Photon Energy Dependence Research Articles

Assessment of photon energy and dose rate dependence of U-NIPAM polymer gel dosimeter

One of the main drawbacks of NPAM gel dosimeter is its relatively low R2-dose sensitivity. Recently, the sensitivity of NIPAM gel dosimeter has been improved by adding urea and this new formula gel dosimeter was named U-NIPAM (Urea and NIPAM). In the current project, photon energy and dose rate dependence of optimized U-NIPAM gel dosimeter (with 3% urea) was assessed by MRI technique. The gel samples were fabricated under normal atmospheric conditions and after 1 day, they were exposed to photon energies of 6 and 15 MV and dose rates of 300, 400, and 500 cGy/min. Then, MRI responses (R2) of the U-NIPAM gel samples were analyzed at 12, 36, 72, 168 h after the irradiation process. The dosimetric evaluation of optimized U-NIPAM gel dosimeter characteristics at 12 h after the irradiation process resulted to following findings: 1) different photon energies (6 and 15 MV) have significant effect on the R2-dose sensitivity of gel dosimeter in 0–7 Gy dose range (P < 0.05), 2) the R2-dose sensitivity of gel dosimeter does not significantly change with different dose rate (300–500 cGy/min) in 0–7 Gy dose range (P˃0.05), 3) the obtained dose resolution for evaluated various photon energies and dose rates ranges from 0.06 Gy to 0.20 Gy. Moreover, it was found that the R2-dose sensitivity dependence of gel dosimeter on photon energy can vary over post irradiation; nevertheless, the R2-dose sensitivity dependence of dosimeter on dose rate dose not vary over post irradiation time.

Cathodoluminescence Phase Extraction of the Coupling between Nanoparticles and Surface Plasmon Polaritons.

Nanoscale gaps between metals can strongly confine electromagnetic fields that enable efficient electromagnetic energy conversion and coupling to nanophotonic structures. In particular, the gap formed by depositing a metallic particle on a metallic substrate produces coupling of localized particle plasmons to propagating surface plasmon polaritons (SPPs). Understanding and controlling the phase of such coupling is essential for the design of devices relying on nanoparticles coupled through SPPs. Here we demonstrate the experimental visualization of the phase associated with the plasmonic field of metallic particle-surface composites through nanoscopically and spectroscopically resolved cathodoluminescence using a scanning transmission electron microscope. Specifically, we study the interference between the substrate transition radiation and the field resulting from out-coupling of SPP excitation, therefore giving rise to angle-, polarization-, and energy-dependent photon emission fringe patterns from which we extract phase information. Our methods should be readily applicable to more complex nanostructures, thus providing direct experimental insight into nanoplasmonic near-fields with potential applications in improving plasmon-based devices.

Theoretical study of the effects of electron-phonon and electron-photon interaction in optoelectronic properties of armchair graphene nano-flakes –a renormalization method

Abstract In this work, we study the effects of electron-phonon and electron-photon interaction on the electronic and optoelectronic properties of nano systems consisting of graphene nano-flakes (small length armchair graphene nanoribbons) connected to two semi-infinite metal electrodes. Our calculations are based on the use of non-equilibrium Green's function formalism. The non-interacting Hamiltonian is written within the nearest neighbor tight binding approximation. The full interacting Hamiltonian is then obtained by addition the electron-phonon and electron –photon interaction to the above non-interacting Hamiltonian. Using unitary transformations the interacting Hamiltonian is renormalized into a non-interacting tight-binding form with effective onsite energy and hopping parameter which contain the interacting effects. The Landuer-Buttiker formalism can now be used for the system with renormalized non-interacting Hamiltonian for calculating the electronic current. Within the above framework, we calculate the electron current, density of states (DOS) and photocurrent in the presence and absence of electron-phonon interaction in a nano-system consisting of graphene nano-flake with different width as central molecule. Our results show that electron-phonon interaction leads to decreasing the number of peaks in the DOS. Also the photocurrent has oscillatory behavior versus electron-photon coupling for different widths of the nano-flake. Finally, incident photon energy dependence of threshold electron-photon coupling (i.e photocurrent becomes larger than Ballistic case) and its increasing trend versus photon energy are studied.

