Radiative Spectra Research Articles

Medium-induced radiative kernel with the Improved Opacity Expansion

We calculate the fully differential medium-induced radiative spectrum at next-to-leading order (NLO) accuracy within the Improved Opacity Expansion (IOE) framework. This scheme allows us to gain analytical control of the radiative spectrum at low and high gluon frequencies simultaneously. The high frequency regime can be obtained in the standard opacity expansion framework in which the resulting power series diverges at the characteristic frequency ωc ∼ hat{q} L2. In the IOE, all orders in opacity are resumed systematically below ωc yielding an asymptotic series controlled by logarithmically suppressed remainders down to the thermal scale T « ωc, while matching the opacity expansion at high frequency. Furthermore, we demonstrate that the IOE at NLO accuracy reproduces the characteristic Coulomb tail of the single hard scattering contribution as well as the Gaussian distribution resulting from multiple soft momentum exchanges. Finally, we compare our analytic scheme with a recent numerical solution, that includes a full resummation of multiple scatterings, for LHC-inspired medium parameters. We find a very good agreement both at low and high frequencies showcasing the performance of the IOE which provides for the first time accurate analytic formulas for radiative energy loss in the relevant perturbative kinematic regimes for dense media.

Free switch between bound states in the continuum (BIC) and quasi-BIC supported by graphene-metal terahertz metasurfaces

An important topic of modern nanophotonics focuses on the bound states in the continuum (BIC) that the non-radiative localized waves lie within the radiative continuous spectrum, which has been introduced into metasurfaces to construct a strong resonance system. The development of theory and application promotes the advanced physics research on metasurfaces supporting dynamic BIC. However, the dynamic conversion between quasi-BIC and BIC of the metasurfaces is still not realized in strict sense. Herein, we report a significant progress that the free switch between BIC and quasi-BIC is successfully achieved by introducing the structured graphene into the metal terahertz metasurfaces, which is based on free-electron concentration of graphene changing from semiconductor-like to metal-like with adjustment of Fermi energy. We propose the efficient-structure model to explain these phenomena, and prove the wide applicability of this model by investigating seven class metal structures which includes a parameter-tuned metasurface and six common symmetry-protected metasurfaces. The further investigation shows such graphene-metal metasurface array has potential application in dynamic display, and it only needs to change the bias voltage applied on each structure to obtain different display contents, without the requirement of preparing additional metasurface array, which is impossible to achieve via traditional static metasurfaces.

Electronic heat current fluctuations in a quantum dot

The fluctuations of the heat current in a quantum dot coupled to electron reservoirs are calculated at finite frequency, voltage, and temperature using the nonequilibrium Green function technique. The nonsymmetrized heat noise is expressed as an integral on energy containing four contributions, each of which includes transmission amplitudes, electron-hole pair distribution functions, and energy difference factors. The effect of the asymmetry of the couplings between the quantum dot and the reservoirs is studied. Features of the heat noise are highlighted and discussed for an equilibrium and an out-of-equilibrium quantum dot. In the latter case and within the high transmission limit, the heat noise is closely related to the radiative power spectrum, leading to an out-of-equilibrium Planck's law. Proposals for the measurement of the heat noise are discussed.

Monte-Carlo calculated detector response functions to unfold radiative neutron capture spectra

Results of geant4 Monte Carlo simulations carried out on the Budapest PGAA detector are presented in this paper. A complete set of detector response functions, i.e. the gamma spectra corresponding to incremental gamma-ray energies up to 12 MeV, were obtained by simulations and used to unfold the experimental gamma spectra of60Co and152Eu. The unfolding successfully removed the continuous Compton-background and the escape peaks related to a full-energy peak but preserved the shape and area of the peak itself. We finally demonstrated the applicability of this approach in determining the total radiative neutron capture cross-sections of the14N(n, γ) reaction, where an excellent agreement with literature data was found.

