Measurements Of Energy Dependence Research Articles

Experimental determination of the hafnium L-subshell fundamental parameters using the holistic approach

By employing the recently demonstrated new holistic approach, the atomic fundamental parameters (FPs) of the three Hf-L subshells were experimentally determined using the radiometrically calibrated instrumentation of the Physikalisch-Technische Bundesanstalt. The Coster–Kronig factors, the L-subshell fluorescence yields, the L-subshell Auger yields, the subshell-photoionization cross sections, and the subshell fluorescence production cross sections were determined by means of photon energy dependent x-ray fluorescence and transmission measurements. The recently demonstrated new holistic evaluation approach allows to determine the FPs with significantly lower uncertainties as compared to the former data evaluation scheme, where only a limited regime of incident photon energies is being probed and the data evaluation scheme is performed in a sequential manner.

Treatment of multiple-beam X-ray diffraction in energy-dependent measurements.

During X-ray diffraction experiments on single crystals, the diffracted beam intensities may be affected by multiple-beam X-ray diffraction (MBD). This effect is particularly frequent at higher X-ray energies and for larger unit cells. The appearance of this so-called Renninger effect often impairs the interpretation of diffracted intensities. This applies in particular to energy spectra analysed in resonant experiments, since during scans of the incident photon energy these conditions are necessarily met for specific X-ray energies. This effect can be addressed by carefully avoiding multiple-beam reflection conditions at a given X-ray energy and a given position in reciprocal space. However, areas which are (nearly) free of MBD are not always available. This article presents a universal concept of data acquisition and post-processing for resonant X-ray diffraction experiments. Our concept facilitates the reliable determination of kinematic (MBD-free) resonant diffraction intensities even at relatively high energies which, in turn, enables the study of higher absorption edges. This way, the applicability of resonant diffraction, e.g. to reveal the local atomic and electronic structure or chemical environment, is extended for a vast majority of crystalline materials. The potential of this approach compared with conventional data reduction is demonstrated by the measurements of the Ta L3 edge of well studied lithium tantalate LiTaO3.

Collision-Induced Dissociation of Citrullinated Peptide Anions.

Peptide identification by positive electrospray ionization (ES+) tandem mass spectrometry (MS/MS) is a well-established strategy in proteomics. Several research groups reported the usefulness of negative electrospray ionization (ES-) for gaining complementary structural information on peptides and their post-translational modifications (PTM) compared to ES+. Fragmentation of citrullinated peptides has not been previously explored in ES-. In this study, 9 peptides containing citrulline residues were investigated in ES- by stepwise collision energy-dependent measurements on a QTOF instrument and a Q-Orbitrap instrument. Our results of high resolution and mass accuracy show the favored citrulline-selective loss of HNCO from these peptide precursors and their fragments─similarly to that in ES+─along with y-NH3/z, c, c-NH3/b sequence ions. Loss of HNCO from citrullinated peptides in ES- and a proposed mechanism for the reaction have been described here for the first time. HNCO loss intensities from precursors were generally even higher than that in ES+. Interestingly, the most intense fragments corresponded to neutral losses from sequence ions while intact sequence ions were usually minor components of the spectra. High-intensity ions related to cleavages N-terminal to Asp and Glu residues that have been previously reported were also observed. On the other hand, a relatively high number of peaks were observed, possibly due to internal fragmentation and/or scrambling events. While (ES-) MS/MS spectra always require manual inspection and the annotation may be ambiguous, the favorable loss of HNCO and the preferable cleavage N-terminal to Asp residues can be used to differentiate between citrullinated/deamidated sequences.