Visible photodissociation study of NO dimer cation using ion imaging technique combined with theoretical calculations

Visible photodissociation of (NO)2+ at 532–692 nm was investigated by ion imaging experiments and theoretical calculations. From images of photofragment NO+, we obtained the translational energy, which increased with excitation energy. Anisotropy parameter increased from β = 0.7 at 692 nm to 1.4 at 532 nm. These results qualitatively agreed with theoretical calculations, in which (NO)2+ was predicted to be directly dissociated after the C∼←X∼ transition. However, a simple description with an impulsive dissociation model cannot reproduce photon energy dependence of the photofragment energy and angular distributions, implying the importance of multidimensional analyses of potential energy surfaces.

Orbital Character Effects in the Photon Energy and Polarization Dependence of Pure C60 Photoemission.

Recent direct experimental observation of multiple highly dispersive C60 valence bands has allowed for a detailed analysis of the unusual photoemission traits of these features through photon energy- and polarization-dependent measurements. Previously obscured dispersions and strong photoemission traits are now revealed by specific light polarizations. The observed intensity effects prove the locking in place of the C60 molecules at low temperatures and the existence of an orientational order imposed by the substrate chosen. Most importantly, photon energy- and polarization-dependent effects are shown to be intimately linked with the orbital character of the C60 band manifolds which allow for a more precise determination of the orbital character within the third highest occupied molecular orbital (HOMO-2). Our observations and analysis provide important considerations for the connection between molecular and crystalline C60 electronic structure, past and future band structure studies, and for increasingly popular C60 electronic device applications, especially those making use of heterostructures.

Accuracy of analytical theories for relativistic bremsstrahlung

Angular and energy-dependent photon distributions emitted from $50\text{\ensuremath{-}}\mathrm{keV}$ to $2.5\text{\ensuremath{-}}\mathrm{MeV}$ electrons colliding with targets ranging from Cu to Pb are calculated using a consistent higher order analytical approach as well as the exact Dirac partial-wave theory. By comparison with exact numerical results for neutral atoms, the validity of the Olsen-Maximon-Wergeland additivity rule for handling the screening correction is probed, and its applicability for collision energies above $100\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$, if the photon frequency and emission angle are both small, is established. The comparison for a point-Coulomb field, aimed at testing the accuracy of the underlying analytical theory, reveals in most cases a deviation below $60%$ for Au, decreasing with target nuclear charge down to $10--20%$ for Cu.

Limitations of In2O3 as a transparent conducting oxide

Sn-doped In2O3 or ITO is the most widely used transparent conducting oxide. We use first-principles calculations to investigate the limitations to its transparency due to free-carrier absorption mediated by phonons or charged defects. We find that the main contribution to the phonon-assisted indirect absorption is due to emission (as opposed to absorption) of phonons, which explains why the process is relatively insensitive to temperature. The wavelength dependence of this indirect absorption process can be described by a power law. Indirect absorption mediated by charged defects or impurities is also unavoidable since doping is required to obtain conductivity. At high carrier concentrations, screening by the free carriers becomes important. We find that charged-impurity-assisted absorption becomes larger than phonon-assisted absorption for impurity concentrations above 1020 cm–3. The differences in the photon-energy dependence of the two processes can be explained by band structure effects.

Energy scale of nematic ordering in the parent iron-based superconductor BaFe2As2

Nematicity plays an important role in the physics of iron-based superconductors (IBS). Its microscopic origin and in particular its importance for the mechanism of high-temperature superconductivity itself are highly debated. A crucial knowledge in this regard is the degree to which the nematic order influences the electronic structure of these materials. Earlier angle-resolved photoemission spectroscopy (ARPES) studies found that the effect is dramatic in three families of IBS including 11, 111 and 122 compounds: energy splitting reaches 70 meV and Fermi surface becomes noticeably distorted. More recent experiments, however, reported significantly lower energy scale in 11 and 111 families, thus questioning the degree and universality of the impact of nematicity on the electronic structure of IBS. Here we revisit the electronic structure of undoped parent BaFe2As2 (122 family). Our systematic ARPES study including the detailed temperature and photon energy dependencies points to the significantly smaller energy scale also in this family of materials, thus establishing the universal scale of this phenomenon in IBS. Our results form a necessary quantitative basis for theories of high-temperature superconductivity focused on the nematicity.

