Photon Energy Scale Research Articles

First principles calculation of structural, electronic and optical properties of (001) and (110) growth axis (InN)/(GaN)n superlattices

Based on the full potential linear muffin-tin orbitals (FPLMTO) calculation within density functional theory, we systematically investigate the electronic and optical properties of (100) and (110)-oriented (InN)/(GaN)n zinc-blende superlattice with one InN monolayer and with different numbers of GaN monolayers. Specifically, the electronic band structure calculations and their related features, like the absorption coefficient and refractive index of these systems are computed over a wide photon energy scale up to 20 eV. The effect of periodicity layer numbers n on the band gaps and the optical activity of (InN)/(GaN)n SLs in the both growth axis (001) and (110) are examined and compared. Because of prospective optical aspects of (InN)/(GaN)n such as light-emitting applications, this theoretical study can help the experimental measurements.

Visible-Light-Driven Methane Conversion with Oxygen Enabled by Atomically Precise Nickel Catalyst

It remains an extreme challenge to activate thermodynamically unfavorable, chemically inert methane molecules under mild conditions. Herein, we report a molecular-like nickel-thiolate hexameric clu...

Few-cycle high-harmonic generation in liquids: in-operando thickness measurement of flat microjets

Extreme ultraviolet high-harmonic generation (HHG) from bulk liquids has only recently been demonstrated (T.T. Luu, Z. Yin et al, Nat. Comm. 9, 3724, (2018)). This has opened new prospects for the development of bright high-harmonic sources and the development of liquid-phase high-harmonic spectroscopy (HHS). Here, we report on the first observation of HHG in liquids driven by few-cycle (∼7 fs) pulses. We observe the emission of a broad quasi-continuum in the extreme ultraviolet, which is strongly modulated on a photon-energy scale much finer than the central photon energy of the driver. We show that these modulations arise from an etalon effect inside the flat microjet, which we use to perform an in-operando measurement of the flat-jet’s thickness. The possibility to directly characterize flat microjets during HHG will greatly support their optimization for light-source applications and facilitate the development of liquid-phase HHS.

Photoionisation of Ca+ ions in the valence-energy region 20–56 eV: experiment and theory

Relative cross sections for the valence shell photoionisation (PI) of ground level and metastable Ca+ ions were measured with high energy resolution by using the ion–photon merged-beams technique at the Advanced Light Source. Overview measurements were performed with a full width at half maximum bandpass of meV, covering the energy range 20–56 eV. Details of the PI spectrum were investigated at energy resolutions reaching the level of meV. The photon energy scale was calibrated with an uncertainty of ±5 meV. By comparison with previous absolute measurements the present experimental high-resolution data were normalized to an absolute cross-section scale and the fraction of metastable Ca+ ions that were present in the parent ion beam was determined to be 18% ± 4%. Large-scale R-matrix calculations using the Dirac Coulomb approximation and employing 594 levels in the close-coupling expansion were performed for the and levels. The experimental data are compared with the results of these calculations and previous theoretical and experimental studies.

Quantum confinement effect on the electronic and optical features of InGaN-based solar cells with InGaN/GaN superlattices as the absorption layers

The short period group III-V nitrides superlattices (SLs), have turned out contemporary in the technology of optoelectronics and solar cell applications. Our theoretical simulation is carried out by means of first-principles full potential linearized augmented plane waves (FP-LAPW) methodology within generalized gradient approximations (GGA) in conjunction with the modified Becke–Johnson (mBJ) potential. In this respect, a comprehensive study for the electronic structures and optical aspects of (InxGa1−xN)n/(GaN)n (001) zinc-blende superlattices (SLs) (x=0.5 and n=3–4), is carried out. Specifically, the electronic band structure calculations and their related features, like the absorption coefficient, reflectivity, refractive index and electron energy loss function spectra of these systems are computed over a wide photon energy scale up to 25eV. The effect of periodicity layer numbers and In composition (x=50%) on the band gaps of (InxGa1−xN)n/(GaN)n SLs is examined. The tailoring of the underlying energy band gap relies on the In content and the periodicity of the superlattices. All these ultrathin superlattices (n=3–4) possess a direct energy band gap. The InGaN layers have immense prominence in ascertaining the underlying energy gap of these superlattice because of the distinctive quantum confinement impact. Furthermore, the flexible energy gap of these ultrathin-period SLs leads to the alteration of the absorption coefficients and static refractive indices. It is viable to attain InGaN solar cells possessing high efficiencies since the energy gap covers all the spectrum optical regime. Predominantly, it is feasible to tune the optical characteristics of these short-period SLs and provide plausible results for the optoelectronic devices applications.

