Function Of Photon Energy Research Articles

Comparative DFT study of vibrational, electronic, and optical properties of energetic alkali metal salts based on nitrogen-rich 5-aminotetrazole.

This article presents a thorough density functional theory based comparative study on nitrogen-rich 5-aminotetrazole alkali metal salts M 5-At (M = Li, K, Rb, Cs). The calculated structural parameters using plane-wave pseudopotential method are consistent with the experimental results. The computed vibrational frequencies at ambient pressure show that vibrational modes in high energy region are due the NH bond of NH2 group. Pressure variation IR spectra of these materials show clear frequency shifts where Li 5-At shows an overall red shift below 900 cm-1 contrary to the blue shift seen in other materials in this range. The born effective charge values reveal the presence of strong covalency between N, H, and C atoms whereas an increased ionic nature is seen as the atomic number of metal atoms increases. Furthermore, we used full potential linear augmented plane wave (FP-LAPW) method for calculating electronic structure and optical properties with TB-mBJ potential which provides an enhanced band gap for all materials compared to standard GGA functional. Electronic structure calculation reveals that all the compounds are indirect band gap insulators with the exception of Li 5-At. The computed partial density of states show mixed ionic-covalent nature in metal-N/C bonds and covalent nature in NC bonds. In addition, we are also presenting the optical properties such as real and imaginary dielectric constant, absorption, refraction, reflection, loss spectrum as functions of photon energy. From the optical properties we can conclude that all the studied compounds are optically anisotropic in nature and are good absorbers in the ultraviolet (UV) region.

Polyethylene Identification in Ocean Water Samples by Means of 50 keV Energy Electron Beam

This study presents a new methodology to reveal traces of polyethylene (the most common microplastic particles, known as a structure of C2H4) in a sample of ocean water by the irradiation of a 50 keV, 1 µA electron beam. This is performed by analyzing the photon (produced by the electrons in water) fluxes and spectra (i.e., fluxes as a function of photon energy) with different types of contaminated water using an adequate device and in particular looking at the peculiar interactions of electrons/photons with the potential abnormal atomic hydrogen (H), oxygen (O), carbon (C), and phosphorus (P) compositions present in the water, as a function of living and nonliving organic organisms with PO4 group RNA/DNA strands in a cluster configuration through a volumetric cells grid.

Ab initio study of the structural, electronic, optical and elastic properties of promising optoelectronic and thermoelectric compounds MgSc2X4 (X = S; Se)

In this paper, we study theoretically the structural, electronic, optical, elastic and thermoelectric properties of MgSc2X4 (X ​= ​S; Se) compounds using first-principles calculations. The calculated band structure reveals that MgSc2S4 and MgSc2Se4 are direct-gap semiconductors. The optical features of MgSc2S4 and MgSc2Se4 as functions of photon energy were calculated for energy range of 0–20 ​eV. MgSc2X4 (X ​= ​S; Se) are promising optoelectronic, and thermoelectric compounds whose absorption rates can reach 4 orders of magnitude in the visible range, and 6 orders of magnitude in ultraviolet range. Both chalcogenides possess high brittleness with the Pugh’s ratio (B/G), and Cauchy pressure C12−C44 of 1.60 and −4.485 GPa (for MgSc2Se4); 1.61 and −3.655 GPa (for MgSc2S4). MgSc2S4 and MgSc2Se4 reveal similar ZTe values. The obtained properties of MgSc2X4 (X ​= ​S; Se) are well confirmed by experimental results.