Depth profiling of LE-µSR parameters with musrfit

The study of thin-film and multi-layered structures with nanometer resolution is possible with low energy µSR (LE-µSR). Modeling of the measured µSR parameters such as diamagnetic asymmetry and relaxation rate as a function of sample depth can be obtained from a series of experimental implantation energy measurements and its correlation with the simulated stopping profiles. The fitting approach assumes a sharp transition between regions with distinct properties. The fitting method, previously developed in matlab, was implemented in musrfit, a free µSR data analysis framework written in C++. The main goal is to make this fitting method widely available for energy dependent measurements and to increase the modeling possibilities within musrfit.

Few-layer hexagonal boron nitride / 3D printable polyurethane composite for neutron radiation shielding applications

Functional polymer composites can confer a range of benefits in practical applications that go beyond the individual properties of the constituent materials. Here we investigate and characterize the neutron absorbing capability of few-layer hexagonal boron nitride (h-BN) in composite with a 3D-printable thermoplastic polyurethane, and present experiment and simulation data to understand the processes and mechanisms in play. Shielding and protection from neutrons can be necessary in a range of terrestrial and space-based applications. The neutron absorption of composites with varying fractions of h-BN is strongly energy-dependent in the low-energy regime below 10 meV, and a composite containing 20 wt% h-BN shows a 70-fold reduction in the transmission relative to pure polyurethane at 0.5 meV neutron energies. This is attributed to the strong neutron capture cross-section of the naturally abundant boron-10 isotope, with energy-dependent measurements up to 100 meV confirming this point. Using inelastic neutron spectroscopy, we identify additional effects from the hydrogen in the polyurethane which both scatters diffusively and moderates neutrons inelastically via its phonon spectrum, enhancing the neutron absorption characteristics. Two models – based on analytic functions and Monte Carlo numerical techniques – are presented, and show excellent agreement with experiment results. The 3D-printability of the composite is demonstrated, and the opportunities and challenges for deploying these composites in neutron radiation protection applications are discussed.

Sn intercalation into the BL/SiC(0001) interface: A detailed SPA-LEED investigation

Growth of epitaxial graphene (EG) with desired properties and formation of new 2D interfaces can be achieved by intercalation of new elements into the carbon buffer layer (BL)/SiC(0001) interface. In this study, we investigated the formation of various ordered Sn-interface structures and, associated with it, the formation of quasi-free monolayer graphene by means of high-resolution spot profile analyzing low energy electron diffraction (SPA-LEED). The analyses of the diffraction patterns and the diffraction spot intensities allowed us to identify various long-range ordered interface structures as a function of the annealing temperature, e.g. (1 × 1), (3×3)R30° and (39×39)R16.1°. The ordering process at the interface and accompanying onset of deintercalation was controlled and monitored by high angular resolution and energy dependent measurements. Besides, for Sn sub-monolayer intercalation coverages, also a (3×3)R30° reconstruction with respect to EG was found, which indicates a broken Kekulé structure of the π-electrons in EG.

Electronic band structure in pristine and sulfur-doped Ta2NiSe5

We present an angle-resolved photoemission study of the electronic band structure of the excitonic insulator Ta$_2$NiSe$_5$, as well as its evolution upon Sulfur doping. Our experimental data show that while the excitonic insulating phase is still preserved at a Sulfur-doping level of 25$\%$, such phase is heavily suppressed when there is a substantial amount, $\sim$ 50$\%$, of S-doping at liquid nitrogen temperatures. Moreover, our photon energy-dependent measurements reveal a clear three dimensionality of the electronic structure, both in Ta$_2$NiSe$_5$ and Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$ ($x=0.25, 0.50$) compounds. This suggests a reduction of electrical and thermal conductivities, which might make these compounds less suitable for electronic transport applications.