Electron and photon performance with the CMS detector at [formula omitted

The performance of the electron and photon reconstruction and selection with the CMS detector at s=8 TeV is presented. Selection criteria and efficiency measurements in data and simulation are shown. The electron and photon energy scale calibration and resolution are also discussed, with particular emphasis on their role in Higgs analyses.

Numerical analysis of carrier multiplication mechanisms in nanocrystalline and bulk forms of PbSe and PbS

We report on a systematic numerical study of carrier multiplication (CM) processes in spherically symmetric nanocrystalline and bulk forms of PbSe and PbS representing the test bed for understanding basic aspects of CM dynamics. The adopted numerical method integrates our previously developed interband exciton scattering model and the effective mass based electronic structure model for the lead chalcogenide semiconductors. The analysis of CM pathways predicted by the interband exciton scattering model shows complete lack of their interference during the biexciton photogeneration. This allows us to interpret this process as a single impact ionization event and to explain a major contribution of the multiple impact ionization events during the phonon-assited population decay into the total quantum efficiency (QE). We investigate the role of quantum confinement on QE and find that the reduction in the biexciton density of states (DOS) overruns weak enhancement of the Coulomb interactions leading to lower QE values in nanocrystals as compared to the bulk on the absolute photon energy scale. However, represented on the photon energy scale normalized by corresponding band gap energies, the trend in QE is opposite demonstrating the advantage of nanocrystals for the photovoltaic applications. Comparison to published experimental data allows us to interpret the observed features and to validate the applicability range of our model. Modeling of QE as a function of pulse duration shows weak dependence for the Gaussian pulses. Finally, comparison of the key quantities determining QE in PbSe and PbS demonstrates the enhancement of impact ionization rate in the latter materials. However, the fast phonon-assisted population decay in PbS nanocrystals can lead to experimentally observed reduction in QE as compared to PbSe nanocrystals.

Correlation effects on optical properties of TbOFeAs

Based on local density approximation and Hubbard-U corrections (LDA+U), we study the influence of Coulomb interaction for Tb 4f states on the optical properties of the recently discovered superconductor, such as TbOFeAs. Within the incorporation of onsite Hubbard effect in TbOFeAs, we discuss the electronic structure, as well as the optical spectra and we compared them to LDA calculations. For non-magnetic (NM) configuration, the electronic structure exhibits high density of states, N(EF) in the proximity of Fermi level. With and without the electronic correlation effects, we carried out the calculations for the optical properties such as the optical conductivity, joint density of states (JDOS), optical absorption, the electron energy loss function and reflectivity of TbOFeAs in a large photon energy scale of 30 eV. Despite the absence of a Mott insulator transition, we infer that the electronic correlation effects are prominent in the recently discovered superconductor, like TbOFeAs. We also predict the in-plane anisotropy of plasma frequency that has been evaluated recently in the other ReOFeAs systems.