Improved opacity expansion for medium-induced parton splitting

We revisit the calculation of the medium-induced gluon radiative spectrum and propose a novel expansion scheme that encompasses the two known analytic limits: i) the high frequency regime dominated by a single hard scattering that corresponds to the leading order in the standard opacity expansion, ii) the low frequency regime that is dominated by multiple soft scatterings. Our approach is based on expanding around the harmonic oscillator instead of vacuum in the leading logarithmic approximation. We compute the first two orders in this improved opacity expansion and show that they account for the aforementioned limits.

Measurement of Møller scattering at 2.5 MeV

Moller scattering is one of the most fundamental processes in QED. Understanding it to high precision is necessary for a variety of modern nuclear and particle physics experiments. In a recent calculation, existing soft-photon radiative corrections were combined with new hard-photon bremsstrahlung calculations to take into account the effect of photon emission at any photon energy, where the electron mass was included at all steps. To test the calculation, an experiment was carried out using the 3 MV Van de Graaff electrostatic accelerator at the MIT High Voltage Research Laboratory. Momentum spectra at three scattering angles at an incident electron energy of 2.5 MeV are reported here, and compared to the simulated radiative Moller spectra, based on our previous calculation. Good agreement between the measurements and our calculation is observed in the momentum spectrum at the three angles.

Spectral Diversities of Gamma-Ray Bursts in High-energy Bands: Hints from Turbulent Cascade

Abstract We statistically examine the gamma-ray burst (GRB) photon indices obtained by the Fermi-GBM and Fermi-LAT observations and compare the LAT GRB photon indices to the GBM GRB photon indices. We apply the jitter radiation to explain the GRB spectral diversities in the high-energy bands. In our model, the jitter radiative spectral index is determined by the spectral index of the turbulence. We classify GRBs into three classes depending on the shape of the GRB high-energy spectrum when we compare the GBM and LAT detections: the GRB spectrum is concave (GRBs turn out to be softer and are labeled as S-GRBs), the GRB spectrum is convex (GRBs turn out to be harder and are labeled as H-GRBs), and the GRBs have no strong spectral changes (labeled as N-GRBs). A universal Kolmogorov index 7/3 in the turbulent cascade is consistent with the photon index of the N-GRBs. The S-GRB spectra can be explained by the turbulent cascade due to the kinetic magnetic reconnection with the spectral index range of the turbulence from 8/3 to 3.0. The H-GRB spectra originate from the inverse turbulent cascade with the spectral index range of the turbulence from 2.0 to 3.5 that occurred during the large lengthscale magnetic reconnection. Thus, the GRB radiative spectra are diversified because the turbulent cascade modifies the turbulent energy spectrum. More observational samples are expected in the future to further identify our suggestions.

Radiative and Auger cascade following [formula omitted] vacancy creation in Fe2＋

Radiative and Auger cascade following the 2p shell vacancy creation in the Fe2＋ ion is studied by analysing transitions among energy levels and subconfigurations. Branching ratios of the cascade are analysed and main decay mechanisms are identified for all levels of the Fe3＋2p53d6 configuration. The study shows that ion yield strongly depends on the level the cascade starts from. The cascade produces the Fe4＋ and Fe5＋ ions with the highest population. The ion yields are presented for all levels of the Fe3＋2p53d6 configuration. The time-dependent study for population of the ions demonstrates that it takes approximately 10−16 s for the Fe4＋ ions to appear. The final states of the cascade are reached in ∼10−3 s. Radiative and Auger spectra produced by cascade span the energy region up to ∼780eV.

Depopulation mechanism for incoherent terahertz source – THz torch – based on GaAsBi/GaAs quantum well in GaAs/AlGaAs parabolic quantum well

Parabolic quantum wells (PQWs) are known as a promising candidate for a compact terahertz (THz) source. PQWs have equidistant subbands that can be designed to be separated by few meV to meet the THz frequency range. To enhance the efficiency and power of THz emission from PQWs, a new approach is proposed by employing depopulation of the lowest subbands of PQW. In this work, the theoretical analysis of an incoherent THz torch device is presented. The findings suggest that the introduction of narrower band-gap GaAsBi/GaAs rectangular quantum well within the GaAs/AlGaAs PQW can alter subbands arrangement to enable a faster depopulation mechanism exploiting LO phonon scattering. The calculated radiative power spectra show the increase of oscillator strength between the rearranged subbands of PQW due to the added GaAsBi rectangular potential. The increased intersubband radiative transition probability can lead to an efficient compact incoherent THz source – THz torch.