Angle-resolved photoemission spectroscopy study of the Möbius Kondo insulator candidate CeRhSb

The electronic structure of a M\obius Kondo insulator candidate of CeRhSb has been investigated by employing angle-resolved photoemission spectroscopy, density functional theory (DFT) band calculations, and dynamical mean-field theory (DMFT) band calculations. Fermi surfaces (FSs) and band structures are successfully measured for three orthogonal crystallographic directions. A sharp Ce $4f$ peak is observed at the Fermi level (${E}_{\text{F}}$), and its temperature ($T$) evolution agrees with that of the Ce $4f$ Kondo resonance. The metallic FSs are obtained for all three different (100), (010), and (001) planes. The Ce $4f$ FSs are described properly by the unfolded DFT calculations considering the reduced Ce-only unit cell. The $T$ dependence of Ce $4f$ states as well as the dispersive coherent Ce $4f$ bands are described well by the DMFT calculations and reveal the anisotropic $c\text{\ensuremath{-}}f$ hybridization. The photon energy dependence of the Fermi-edge states in CeRhSb reveals the three-dimensional character, consistent with the bulk states dispersing to ${E}_{\text{F}}$ over a larger energy scale rather than the predicted M\obius topological surface states.

Low-dose X-ray structure analysis of cytochrome c oxidase utilizing high-energy X-rays.

To investigate the effect of high-energy X-rays on site-specific radiation-damage, low-dose diffraction data were collected from radiation-sensitive crystals of the metal enzyme cytochrome c oxidase. Data were collected at the Structural Biology I beamline (BL41XU) at SPring-8, using 30 keV X-rays and a highly sensitive pixel array detector equipped with a cadmium telluride sensor. The experimental setup of continuous sample translation using multiple crystals allowed the average diffraction weighted dose per data set to be reduced to 58 kGy, and the resulting data revealed a ligand structure featuring an identical bond length to that in the damage-free structure determined using an X-ray free-electron laser. However, precise analysis of the residual density around the ligand structure refined with the synchrotron data showed the possibility of a small level of specific damage, which might have resulted from the accumulated dose of 58 kGy per data set. Further investigation of the photon-energy dependence of specific damage, as assessed by variations in UV-vis absorption spectra, was conducted using an on-line spectrometer at various energies ranging from 10 to 30 keV. No evidence was found for specific radiation damage being energy dependent.

Comprehensive analysis of thermoluminescence signals in MgB4O7:Dy,Na dosimeter

Magnesium borate (MgB4O7) is a material of interest for thermoluminescence (TL) dosimetry of ionizing radiation, due to its low photon energy dependence and the possibility of developing neutron dosimeters, and its potential as a temperature sensor in passive temperature sensing applications. Recent experimental and modeling work has shown that TL glow curves and isothermal luminescence signals in MgB4O7:Dy,Na can be analyzed using a radiative tunneling model, based on localized recombination processes. This paper presents a comprehensive analysis of TL signals in MgB4O7:Dy,Na dosimeters, based on a recently published model which incorporates simultaneous irradiation and tunneling processes. Four different types of TL data are presented and analyzed using the proposed model: (a) TL glow curves at different irradiation doses, (b) TL signals after sample irradiation at different temperatures, (c) TL after isothermal heating for 500 s at various temperatures, and (d) TL after room temperature irradiation, followed by heating to a temperature Tstop. We also present an alternative fast and efficient Monte Carlo method of analyzing the data, which is very useful for quick analysis of experimental data. The experimental data and modeling results show that the shape of the TL glow peaks remains essentially the same in all four experimental protocols, while the height and temperature of maximum TL intensity depends on the irradiation and preheating conditions.

Active Species in Photocatalytic Reactions of Methanol on TiO2(110) Identified by Surface Sum Frequency Generation Vibrational Spectroscopy

The photocatalytic reactivity of one monolayer of methanol on TiO2(110) was measured with the surface sum frequency generation vibrational spectroscopy in the C-H stretching range, which shows high photon energy dependence. The photocatalytic reactivity was systematically investigated for different active species on TiO2(110), dissociative adsorbed methanol at five-coordinated Ti sites (Ti-5c), methoxy without adsorbed methanol at Ti-5c sites, and methoxy O-b sites. The results show that the methoxy at Ti-5c sites has much higher photocatalytic reactivity than that of dissociative adsorbed methanol at Ti-5c sites, and the methoxy at O-b sites might be inactive. This work demonstrates that methoxy at Ti-5c sites is the most active species on TiO2(110) and suggests that effectively removing surface OH could enhance the photodegradation of surface organics.