Photon energy scale determination and commissioning with radiative Z decays

The CMS electromagnetic calorimeter (ECAL) is composed of 75848 lead-tungstate scintillating crystals. It has been designed to be fast, compact, and radiation hard, with fine granularity and excellent energy resolution. Obtaining the design resolution is a crucial challenge for the SM Higgs search in the two photon channel at the LHC, and more generally good photon calibration and knowledge of the photon energy scale is required for analyses with photons in the final state. The behavior of photons and electrons in the calorimeter is not identical, making the use of a dedicated standard candle for photons, complementary to the canonical highyield Z decay to electrons, highly desirable.The use of Z decays to a pair of muons, where one of the muons emits a Bremsstrahlung photon, can be such a standard candle. These events, which can be cleanly selected, are a source of high-purity, relatively high-pt photons. Their kinematics are well-constrained by the Z boson mass and the precision on the muon momenta, and can be used for numerous calibration and measurement purposes. This proceeding presents the event selection method and the results of the photon energy scale measurement via Z 0 → μμγ events as well as their use in evaluating the efficiency of photon identification requirements, based on data recorded by the CMS experiment in 2010.

Ba 5s photoionization in the region of the second Cooper minimum

We investigate the 5s angular distribution parameter and partial photoionization cross section of atomic Ba in the region of the second Cooper minimum covering a photon energy region from 120 to 260 eV. We observe a strong drop in the Ba 5s β value from 2.0, reaching a minimum of 1.57 ± 0.07 at a photon energy of 150 eV. The β value then slowly rises back towards its nominal value of 2.0 at photon energies beyond the minimum. Our measured 5s partial cross section also shows a pronounced dip around 170 eV due to interchannel coupling with the Ba 4d photoelectrons. After combining our measurements with previous experimental values at lower photon energies, we obtain a consistent data set spanning the photon energy range prior to the onset of the partial cross section maximum and through the cross section minimum. We also calculate the 5s partial cross section under several different levels of approximation. We find that the generalized random-phase approximation with exchange calculation models the shape and position of the combined experimental cross section data set rather well after incorporating experimental ionization energies and a shift in the photon energy scale.

Calibration of the CMS electromagnetic calorimeter at the LHC start up

The electromagnetic calorimeter of the CMS experiment is designed to reach excellent energy resolution, essential for the Higgs search in the H → γ γ decay channel. The non-uniformity of the individual channel responses directly contributes to the energy resolution. The target precision of the detector calibration, both of the relative response of the channels (inter-calibration) and of the electron and photon energy scale, at the 0.5% level, will be reached employing the collision events collected at the LHC. One-quarter of the barrel ECAL has been calibrated at this level of precision on electron beams. The entire barrel was calibrated with cosmic rays before installation in CMS with an accuracy at the 2% level, giving acceptable performance at the startup. During the commissioning phase in CMS about 600 million muon events have been recorded allowing to verify that the working conditions used during the pre-calibration were reproduced in CMS. The different phases of the pre-calibration procedures are summarized and the techniques for the calibration with the first LHC data are presented.

Current sign inverter guided by polarization

In this work, there are studied the photodetectors’ properties (photocurrent sign inverter) of a linear polarized radiation realized on the basis of anisotropic semiconductors. The photocurrent sign guiding is made by the changing of polarization plane of a linear polarized light wave. It was determined that the inversion point can be moved over the photon energy scale while applying an external displacement. It is shown in the work the influence of the light wave interference on the properties of the photodetector’s photosensibility. It was revealed the possibility of creating narrow band photodetectors with sensibility maximums suitable to be used in fiber optic communications. There had been analyzed some cases of using the guided photocurrent sign inverter in which the active elements are the p–n structures, Schottky diodes and twins.

High-resolution absolute photoabsorption cross sections for Ne in the 1s2s and 1s2p resonant double excitation

The high-resolution absolute photoabsorption cross sections with an absolute photon energy scale for Ne 1s2p/1s2s double excitation in the energy region of 900–940 eV have been measured using a multi-electrode ionization chamber and monochromatized synchrotron radiation. The Ne [1s2p](3,1P)nln′l′ doubly excited states observed in the energy range of 900–920 eV are analysed with the aid of theoretical calculations based on the multiconfiguration Dirac–Fock method. The second strongest peak in this energy region is assigned to be [1s2p](3P)3p4p.