Analysis of various electron blocking layers to improve efficiency in green light-emitting diodes

Efficiency of GaN-based green light-emitting diodes with various electron blocking layers has been studied numerically. By employing conventional AlGaN, graded AlGaN, quaternary AlGaInN and ternary AlInN electron blocking layers, performance of each device has been evaluated. The device characteristics presented include carrier transport, radiative recombination rate, electrostatic field, emission spectra and efficiency.

Radiative and Auger cascades in Fe3+

SynopsisRadiative and Auger cascades following creation of the 2s or 2p shell vacancy in the Fe2+ ion are studied by performing calculations for energy levels and subconfigurations. Radiative and Auger spectra and charge state distribution determined by cascade are investigated. Branching ratios of the cascade are analyzed and main decay mechanisms are identified for all levels of the studied configurations.

Cascade of elementary processes in Se1+

SynopsisAnalysis of radiative and Auger cascade following creation of the K shell vacancy in the Se atom is presented. Calculations of radiative and Auger transition probabilities are performed by using Dirac-Fock-Slater approximation. Radiative and Auger spectra as well as ion yield produced by cascade are studied.

The Belgian Repository of Fundamental Atomic Data and Stellar Spectra (BRASS)

Background: BRASS (Belgian Repository of Fundamental Atomic Data and Stellar Spectra) is an international networking project for the development of a new public database providing accurate fundamental atomic data of vital importance for stellar spectroscopic research. We present an overview of research results obtained in the past four years. Methods: The BRASS database offers atomic line data we thoroughly tested by comparing theoretical and observed stellar spectra. We perform extensive quality assessments of selected atomic input data using advanced radiative transfer spectrum synthesis calculations, which we compare to high-resolution Mercator-HERMES and ESO-VLT-UVES spectra of F-, G-, and K-type benchmark stars observed with very high signal-to-noise ratios. We have retrieved about half a million atomic lines required for our detailed spectrum synthesis calculations from the literature and online databases such as VAMDC, NIST, VALD, CHIANTI, Spectr-W 3 , TIPbase, TOPbase, SpectroWeb. Results: The atomic datasets have been cross-matched based on line electronic configuration information and organized in a new online repository called BRASS. The validated atomic data, combined with the observed and theoretical spectra are also interactively offered in BRASS. The combination of these datasets is a novel approach for its development providing a universal reference for advanced stellar spectroscopic research. Conclusion: We present an overview of the BRASS Data Interface developments allowing online user interaction for the combined spectrum and atomic data display, line identification, atomic data accuracy assessments including line log(gf)-values, and line equivalent width measurements.

DFT Study of Photochemical Properties and Radiative Forcing Efficiency Features of the Stereoisomers cis- and trans-CHCl═CH-CF3.