Measurement of non-linearity in the cathodoluminescence yield for non-doped scintillators

We propose a method for studying nonlinearity in the cathodoluminescence (CL) characteristics of wide-bandgap materials based on the measurements of the CL energy dependence on the total energy of a pulsed electron beam Eb using its bremsstrahlung. It is shown that the x-ray radiant energy produced by a high-power (∼10MW/cm2) electron beam with particle energies of 50–300 keV is proportional to Eb in the case of weak variations in the electron energy distribution of the beam. This direct proportionality between Eb and the x-ray radiant energy is experimentally confirmed in the current experiment by measuring the dependence of the molecular nitrogen emission radiant energy at 337 nm excited by a direct electron impact (the 0-0 vibrational transition of the second positive system of the emission bands of N2 molecule) on the total electron beam energy Eb. Using this result, the dependencies of the CL radiant energy on Eb are studied for undoped Bi4Ge3O12, PbWO4, CeF3, and BaF2 crystals with bright intrinsic luminescence. An interpretation of these dependencies is given using a simple theoretical model and photoluminescence nonlinearity data published in the literature. We estimate the average concentration of the electronic excitations (EE) provided by the electron beam (1018–1019 e.–h.p./cm3) and obtain the approximate dependencies of the CL yield on the EE density for the studied materials. For the CeF3 crystal, different CL yield dependencies on the EE density are found for the bands at 300 and 350 nm.

Polarization-Resolved Extreme-Ultraviolet Second-Harmonic Generation from LiNbO3

Second harmonic generation (SHG) spectroscopy ubiquitously enables the investigation of surface chemistry, interfacial chemistry, as well as symmetry properties in solids. Polarization-resolved SHG spectroscopy in the visible to infrared regime is regularly used to investigate electronic and magnetic order through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hampers the interpretation of experiments solely based on the investigation of hybridized valence states. Here, polarization-resolved SHG in the extreme ultraviolet (XUV-SHG) is demonstrated for the first time, enabling element-resolved angular anisotropy investigations. In noncentrosymmetric LiNbO_{3}, elemental contributions by lithium and niobium are clearly distinguished by energy dependent XUV-SHG measurements. This element-resolved and symmetry-sensitive experiment suggests that the displacement of Li ions in LiNbO_{3}, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO_{6} octahedron as well. Our simulations show that the measured second harmonic spectrum is consistent with Li ion displacements from the centrosymmetric position while the Nb─O bonds are elongated and contracted by displacements of the O atoms. In addition, the polarization-resolved measurement of XUV-SHG shows excellent agreement with numerical predictions based on dipole-induced SHG commonly used in the optical wavelengths. Our result constitutes the first verification of the dipole-based SHG model in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems.

Flexible Polymer X-ray Detectors with Non-fullerene Acceptors for Enhanced Stability: Toward Printable Tissue Equivalent Devices for Medical Applications.

There is growing interest in the development of novel materials and devices capable of ionizing radiation detection for medical applications. Organic semiconductors are promising candidates to meet the demands of modern detectors, such as low manufacturing costs, mechanical flexibility, and a response to radiation equivalent to human tissue. However, organic semiconductors have typically been employed in applications that convert low energy photons into high current densities, for example, solar cells and LEDs, and thus existing design rules must be re-explored for ionizing radiation detection where high energy photons are converted into typically much lower current densities. In this work, we report the optoelectronic and X-ray dosimetric response of a tissue equivalent organic photodetector fabricated with solution-based inks prepared from polymer donor poly(3-hexylthiophene) (P3HT) blended with either a non-fullerene acceptor (5Z,5'Z)-5,5'-((7,7'-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (o-IDTBR) or a fullerene acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Indirect detection of X-rays was achieved via coupling of organic photodiodes with a plastic scintillator. Both detectors displayed an excellent response linearity with dose, with sensitivities to 6 MV photons of 263.4 ± 0.6 and 114.2 ± 0.7 pC/cGy recorded for P3HT:PCBM and P3HT:o-IDTBR detectors, respectively. Both detectors also exhibited a fast temporal response, able to resolve individual 3.6 μs pulses from the linear accelerator. Energy dependence measurements highlighted that the photodetectors were highly tissue equivalent, though an under-response in devices compared to water by up to a factor of 2.3 was found for photon energies of 30-200 keV due to the response of the plastic scintillator. The P3HT:o-IDTBR device exhibited a higher stability to radiation, showing just an 18.4% reduction in performance when exposed to radiation doses of up to 10 kGy. The reported devices provide a successful demonstration of stable, printable, flexible, and tissue-equivalent radiation detectors with energy dependence similar to other scintillator-based detectors used in radiotherapy.