Photo-neutron cross-section of $${}^{{\mathrm {nat}}}{\mathrm {Dy}}$$ in the bremsstrahlung end-point energies of 12, 14, 16, 65, and 75 MeV

The flux-weighted average cross-sections of $${}^{\mathrm {nat}}{\mathrm {Dy}}(\upgamma , {\mathrm {xn}}){}^{{159,157,155}}{\mathrm {Dy}}$$ reactions were measured at the bremsstrahlung end-point energies of 12, 14, 16, 65 and 75 MeV with the activation and off-line $$\upgamma $$ -ray spectrometric technique using the 20 MeV Electron Linac for beams with high Brilliance and low Emittance (ELBE) at Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany and the 100 MeV electron linac at the Pohang Accelerator Laboratory, Korea. The $${}^{\mathrm {nat}}{\mathrm {Dy}}(\upgamma , {\mathrm {xn}}){}^{{157,155}}{\mathrm {Dy}}$$ reaction cross-sections as a function of photon energy were also calculated theoretically using TALYS 1.9 code. Then the flux-weighted average values at different end-point energies were obtained based on the theoretical values of mono-energetic photons. These values were compared with the flux-weighted values of present work and are found to be in general agreement. It was also found that the experimental and theoretical formation cross sections of $${}^{159}{\mathrm {Dy}}$$ , $${}^{157}{\mathrm {Dy}}$$ and $${}^{155}{\mathrm {Dy}}$$ from the $${}^{\mathrm {nat}}{\mathrm {Dy}}(\upgamma , {\mathrm {xn}})$$ reactions increased from their respective threshold values to a certain energy where other reaction channels opened. After reaching a maximum value, the individual reaction cross-sections slowly decreased with the increase of the bremsstrahlung energy due to the initiation of other competing reactions at higher energy, which indicates the impact of the excitation energy. However, the production cross sections of $${}^{157}{\mathrm {Dy}}$$ and $${}^{155}{\mathrm {Dy}}$$ from the $${}^{\mathrm {nat}}{\mathrm {Dy}}(\upgamma , {\mathrm {xn}})$$ reactions slightly increase in between and then decreased slowly with bremsstrahlung energy, which is due to the contributing reactions of higher mass isotopes.

Extension of the Stoichiometric Calibration of CT Hounsfield values to Metallic Materials

Abstract Nowadays, patients with metallic implants undergoing radiotherapy may suffer from inaccuracy in the treatment plan caused by the implant. To ensure a precise plan an accurate relation between Hounsfield values of the computer tomographic (CT) images and the electron density of the elements and material mixtures is indispensable. In order to extend the stoichiometric calibration approach known for tissues to the regime of metallic materials, the basic physical equations as well as approximations in the parametrization and fitting are carefully reviewed. CT images of a standard calibration phantom and pure metallic samples up to the atomic number Z = 29 were acquired for various energies. Hounsfield values were determined on an extended Hounsfield scale which allows the mapping of material having high atomic number Z. It is found that from basic physics an empirical factorization of the cross-sections into a function of Z and a function of photon energy E is not allowed over a wide range of Z. Specifically, the parameterization for tissue like materials cannot be prolonged to materials with high-Z. Thus, the calibration is subdivided into regions of materials and its accuracy is quantified in each region. It depends, among others, on the knowledge of the X-ray photon spectra, the segmentation of the material samples and the empirical parameterization of the linear-attenuation coefficient.

Uniaxial Strain and Optical properties of GaP Nanowires

The optical properties for wurtzite (WZ) Bulk GaP and GaP nanowires (GaP-NWs) in hexagonal(circular), triangular and square cross sectional are investigated by using full potential linear augmented plane waves and local orbitals (FP-LAPW-LO) method based on density functional theory (DFT). The wurtzite nanowires are investigated with diameters limited to ~27 oA. The optical functions as a function of photon energy have been investigated for unstrained bulk WZ-GaP and WZ-GaP-NWs in the first part. Finally, the optical functions are investigated for WZ-GaP-NWs by applying uniaxial strain from -5% to +5%. The effect of cross sectional variation in the area and shape of the NWs on the optical functions is clear when compared with bulk GaP. GaP-NWs kept real dielectric constant positive and refractive index close to vacuum value for a wider range of energy on the contrary of bulk GaP. It is found that the dielectric constants are slightly increased from the unstrained GaP-NWs values with strain increases. A shift in the threshold energy for toward higher energy as the strain changes from tensile to compressive. The refractive index shows a small change when passing from compressive toward tensile strain. It is found that GaP-NWs could play a role in solar cells structure and high speed electronic circuits.