The accurate assessment of radiative forcing efficiency (RFE) of a greenhouse gas is based on the precise knowledge of its structure and infrared absorption spectrum. The present work investigates the UV-vis absorption spectra and IR absorption spectra that are used for the determination of RFE of the short-lived compounds, cis- and trans-CHCl═CHCF3 (CTFP). These investigations were carried out with six different density functional theory (DFT) methods B3LYP, CAM-B3LYP, M06, M06-2X, TPSS0, and ωB97X-D associated to the basis set 6-31G(3df). Therefrom, the relative populations of the two states cis and trans for temperatures over the range 220-370 K at 1 atm and along the atmospheric altitude were assessed. It turns out that trans-CTFP is the abundant component between the two states. This review reveals that B3LYP and M06 reproduce well the experimental results of UV-vis spectra of trans-CTFP. As for cis-CTFP, EOM-CCSD is not well fitted by DFT methods. The cis- to trans-CTFP isomerization leads to the red shift for DFT methods and to the blue shift in regard to EOM-CCSD and experimental results. The IR absorption spectra are well fitted by B3LYP over the range 500-1600 cm-1 and TPSS0 over 1300-2000 cm-1 for both stereoisomers. Moreover, the root-mean-square errors (RMSEs) of frequencies from experimental data are lower for B3LYP and TPSS0 for both systems. The computed IR absorption band strengths over 500-2000 cm-1 for cis- and over 600-1800 cm-1 for trans-CTFP are consistent with the experiment. The relevant descriptor RFEs of the climate effect were calculated using a narrow band model for a constant vertical profile and then corrected with a lifetime factor for different computational methods. The computed values correlate well with the experimental results for both stereoisomers except M06-2X and TPSS0. It is worth noting that, for both systems, the intense radiative forcing spectra are located at frequencies ranging in 1000-1200 cm-1. The lower forcings of trans-CTFP lying in the atmospheric window region 800-1000 cm-1 are greater than those of cis-CTFP. Therefore, RFE(trans-CTFP) = 1.127 RFE(cis-CTFP).

Parton energy loss and the generalized jet transport coefficient

We revisit radiative parton energy loss in deeply inelastic scattering (DIS) off a large nucleus within the perturbative QCD approach. We calculate the gluon radiation spectra induced by double parton scattering in DIS without collinear expansion in the transverse momentum of initial gluons as in the original high-twist approach. The final radiative gluon spectrum can be expressed in terms of the convolution of hard partonic parts and unintegrated or transverse momentum dependent (TMD) quark-gluon correlations. The TMD quark-gluon correlation can be factorized approximately as a product of initial quark distribution and TMD gluon distribution which can be used to define the generalized or TMD jet transport coefficient. Under the static scattering center and soft radiative gluon approximation, we recover the result by Gylassy-Levai-Vitev (GLV) in the first order of the opacity expansion. The difference as a result of the soft radiative gluon approximation is investigated numerically under the static scattering center approximation.

Effect of atmospheric water vapor on radiative cooling performance of different surfaces

The sub-ambient clear sky daytime radiative cooling has been achieved recently by applying advanced nanofabrication technologies, which attracted significant research attention in this area. However, there are also several researches which reported that radiative coolers with excellent thermal radiative spectrum failed to achieve sub-ambient radiative cooling. To elucidate the difference, in this work, we perform a detailed investigation of ambient conditions, in particular, water vapor density, on the performance of radiative coolers. We first present the details of how to estimate the performance of radiative coolers under different water vapor concentration and simulate the transmittance and emitted radiation of atmosphere as well as the cooling performance of the ideal selective emitter, which finds the cooling power decreases by 86.6 W/m2 with increasing total water vapor column. Furthermore, the cooling performance of an integrated photonic solar reflector and thermal emitter with atmospheric profiles in California and Hong Kong were calculated and compared with experiment result. To verify our calculation, nighttime on-site cooling performance of a reference film (Al foil) and two selective emitters (DESR-M and double-layer coating) was measured in several ambient conditions. Our research reveals how the ambient humidity affects the radiative cooling performance by both theoretical and experimental analysis and is important for the further application development of radiative coolers.

Characterization of radiative heat transfer in a spark-ignition engine through high-speed experiments and simulations

A combined experimental and Large-Eddy Simulation (LES) study of molecular radiation is presented for combustion in a homogeneous pre-mixed spark-ignition engine. Molecular radiation can account for ~10% of the engine heat loss and could have a noticeable impact on the local conditions within the combustion chamber. The Transparent Combustion Chamber (TCC) engine, a single-cylinder two-valve research engine with a transparent liner and piston for optical access, was used for this study. High-speed infrared emission spectroscopy and radiative post-processing of LES calculations have been performed to gain insight into the timescales and magnitude of radiative emissions of molecular gases during the combustion process. Both the measurements and simulations show significant Cycle-to-Cycle Variations (CCV) of radiative emission. There is agreement in the instantaneous radiative spectrum of experiment and simulation, but the crank-angle development of the radiative spectrum shows disagreement. The strengths and limitations of the optical experiments and radiative simulations are seen in the results and suggest pathways for future efforts in characterizing the influence of molecular radiation. In particular, focusing on the relative changes of the spectral features will be important as they contain information about the thermochemical properties of the gas mixture.