Effect of surface oxidation on soft x-ray optical properties of ion beam sputter deposited amorphous AlN thin film

In the present study, structural and compositional analyses of reactive ion beam sputter deposited aluminum nitride (AlN) thin film of thickness 100 Å are carried out using x-ray reflectivity and x-ray photoelectron spectroscopy techniques. Soft x-ray optical response of the film is derived from energy dependent soft x-ray reflectivity measurements performed in photon energy region of 380–1700 eV. Optical constants (δ and β) obtained from the reflectivity spectra show features corresponding to absorption edges of the constituent elements. Observed fine features in the β profile are further confirmed from x-ray absorption (XAS) measurements carried out in the total electron yield mode. The measured XAS spectra are correlated with electronic and compositional properties of the AlN film. The effects of surface oxidation on soft x-ray optical properties of the AlN thin film are discussed.

Empirical Evidence for Latitude and Altitude Variation of the In Situ Cosmogenic 26Al/10Be Production Ratio

We assess if variations in the in situ cosmogenic 26Al/10Be production ratio expected from nuclear physics are consistent with empirical data, knowledge critical for two-isotope studies. We do this using 313 samples from glacially transported boulders or scoured bedrock with presumed simple exposure histories in the Informal Cosmogenic-nuclide Exposure-age Database (ICE-D) from latitudes between 53°S to 70°N and altitudes up to 5000 m above sea level. Although there were small systematic differences in Al/Be ratios measured in different laboratories, these were not significant and are in part explained by differences in elevation distribution of samples analyzed by each laboratory. We observe a negative correlation between the 26Al/10Be production ratio and elevation (p = 0.0005), consistent with predictions based on the measured energy dependence of nuclear reaction cross-sections and the spatial variability in cosmic-ray energy spectra. We detect an increase in the production ratio with increasing latitude, but this correlation is significant only in a single variate model, and we attribute at least some of the correlation to sample elevation bias because lower latitude samples are typically from higher elevations (and vice versa). Using 6.75 as the 26Al/10Be production ratio globally will bias two-isotope results at higher elevations and perhaps higher latitudes. Data reported here support using production rate scaling that incorporates such ratio changes, such as the LSDn scheme, to minimize such biases.

High-resolution model-based material decomposition in dual-layer flat-panel CBCT.

Spectral CT uses energy-dependent measurements that enable material discrimination in addition to reconstruction of structural information. Flat-panel detectors (FPDs) have been widely used in dedicated and interventional systems to deliver high spatial resolution, volumetric cone-beam CT (CBCT) in compact and OR-friendly designs. In this work, we derive a model-based method that facilitates high-resolution material decomposition in a spectral CBCT system equipped with a prototype dual-layer FPD. Through high-fidelity modeling of multilayer detector, we seek to avoid resolution loss that is present in more traditional processing and decomposition approaches. A physical model for spectral measurements in dual-layer flat-panel CBCT is developed including layer-dependent differences in system geometry, spectral sensitivities, and detector blur (e.g., due to varied scintillator thicknesses). This forward model is integrated into a model-based material decomposition (MBMD) method based on minimization of a penalized weighted least-squared (PWLS) objective function. The noise and resolution performance of this approach was compared with traditional projection-domain decomposition (PDD) and image-domain decomposition (IDD) approaches as well as one-step MBMD with lower-fidelity models that use approximated geometry, projection interpolation, or an idealized system geometry without system blur model. Physical studies using high-resolution three-dimensional (3D)-printed water-iodine phantoms were conducted to demonstrate the high-resolution imaging performance of the compared decomposition methods in iodine basis images and synthetic monoenergetic images. Physical experiments demonstrate that the MBMD methods incorporating an accurate geometry model can yield higher spatial resolution iodine basis images and synthetic monoenergetic images than PDD and IDD results at the same noise level. MBMD with blur modeling can further improve the spatial-resolution compared with the decomposition results obtained with IDD, PDD, and MBMD methods with lower-fidelity models. Using the MBMD without or with blur model can increase the absolute modulation at 1.75 lp/mm by 10% and 22% compared with IDD at the same noise level. The proposed model-based material decomposition method for a dual-layer flat-panel CBCT system has demonstrated an ability to extend high-resolution performance through sophisticated detector modeling including the layer-dependent blur. The proposed work has the potential to not only facilitate high-resolution spectral CT in interventional and dedicated CBCT systems, but may also provide the opportunity to evaluate different flat-panel design trade-offs including multilayer FPDs with mismatched geometries, scintillator thicknesses, and spectral sensitivities.