Optical band gap and dispersion of optical constants of Cu-Ga-S thin films

Thermally deposited Cu-Ga-S thin films were optically characterized by means of experimental techniques of transmission measurements. The analyses of transmittance spectra were accomplished by derivative spectrophotometry analyses to get gap energies of thin films. The transmittance spectra of thin films annealed at different temperatures presented interference fringes which were analyzed by Swanepoel envelope method. The wavelength dependencies of optical parameters; refractive index (n), real part of complex dielectric function (εre) and extinction coefficient (k) were reported in the weak absorption region. The photon energy dependencies of n and εre were analyzed using single-oscillator and Spitzer-Fan models, respectively.

Study on anomalous photoemission of LaB6 at high temperatures

Photoemission is used for a large variety of experimental techniques for study of material properties. Further applications photoemission finds for electron beam generation. Development of photocathodes with high quantum efficiency requires sufficient microscopic understanding of photoemission processes. The LaB6 is a well-known thermionic emitter, which can be used as a photocathode. Anomalous, thermally assisted increase of quantum efficiency was previously demonstrated for this material. The increase shows a bi-exponential rise with photon-energy dependent slopes. The photon-energy dependency indicates correlation of extracted current with electronic band structure. In this work, we have investigated possible mechanisms, which could be the reason for the anomalous dependence of photoemission current on the cathode temperature. In particular, the Fowler–DuBridge theory was modified to account for changes in the electron density of states. The results show negligible low influence of density of states structure. Finally, we make a suggestion regarding the modification of the electron escape probability, which can cause the increase in photoemission.

Ultrafast spectroscopy of shift-current in ferroelectric semiconductor Sn2P2S6

We report sub-picosecond photocarrier dynamics observed via emitted terahertz waves in a ferroelectric semiconductor Sn2P2S6. Excitation photon energy, intensity and polarization dependences of the photocarrier dynamics testify that the ultrafast photocurrent originates from the shift-current under interband excitation. The photocurrent excitation spectrum shows a quantitative agreement with that derived from first-principles calculations with the Berry connection integrated, showing the shift-current to be a sensitive feature in ferroelectrics. The terahertz emission spectroscopy is a powerful tool to identify the intricate carrier dynamics under pulsed photoexcitation.

Incident angle and photon energy dependence of polariton lasing in an organic microcavity

Organic semiconductor microcavities forming strongly coupled exciton-polaritons are interesting systems due to their nonlinear coherent emission observed even at room temperature. Here, we investigate the incident angle and photon-energy-dependent photoemission from a planar organic microcavity to evaluate the relaxation efficiency towards the bottom of the lower polariton branch. We observe nonlinear increase in the vertical emission from the microcavity when the sample is pumped resonantly to the lower polariton branch with an incident angle of 30 to 55°. The observed results support the fast and efficient relaxation of organic microcavity polaritons within the lower polariton branch.

Half-metallicity of the ferrimagnet Mn2VAl revealed by resonant inelastic soft x-ray scattering in a magnetic field

Detailed information on the electronic states of both V and Mn $3d$ electrons in the ferrimagnet $\mathrm{M}{\mathrm{n}}_{2}\mathrm{VAl}$ is obtained by bulk sensitive resonant inelastic soft x-ray scattering (SX-RIXS) excited with circularly polarized light under an external magnetic field. The results under the V $L$-edge excitation have revealed the negligible partial density of states (PDOS) of the V $3d$ states around the Fermi energy as well as their rather localized character. Under the Mn $L$-edge excitation, on the other hand, the spectra are dominated by fluorescence with clear magnetic circular dichroism with noticeable excitation photon energy dependence. Compared with the theoretical prediction of the RIXS spectra based on the density-functional-theory band structure calculation, an itinerant, spin-dependent character of the Mn $3d$ states and decays of the Mn $2p$ core states are confirmed in consistence with the half-metallicity of the Mn $3d$ states.

