advanced evolutionary state and that He and Si peculiarities, preferentially found in the hotter, and thus more massive, CP stars, produce larger spots or spots of higher contrast.

New Type of Short High‐Frequency VLF Patches (“VLF Birds”) Above 4–5 kHz

AbstractThe new type of daytime natural very low frequency (VLF) whistler mode emissions of the magnetospheric origin was found in the VLF observations at Kannuslehto station (L ∼ 5.5) in Northern Finland. The events occurred at the frequencies above 4–5 kHz even up to 15 kHz. These emissions have never been observed earlier in the spectrograms because they were hidden by strong impulsive atmospherics (sferics) in the same frequency range originating in lightning discharges. After filtering out the sferics, we surprisingly discovered completely new types of high frequency right‐hand polarized peculiar VLF emissions with various unusual dynamic spectra. These new VLF emissions typically occur as a sequence of a number of short (∼1–3 min) burst‐like structures or single short patches with the total duration up to a few hours. The spectral peculiarities of several long‐lasting VLF events as well as individual short VLF patches are discussed. The dynamic spectra of these new VLF patches raise the question of the temporal and spatial details of the wave‐particle interactions in the magnetospheric plasma. These emissions are observed only under quiet or weakly disturbed space weather conditions, but during small negative values of the Dst index, indicating the presence of a certain excess of the radiation belt electrons. Note, these high frequency VLF patches are observed at the frequencies much higher than the half of the equatorial electron gyrofrequency which is equal to ∼2.7 kHz at L ∼ 5.5. It seems that these emissions are generated deep in the magnetosphere, but the detailed nature, generation region, and propagation behavior of these newly discovered VLF emissions remain unknown. An appearance of high frequency VLF patches could be an indirect indicator of a local enhancement of electron fluxes in the radiation belt that are not directly measured and may occur even in the absence of visible ground‐based geomagnetic disturbances. Further researches may shed new light on wave‐particle interactions occurring in the Earth's radiation belts.

The GALAH survey: characterization of emission-line stars with spectral modelling using autoencoders

ABSTRACT We present a neural network autoencoder structure that is able to extract essential latent spectral features from observed spectra and then reconstruct a spectrum from those features. Because of the training with a set of unpeculiar spectra, the network is able to reproduce a spectrum of high signal-to-noise ratio that does not show any spectral peculiarities, even if they are present in an observed spectrum. Spectra generated in this manner were used to identify various emission features among spectra acquired by multiple surveys using the HERMES spectrograph at the Anglo-Australian telescope. Emission features were identified by a direct comparison of the observed and generated spectra. Using the described comparison procedure, we discovered 10 364 candidate spectra with varying intensities (from partially filled-in to well above the continuum) of the Hα/Hβ emission component, produced by different physical mechanisms. A fraction of these spectra belong to the repeated observation that shows temporal variability in their emission profile. Among the emission spectra, we find objects that feature contributions from a nearby rarefied gas (identified through the emission of [N ii] and [S ii] lines) that was identified in 4004 spectra, which were not all identified as having Hα emission. The positions of identified emission-line objects coincide with multiple known regions that harbour young stars. Similarly, detected nebular emission spectra coincide with visually prominent nebular clouds observable in the red all-sky photographic composites.

Modeling of Electronic Spectra of Ionic Forms of Eosin and Erythrosin

Multistage dissociation of fluoroscein dyes, widely used in biological labeling, yields a variety of ionic and tautomeric forms in a wide range of pH values. In contrast to well-studied absorption spectra, the emission spectra are not quite readily interpreted due to their strong overlapping and proton transfer in electronically excited states. The least studied are the fluorescent properties of eosin and erythrosin dyes containing heavy atoms (Br, I), in which the characteristics of the dianionic form only are reliably determined. In the framework of the density functional theory using the B3LYP-functional including nonequilibrium solvation, the geometries of the series of ionic forms of eosin and erythrosin in the ground and excited states are found, and the electronic spectra are calculated. Based on the identified linear regression of the calculated and experimental data for the earlier resolved electronic spectra, for the first time, the emission spectrum maxima of the monoanionic, neutral quinoid, and cationic forms of the dyes are determined. The spectral peculiarities (Stokes shifts) are discussed in terms of variation of the molecule and ion geometries in the ground and excited states.

Spectral Peculiarities of Multilayer Cholesteric Wedge-Cell System with Dye-Doped Polymer Layer

Spectral properties of the three-layered wedge-cell system of two identical cholesteric layers with an isotropic defect (dye-doped polymer layer) between them were investigated experimentally and theoretically. It was shown that multiple defect modes can be observed in this kind of system’s photonic band gap which widen the application range of system mentioned above such as low threshold lasing, multi-position trigger, multiwavelength filters, light shutters, etc. The supporting simulation was also provided showing an agreement between experimental results and theoretical calculations. The problem was solved by Ambartsumian’s layer addition modified method.

Spectral behavior peculiarities of the carboxyl functional group in iodine encirclement - the case of 2,6-diiodo-3,4,5-trimethoxybenzoic acid

A new diiodo-containing compound, fully substituted derivative of benzoic acid was successfully synthesized in quantitative yield using iodine/silver trifluoroacetate as an efficient iodinium donating reagent. The obtained 2,6-diiodo-3,4,5-trimethoxybenzoic acid was analyzed structurally by FT-IR, 1H NMR, and 13C NMR spectroscopic techniques and was additionally compared with optimized structure and harmonic vibrational frequencies predicted by quantum chemical calculations. Peculiar spectral features as a result of intramolecular interactions between neighboring iodo and carboxyl functionalities were in a particular focus in the present study.

Photoluminescence of two-dimensional plasmonic structures: enhancement, spectral and lifetime peculiarities below the lasing threshold

We study the interplay between optical properties and photoluminescence spectra of structures based on square lattices of silver nanodiscs or nanoholes in silver embedded in or adjoined to a dye-doped polymer waveguiding layer. The lattices and the waveguiding gain layer were designed to overlap the spectral features of such two-dimensional (2D) plasmonic structures. By varying the lattice constant, we demonstrate a condition for a significant increase of photoluminescence intensity and compare lifetime reduction for both nanodiscs- and nanoholes-based structures, when pumping them below lasing threshold. These enhancements are reached by matching the dark modes of the structure with the gain medium fluorescence spectrum.

Gamma-Ray Dark Matter Searches in Milky Way Satellites—A Comparative Review of Data Analysis Methods and Current Results

If dark matter is composed of weakly interacting particles with mass in the GeV-TeV range, their annihilation or decay may produce gamma rays that could be detected by gamma-ray telescopes. Observations of dwarf spheroidal satellite galaxies of the Milky Way (dSphs) benefit from the relatively accurate predictions of dSph dark matter content to produce robust constraints to the dark matter properties. The sensitivity of these observations for the search for dark matter signals can be optimized thanks to the use of advanced statistical techniques able to exploit the spectral and morphological peculiarities of the expected signal. In this paper, I review the status of the dark matter searches from observations of dSphs with the current generation of gamma-ray telescopes: Fermi-LAT, H.E.S.S, MAGIC, VERITAS and HAWC. I will describe in detail the general statistical analysis framework used by these instruments, putting in context the most recent experimental results and pointing out the most relevant differences among the different particular implementations. This will facilitate the comparison of the current and future results, as well as their eventual integration in a multi-instrument and multi-target dark matter search.