Uniaxial Strain and Optical properties of GaP Nanowires

The optical properties for wurtzite (WZ) Bulk GaP and GaP nanowires (GaP-NWs) in hexagonal(circular), triangular and square cross sectional are investigated by using full potential linear augmented plane waves and local orbitals (FP-LAPW-LO) method based on density functional theory (DFT). The wurtzite nanowires are investigated with diameters limited to ~27 oA. The optical functions as a function of photon energy have been investigated for unstrained bulk WZ-GaP and WZ-GaP-NWs in the first part. Finally, the optical functions are investigated for WZ-GaP-NWs by applying uniaxial strain from -5% to +5%. The effect of cross sectional variation in the area and shape of the NWs on the optical functions is clear when compared with bulk GaP. GaP-NWs kept real dielectric constant positive and refractive index close to vacuum value for a wider range of energy on the contrary of bulk GaP. It is found that the dielectric constants are slightly increased from the unstrained GaP-NWs values with strain increases. A shift in the threshold energy for toward higher energy as the strain changes from tensile to compressive. The refractive index shows a small change when passing from compressive toward tensile strain. It is found that GaP-NWs could play a role in solar cells structure and high speed electronic circuits.

Astrochemical relevance of VUV ionization of large PAH cations.

As a part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) are processed by the interaction with vacuum ultra-violet (VUV) photons that are emitted by hot young stars. This interaction leads to the emission of the well-known aromatic infrared bands but also of electrons, which can significantly contribute to the heating of the interstellar gas. Our aim is to investigate the impact of molecular size on the photoionization properties of cationic PAHs. Trapped PAH cations of sizes between 30 and 48 carbon atoms were submitted to VUV photons in the range of 9 to 20 eV from the DESIRS beamline at the synchrotron SOLEIL. All resulting photoproducts including dications and fragment cations were mass-analyzed and recorded as a function of photon energy. Photoionization is found to be predominant over dissociation at all energies, which differs from an earlier study on smaller PAHs. The photoionization branching ratio reaches 0.98 at 20 eV for the largest studied PAH. The photoionization threshold is observed to be between 9.1 and 10.2 eV, in agreement with the evolution of the ionization potential with size. Ionization cross sections were indirectly obtained and photoionization yields extracted from their ratio with theoretical photoabsorption cross sections, which were calculated using time-dependent density functional theory. An analytical function was derived to calculate this yield for a given molecular size. Large PAH cations could be efficiently ionized in H i regions and provide a contribution to the heating of the gas by photoelectric effect. Also, at the border of or in H ii regions, PAHs could be exposed to photons of energy higher than 13.6 eV. Our work provides recipes to be used in astronomical models to quantify these points.

Naturally derived FeTiO3 nanoparticles: analysis of optical properties

Eco-friendly metal titanates, which are abundant in nature, are widely used in our day-to-life in a variety of areas. This work is focused on the optical properties of FeTiO3 (FT) nanoparticles prepared by acid extraction method from ilmenite sand. The prepared FeTiO3 nanoparticles were analyzed by ultraviolet (UV) spectroscopy. Various linear optical parameters such as refractive index, extinction coefficient, absorption coefficient, real and imaginary parts of dielectric constant, loss tangent, optical conductivity, electron energy loss, and dielectric relaxation time were calculated in 1.5–6 eV energy range and discussed as a function of photon energy. The calculated values of the real part of complex permittivity and volume energy loss were higher than those of the imaginary part of complex permittivity and surface energy loss, respectively. The discussed results showed that the prepared FeTiO3 nanoparticles are suitable for the photocatalytic activity and optoelectronic device applications due to their hybridization orbital.