Synergistic effect of (3-Aminopropyl)Trimethoxysilane treated ZnO and corundum nanoparticles under UV-irradiation on UV-cutoff and IR-absorption spectra of acrylic polyurethane based nanocomposite coating

ZnO and corundum (α–Al2O3) nanoparticles were successfully synthesized by aqueous precipitation and sintering techniques respectively. ZnO nanoparticles were effectively coated with (3-aminopropyl)trimethoxysilane (APTMS) by polycondensation method to prevent the photocatalytic activity of ZnO during a UV-weathering study. X-ray diffractogram and FTIR were used to confirm the crystalline structure of as prepared nanoparticles, blue shift of the AlO bond and the formation of a secondary amine via polycondensation of APTMS over ZnO surface. The prepared APTMS-ZnO, corundum and commercially available surface modified hydrophobic SiO2 (M − SiO2) nanoparticles were used to prepare the acrylic polyurethane (AP: Poly-Macrynal® SM 510 N coating resign) bases nanocomposite coating on a polyurethane substrate. Individual and mixed nanoparticles were dispersed into acrylic polyurethane to prepare the coating layer on polyurethane film substrate separately. IR-active and UV–visible regions of the FTIR and UV–Vis spectroscopies were used to investigate the synergistic effect of the nanoparticles on a selected range of the radiative spectrum, especially the UV-resistant and IR-absorption properties of the coated films with and without exposure of UV-irradiations. Polyurethane substrate coated with APTMS-ZnO (2 wt%) based acrylic polyurethane-based nanocomposite coating (APUC) layer containing 2 wt% corundum (D50) and 6 wt% M − SiO2 (F50) exhibited 98.77% and 97.60% of UV-resistant property respectively. These results indicate that the visible light transparency and transmittance ability reduced significantly after 500 h of UV-irradiation exposure. Both of the activity and deformation have great impact on the IR-absorption property of the APUC.

Ion population fraction calculations using improved screened hydrogenic model with l-splitting

Ion population fraction (IPF) calculations are very important to understand the radiative spectrum emitted from the hot dense matter. IPF calculations require detailed knowledge of all the ions and correlation interactions between the electrons of an ion which are present in a plasma environment. The average atom models, e.g., screened hydrogenic model with l-splitting (SHML), now have the capabilities for such calculations and are becoming more popular for in line plasma calculations. In our previous work [Ali A, Shabbir Naz G, Shahzad M S, Kouser R, Rehman A and Nasim M H 2018 High Energy Density Phys.26 48], we have improved the continuum lowering model and included the exchange and correlation effects in SHML. This study presents the calculation of IPF using classical theory of fluctuation for our improved screened hydrogenic model with l-splitting (I-SHML) under local thermodynamic equilibrium conditions for iron and aluminum plasma over a wide range of densities and temperatures. We have compared our results with other models and have found a very good agreement among them.

Correct determination of charge transfer state energy from luminescence spectra in organic solar cells

Generation and recombination of electrons and holes in organic solar cells occurs via charge transfer states located at the donor/acceptor interface. The energy of these charge transfer states is a crucial factor for the attainable open-circuit voltage and its correct determination is thus of utmost importance for a detailed understanding of such devices. This work reports on drastic changes of electroluminescence spectra of bulk heterojunction organic solar cells upon variation of the absorber layer thickness. It is shown that optical thin-film effects have a large impact on optical out-coupling of luminescence radiation for devices made from different photoactive materials, in configurations with and without indium tin oxide. A scattering matrix approach is presented which accurately reproduces the observed effects and thus delivers the radiative recombination spectra corrected for the wavelength-dependent out-coupling. This approach is proven to enable the correct determination of charge transfer state energies.