The intrinsic x-ray energy dependence of beryllium oxide (BeO) ceramic dosimeters

Beryllium oxide (BeO) ceramic is a radioluminescent and optically stimulated luminescent material which has been shown to be quite energy independent. However, BeO ceramic is a highly self-optically attenuating material. The aim of this study is to evaluate the effect self-optical attenuation has on the radioluminescence of the BeO ceramic, and how this effects the measured energy dependence. The energy dependence of a BeO ceramic fibre-coupled luminescent dosimeter was measured using a superficial x-ray unit with beams of 30–150 kVp. The energy dependence was measured with the dosimeter oriented parallel and perpendicular to the beam in order to enhance the effect of the self-optical attenuation from the BeO ceramic. From the measurements, energy dependence curves were generated showing the detector response in both orientations. The largest difference in response was >25% observed at the lowest energy.Monte-Carlo simulations of the dose distribution within the BeO ceramic were convolved with ray tracing modelling of the light collection from the BeO ceramic probe in order to simulate the total detector response. The light collection corrected the Monte Carlo simulation results, and the intrinsic energy dependence was obtained with the trends between the two orientations agreeing within 8%. Therefore, it can be concluded that the measured orientational energy dependence is due to the self-optical attenuation of the BeO ceramic.

Evidence of Weyl fermions in α−RuCl3

Layered honeycomb lattice antiferromagnetic $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ has been studied intensively recently as a Kitaev spin liquid candidate. Here we present strong evidence that this material host realistic Weyl fermions derived from Cl $2p$ electrons. Our theoretical analysis suggests the existence of quadratic Weyl fermions without spin-orbit coupling and linear Weyl fermion with spin-orbit coupling. Angle-resolved photoemission spectroscopy with systematic photon energy-dependent measurements demonstrates that this Weyl cone possesses linear dispersion at bulk Brillouin zone center along all the three reciprocal directions and hyperbolic dispersion off center. Our work reveals the rich topological physics lying in the band structure of Kitaev QSL candidates and will stimulate further investigation not only limited to $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Ru}{\mathrm{Cl}}_{3}$ but also honeycomb iridate family of materials.

Energy dependent XPS measurements on thin films of a poly(vinyl methyl ether)/polystyrene blend concentration profile on a nanometer resolution to understand the behavior of nanofilms.

The composition of the surface layer in dependence from the distance of the polymer/air interface in thin films with thicknesses below 100 nm of miscible polymer blends in a spatial region of a few nanometers is not investigated completely. Here, thin films of the blend poly(vinyl methyl ether) (PVME)/polystyrene (PS) with a composition of 25/75 wt% are investigated by Energy Resolved X-ray Photoelectron Spectroscopy (ER-XPS) at a synchrotron storage ring using excitation energies lower than 1 keV. By changing the energy of the photons the information depth is varied in the range from ca. 1 nm to 10 nm. Therefore, the PVME concentration could be estimated in dependence from the distance of the polymer/air interface for film thicknesses below 100 nm. Firstly, as expected for increasing information depth the PVME concentration decreases. Secondly, it was found that the PVME concentration at the surface has a complicated dependence on the film thickness. It increases with decreasing film thickness until 30 nm where a maximum is reached. For smaller film thicknesses the PVME concentration decreases. A simplified layer model is used to calculate the effective PVME concentration in the different spatial regions of the surface layer.