Interband transitions in narrow-gap carbon nanotubes and graphene nanoribbons

We use the robust nearest-neighbor tight-binding approximation to study the same footing interband dipole transitions in narrow-bandgap carbon nanotubes (CNTs) and graphene nanoribbons (GNRs). It is demonstrated that curvature effects in metallic single-walled CNTs and edge effects in gapless GNRs not only open up bandgaps, which typically correspond to THz frequencies, but also result in a giant enhancement of the probability of optical transitions across these gaps. Moreover, the matrix element of the velocity operator for these transitions has a universal value (equal to the Fermi velocity in graphene) when the photon energy coincides with the bandgap energy. Upon increasing the excitation energy, the transition matrix element first rapidly decreases (for photon energies remaining in the THz range but exceeding two bandgap energies, it is reduced by three orders of magnitude), and thereafter it starts to increase proportionally to the photon frequency. A similar effect occurs in an armchair CNT with a bandgap opened and controlled by a magnetic field applied along the nanotube axis. There is a direct correspondence between armchair GNRs and single-walled zigzag CNTs. The described sharp photon-energy dependence of the transition matrix element, together with the van Hove singularity at the bandgap edge of the considered quasi-one-dimensional systems, makes them promising candidates for active elements of coherent THz radiation emitters. The effect of Pauli blocking of low-energy interband transitions caused by residual doping can be suppressed by creating a population inversion using high-frequency (optical) excitation.

MATHEMATICAL MODELING OF TEMPERATURE EFFECT ON THE FAN CHART OF MAGNETOABSORPTION SPECTRUM IN SEMICONDUCTORS

The article considers the oscillations of interband magneto-optical absorption in semiconductors with the Kane dispersion law. We have compared the changes in oscillations of the joint density of states with respect to the photon energy for different Landau levels in parabolic and non-parabolic zones. An analytical expression is obtained for the oscillation of the combined density of states in narrow-gap semiconductors. We have calculated the dependence of the maximum photon energy on the magnetic field at different temperatures. A theoretical study of the band structure showed that the magnetoabsorption oscillations decrease with an increase in temperature, and the photon energies nonlinearly depend on a strong magnetic field. The article proposes a simple method for calculating the oscillation of joint density of states in a quantizing magnetic field with the non-quadratic dispersion law. The temperature dependence of the oscillations joint density of states in semiconductors with non-parabolic dispersion law is obtained. Moreover, the article studies the temperature dependence of the band gap in a strong magnetic field with the non-quadratic dispersion law. The method is applied to the research of the magnetic absorption in narrow-gap semiconductors with nonparabolic dispersion law. It is shown that as the temperature increases, Landau levels are washed away due to thermal broadening and density of states turns into a density of states without a magnetic field. Using the mathematical model, the temperature dependence of the density distribution of energy states in strong magnetic fields is considered. It is shown that the continuous spectrum of the density of states, measured at the temperature of liquid nitrogen, at low temperatures turns into discrete Landau levels. Mathematical modeling of processes using experimental values of the continuous spectrum of the density of states makes it possible to calculate discrete Landau levels. We have created the three-dimensional fan chart of magneto optical oscillations of semiconductors with considering for the joint density of energy states. For a nonquadratic dispersion law, the maximum frequency of the absorbed light and the width of the forbidden band are shown to depend nonlinearly on the magnetic field. Modeling the temperature dependence allowed us to determine the Landau levels in semiconductors in a wide temperature spectrum. Using the proposed model, the experimental results obtained for narrow-gap semiconductors are analyzed. The theoretical results are compared with experimental results.

Structural and Optical Properties of Ga2Se3 Crystals by Spectroscopic Ellipsometry

Optical and crystalline structure properties of Ga2Se3 crystals were analyzed utilizing ellipsometry and x-ray diffraction (XRD) experiments, respectively. Components of the complex dielectric function (e = e1 + ie2) and refractive index (N = n + ik) of Ga2Se3 crystals were spectrally plotted from ellipsometric measurements conducted from 1.2 eV to 6.2 eV at 300 K. From the analyses of second-energy derivatives of e1 and e2, interband transition energies (critical points) were determined. Absorption coefficient–photon energy dependency allowed us to achieve a band gap energy of 2.02 eV. Wemple and DiDomenico single effective oscillator and Spitzer–Fan models were accomplished and various optical parameters of the crystal were reported in the present work.