Theoretical Investigation of BGaAs/GaAs for Optoelectronic Device Applications

Boron dopant incorporation into host GaAs material to form a novel BGaAs semiconductor has been studied using a 10-band kp model. This model allows us to calculate the electronic band structure along the crystallographic direction. So, from the obtained E–k relation, we calculate the carrier effective mass, intrinsic carrier concentration, and x = 0 (GaAs) are in good agreement with previously reported literature. The intrinsic carrier concentration variation is seen as monotonic with boron composition. On the other hand, the composition-dependent Debye temperature observed reduction along with symmetry directions, Λ by 1.23/%B, Σ by − 2.33/%B, and Δ by − 3.52/%B, which leads to the lower stability of crystalline BGaAs alloy. In addition, the interband absorption coefficient as a function of photon energy shows nonmonotonic dependence with fundamental absorption observed at 1.39 eV. Whereas, in the case of quantum well, the optical gain spectrum peak is blueshifted with the density of injected carriers. Hence, this study provides beneficial guidance in the interpretation of applications such as solar cells, optical modulators, multiple QWs.

Addressing the sensitivity of signals from solid/liquid ambient pressure XPS (APXPS) measurement

Ambient pressure XPS has demonstrated its great potential in probing the solid/liquid interface, which is a central piece in electrocatalytic, corrosion, and energy storage systems. Despite the advantage of ambient pressure XPS being a surface sensitive characterization technique, the ability of differentiating the surface adsorbed species (∼Å scale) and bulk electrolyte (∼10 nm scale) in the spectrum depends on the delicate balance between bulk solution concentration (C), surface coverage (θ), bulk liquid layer thickness (L), and inelastic mean free path (λ) as a function of photon energy. By investigating a model system of gold dissolving in a bromide solution, the connection between theoretical prediction at the atomic resolution and macroscopic observable spectrum is established.

Iterative X-ray spectroscopic ptychography.

Spectroscopic ptychography is a powerful technique to determine the chemical composition of a sample with high spatial resolution. In spectro-ptychography, a sample is rastered through a focused X-ray beam with varying photon energy so that a series of phaseless diffraction data are recorded. Each chemical component in the material under investigation has a characteristic absorption and phase contrast as a function of photon energy. Using a dictionary formed by the set of contrast functions of each energy for each chemical component, it is possible to obtain the chemical composition of the material from high-resolution multi-spectral images. This paper presents SPA (spectroscopic ptychography with alternating direction method of multipliers), a novel algorithm to iteratively solve the spectroscopic blind ptychography problem. First, a nonlinear spectro-ptychography model based on Poisson maximum likelihood is designed, and then the proposed method is constructed on the basis of fast iterative splitting operators. SPA can be used to retrieve spectral contrast when considering either a known or an incomplete (partially known) dictionary of reference spectra. By coupling the redundancy across different spectral measurements, the proposed algorithm can achieve higher reconstruction quality when compared with standard state-of-the-art two-step methods. It is demonstrated how SPA can recover accurate chemical maps from Poisson-noised measurements, and its enhanced robustness when reconstructing reduced-redundancy ptychography data using large scanning step sizes is shown.

Study of Optical and Structural Properties of (NiO)1-x(CuO)x Nanostructures Thin Films

Abstract In this study, mixed Nickel- Copper oxide (NiO)1-x(CuO)x thin films were deposited on glass substrates using chemical spray pyrolysis technique at substrate temperature of 400 °C and thickness of 200 nm. UV-Visible spectroscopy and X- ray diffraction (XRD) were used to study the optical and structural properties of films. The optical energy gap for the thin films was estimated using Tauc's function. It is observed that the band gap decreases with the increase of the mixing ratio of copper oxide. The constants of optical properties contain absorption coefficient as a function of photon energy. Extinction coefficient for all samples was estimated as a wavelength function and the refractive index increased as (CuO) contents increased. The results of XRD showed that these films were polycrystalline in nature with orientation along (111) plane. The average values of crystallite size was estimated by using Scherrer's method and W-H questions are found to increase as the (CuO) contents increased.

X-ray diffraction at the National Ignition Facility.