Electronic structure of non-centrosymmetric PtBi2 studied by angle-resolved photoemission spectroscopy

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), we have systematically studied the electronic structure of non-centrosymmetric PtBi2 with a layered structure. Through photon energy dependent measurements, the surface state and the bulk state were identified. Consistent with the non-centrosymmetric structure, different surface states were observed on opposite PtBi2(001) surfaces. Experimental bulk bands agree nicely with first-principle calculations. By combining ARPES and calculations, energy bands that contribute to triply degenerate point fermion were all detected. We also suggest that there are “canted” tube-like Fermi surfaces that might cause strong anisotropy in electronic transportation in non-centrosymmetric PtBi2.

Femtosecond time-resolved nonlinear optical spectroscopy of charge transfer at the buried GaP/Si(001) interface

The ultrafast charge-carrier dynamics at the buried heterointerface of gallium phosphide on silicon(001) are investigated by means of time-resolved optical second-harmonic generation. Photon energy dependent measurements reveal the existence of electronic interface states in the bandgap of both materials. Charge carriers excited via these interface states are efficiently injected within a few hundred femtoseconds from the GaP/Si interface into the Si substrate, resulting in the build-up of an electric field perpendicular to the interface on a picosecond time scale.

A comprehensive ARPES study on the type-II Dirac semimetal candidate Ir1−xPtxTe2

The transition metal dichalcogenide Ir1−xPtxTe2 displays both superconductivity and a topological band structure. Using angle-resolved photoemission spectroscopy, we obtain a comprehensive understanding of the three-dimensional electronic structure in the normal state of Ir1−xPtxTe2 for doping levels from x = 0.1 to 0.4, which spans the composition range of a superconducting state to a non-superconducting state. Many features of the electronic structure can be attributed to strong Te–Te interactions between the layers of the layered crystal structure and can be resolved by photon energy dependent measurements. We demonstrate that the type-II Dirac fermions can be successfully tuned via Pt doping, where the Dirac point lies close to the Fermi level for x = 0.1. The band evolution vs doping provides a clearer understanding of the relationship between the superconductivity and electronic structure. In addition, the β band in the superconducting samples locates the system close to a type-II van Hove singularity, where spin triplet paring symmetry has been predicted. Our results provide a comprehensive understanding of the band structure of Ir1−xPtxTe2, and we discuss the possibilities of the existence of topological superconductivity in this system.

Electronic structure of La (0001) thin films on W (110) studied by photoemission spectroscopy and first principle calculations

Surface states that have a dz2 symmetry around the center of the surface Brillouin zone (BZ) have been regarded common in closely-packed surfaces of rare-earth metals. In this work, we report the electronic structure of dhcp La (0001) thin films by ultra-high energy resolution angle-resolved photoemission spectroscopy (ARPES) and first principle calculations. Our first principle analysis is based on the many-body approach, therefore, density function theory (DFT) combined with dynamic mean-field theory (DMFT). The experimentally observed Fermi surface topology and band structure close to the Fermi energy qualitatively agree with first principle calculations when using a renormalization factor of between 2 and 3 for the DFT bands. Photon energy dependent ARPES measurements revealed clear k Z dependence for the hole-like band around the BZ center, previously regarded as a surface state. The obtained ARPES results and theoretical calculations suggest that the major bands of dhcp La (0001) near the Fermi level originate from the bulk La 5 d orbits as opposed to originating from the surface states.