We report details of an experimental platform implemented at the National Ignition Facility to obtain in situ powder diffraction data from solids dynamically compressed to extreme pressures. Thin samples are sandwiched between tamper layers and ramp compressed using a gradual increase in the drive-laser irradiance. Pressure history in the sample is determined using high-precision velocimetry measurements. Up to two independently timed pulses of x rays are produced at or near the time of peak pressure by laser illumination of thin metal foils. The quasi-monochromatic x-ray pulses have a mean wavelength selectable between 0.6 Å and 1.9 Å depending on the foil material. The diffracted signal is recorded on image plates with a typical 2θ x-ray scattering angle uncertainty of about 0.2° and resolution of about 1°. Analytic expressions are reported for systematic corrections to 2θ due to finite pinhole size and sample offset. A new variant of a nonlinear background subtraction algorithm is described, which has been used to observe diffraction lines at signal-to-background ratios as low as a few percent. Variations in system response over the detector area are compensated in order to obtain accurate line intensities; this system response calculation includes a new analytic approximation for image-plate sensitivity as a function of photon energy and incident angle. This experimental platform has been used up to 2 TPa (20 Mbar) to determine the crystal structure, measure the density, and evaluate the strain-induced texturing of a variety of compressed samples spanning periods 2-7 on the periodic table.

Measurement of natNi(γ, xn)57,56Ni and natNi(γ, pxn)58−55Co reaction cross sections in bremsstrahlung with end-point energies of 65 and 75 MeV

The cross sections of natNi(γ, xn)57,56Ni and natNi(γ, pxn)58−55Co reactions induced by bremsstrahlung end-point energies of 65 and 75 MeV were measured by using the method of activation along with off-line γ-ray spectrometric technique. The natNi(γ, xn)57,56Ni and natNi(γ, pxn)58−55Co reaction cross sections as a function of photon energy were also calculated by using TALYS 1.9 code. The flux-weighted average values at different end-point energies were obtained from the literature data, as well as from the theoretical values of TALYS and TENDL-2017 data library based on mono-energetic photons. Those were compared with the experimental data of present work and found to be in general agreement. It was also found that the experimental and theoretical cross-sections data increase with bremsstrahlung energy up to a certain value. After that the individual reaction cross sections slightly decrease with the increase of bremsstrahlung energy due to the opening of other reactions. As an example, the natNi(γ, xn)57Ni reaction cross section decreases when the natNi(γ, xn)56Ni reaction opens up. Similarly, the natNi(γ, pxn)58Co reaction cross section decreases when natNi(γ, pxn)57Co reaction opens up and so on. These observations indicate the role of excitation energy and its portion in different reaction channels.

Negative-ion/positive-ion coincidence spectroscopy as a tool to identify anionic fragments: The case of core-excited CHF3.

We have studied the dissociation of the trifluoromethane molecule, CHF3 , into negative ionic fragments at the C 1s and F 1s edges. The measurements were performed by detecting coincidences between negative and positive ions. We observed five different negative ions: F- , H- , C- , CF- , and F2 - . Their production was confirmed by the analysis of triple coincidence events (negative-ion/positive-ion/positive-ion or NIPIPI coincidences) that were recorded with cleaner signals than those of the negative-ion/positive-ion coincidences. The intensities of the most intense NIPIPI coincidence channels were recorded as a function of photon energy across the C 1s and F 1s excitations and ionization thresholds. We also observed dissociation channels involving the formation of one negative ion and three positive ions. Our results demonstrate that negative-ion/positive-ion coincidence spectroscopy is a very sensitive method to observe anions, which at inner-shell edges are up to three orders of magnitude less probable dissociation products than cations.

Femtosecond Pump Probe Reflectivity Spectra in CdTe and GaAs Crystals at Room Temperature.

Ultrafast pump probe reflectivity (PPR) signal near band edge is modeled by taking into account band filling (BF) and band gap renormalization (BGR) effects with the carrier density of ~1017/cm3 in GaAs crystal at room temperature. The calculated results indicate that the transient reflectivity ΔR/R is determined by BF and BGR effects. The most interesting feature is that ΔR/R signal experiences a sign change from photo-bleaching (PB) to photo-absorption (PA) due to the competition between BF and BGR effects. We experimentally measured ΔR as a function of photon energy across band edge with carrier density of ~1017/cm3 in GaAs and CdTe crystals, which has a similar trend as that calculated according to our model. In addition, the reflectivity is very sensitive to electron spin orientation, which is well confirmed by the corresponding experiments with 100 fs pump probe reflectivity spectroscopy in bulk CdTe. Our research in this work provides a method to study optoelectronic properties of conventional semiconductors at moderate carrier density excited by ultrafast laser pulse. Importantly, this model can be used for other novel semiconductor materials beyond GaAs and will provide new insights into the underlying spin dependent photophysics properties for new materials.

Annealing Effect on the Optical Properties of Multi Walled Carbon Nano Tube Doped Cu5Se75Ge10In10 Thin Films Prepared by Thermal Evaporation Method

The optical properties of as prepared and annealed [(Cu5Se75Ge10In10)99 (CNT)1] thin films deposited with thermal evaporation technique has been discussed as a function of photon energy in wavelength range 300–900 nm in present paper. The bulk sample of as prepared glassy alloys were developed using the melt quench technique. Thin films of prepared glassy alloys were deposited on a glass substrate with thermal evaporation unit at room temperature at base pressure of ∼10–5 Pa. The films were annealed at two different temperatures between glass transition temperature and crystallization temperatures. Analysis of the optical data shows that optical bandgap decreases with increasing annealing temperature while extinction coefficient, absorption coefficient and transmission shows increasing behaviour with increase in annealing temperature.

Theoretical calculations on photoelectron angular distribution of sequential two-photon double ionization for Ar atom

With the development of the intense light source, such as free electron lasers, the experiments on the nonlinear process in atomic photo absorption in the XUV and X-ray region became more and more feasible. As one of the simplest possible nonlinear processes, the sequential two-photon double ionization, in which the first photon produces an ion which is subsequently ionized by the second photon, attracts increasing attention of theorists and experimentalists. Study on the angular distributions and angular correlations of the photoelectrons in the sequential two-photon double ionization process are especially attractive, which provides valuable information about the electronic structure of atom or molecule systems and allows the obtaining of additional information about mechanism and pathway of the two-photon double ionization. In this paper, the expression for the photoelectron angular distribution in a sequential two-photon process is given based on the multi-configuration Dirac-Fock method and the density matrix theory. And then, the relativistic calculation program for photoelectron angular distribution is further developed with the help of the program packages GRASP2K and RATIP which are based on the multi-configuration Dirac-Fock method. By using this code, the sequential two-photon double ionization of the 3p shell in atomic argon is studied theoretically. The cross section, magnetic cross section, alignment of residual ions and the asymmetry parameter of the photoelectron angular distribution, each as a function of photon energy, for the first and the second step of sequential two-photon double ionization of argon are presented. The calculations predict that the alignment has a maximum value and the asymmetry parameter has a minimum value in the region of the cooper minimum. The angular distribution of the first step ionization for Ar atom and the second step ionization for Ar<sup>+</sup> ion are given at 33.94 eV and 55.34 eV photon energy, respectively. In addition, the difference in property between the angular distributions of the first photoelectron in sequential two-photon double ionization and in conventional one-photon single ionization is discussed. The present calculated results are compared with other available results, showing that they are in good agreement with each other. The results of this paper will be helpful in studying nonlinear processes in the XUV range.

Temperature dependent dielectric function and direct bandgap of Ge

The dielectric function of bulk Ge is determined between 0.5 and 6.3 eV in a temperature range of 10–738 K using spectroscopic ellipsometry. The authors provide the data in a tabulated format that can be interpolated as a function of photon energy and temperature using commercial software. Another focus of this paper lies on the analysis of critical points, in particular, on the investigation of the temperature dependence of the direct bandgap E0 and the critical point E0+Δ0, where Δ0 is the spin–orbit splitting. To explore the temperature dependence of critical points, the parameters that characterize their line shapes are calculated using three different techniques. First, the common method of numerically calculating and analyzing the second derivatives of the dielectric function works well for critical points at higher energies. Second, an analysis in reciprocal space by performing a discrete Fourier transform and analyzing the resulting Fourier coefficients yields values for the energies of E0 and E0+Δ0. Third, the energy determined from a parametric semiconductor model is shown as a function of temperature. The authors observe a temperature dependent redshift of the E0 and E0+Δ0 critical point energies as well as an increase in the broadening of E0 with temperature.