Normal distribution of confinement energy from a photoluminescence line shape analysis in oxidized porous silicon

Photoluminescence (PL) spectra of oxidized porous silicon (PS) samples plotted against photon energy scale on x-axis would have pure Gaussian form or asymmetric form, depending on the conditions of preparation. At first a pure Gaussian-shape spectrum plotted against photon energy scale on x-axis is analyzed by statistical method on the assumption that the energy of emitted photon would be considered as a random variable simultaneously affected by two independent factors, namely: the statistical spread of wire width and the fluctuation in barrier height of SiOx/Si structures. Comparison of visual plots and statistic characteristic between three spectra plotted against various variables (confinement energy, logarithmic wire width and wire width) scale on x-axis demonstrates the exact description of the spectrum by Gaussian profile using normal distribution of confinement energy. An expression correlating the PL intensity (the number of emitted photons) to the number of wires and quantum efficiency has been developed. Further this expression has been extended to the case of spectrum having asymmetric form, which corresponds to the emission from both wire and dot nano-structures. Thus, our model permits to calculate the structural and spectroscopic parameters such as proportion of wires and dots, integrated intensity, peak energy position, FWHM and standard deviation.

High-accuracy determination of the dependence of the photoluminescence emission energy on alloy composition in AlxGa1−xAs films

In an effort to improve the accuracy of photoluminescence (PL) measurements of the Al mole fraction (x) of AlxGa1−xAs alloys, the PL peak emission energy, EPL,peak, was measured at room temperature for molecular-beam epitaxy-grown AlxGa1−xAs films with 0⩽x<0.37, and correlated with independent measurements of x by in situ reflective high-energy electron diffraction (RHEED) and also by ex situ wavelength-dispersive x-ray spectroscopy in an electron microprobe analyzer (WDS/EMPA). The measurement uncertainty of EPL,peak was minimized through the following procedures: Accurate calibration of the photon energy (or wavelength) scale, correction of the measured spectra for the spectrometer response function, fitting the data with a well-chosen line shape function, and compensation for the effect of ambient temperature drift. With these procedures, the 2σ measurement uncertainty of EPL,peak was of the order 5×10−4 eV for most samples. From correlation of the PL and WDS/EMPA composition data, the slope ∂EPL,peak/∂x near room temperature was determined to be ∂EPL,peak/∂x=(1.4017±0.0090 eV)−[(2.71±0.97)×10−4 eV/K](T−298.3 K). Correlation with the RHEED data gave the same result within measurement uncertainty. Previously published measurements of ∂EPL,peak/∂x were reviewed and compared with the present study. The results of T. F. Kuech et al. [Appl. Phys. Lett. 51, 505 (1987)], based on nuclear resonant reaction analysis of the Al mole fraction, were found to be in good agreement with the present study after the addition of a correction term to account for the sample temperature difference (T=2 K for Kuech et al., T=298 K for the present study).

Two-loop self-energy corrections to the fine structure

We investigate two-loop higher-order binding corrections to the fine structure, which contribute to the spin-dependent part of the Lamb shift. Our calculation focuses on the so-called ``two-loop self-energy'' involving two virtual closed photon loops. For bound states, this correction has proven to be notoriously difficult to evaluate. The calculation of the binding corrections to the bound-state two-loop self-energy is simplified by a separate treatment of hard and soft virtual photons. The two photon-energy scales are matched at the end of the calculation. We explain the significance of the mathematical methods employed in the calculation in a more general context, and present results for the fine-structure difference of the two-loop self-energy through the order of $\alpha^8$.

Crystalline potential for photoelectron scattering phase-shift calculations and X-ray absorption spectra of Ti in crystals

X-ray absorption spectra (XAS) of Ti in crystals of Ti metal, TiC, TiN and TiO2 rutile are calculated in EXAFS and XANES regions. Single, double and triple photoelectron scattering processes are included through the spherical-wave formalism and the crystalline muffin-tin (MT) potential model, which employs a Hartree-Fock exchange interaction between photoelectron and core electrons inside MT spheres. Overall agreement of the theoretical and experimental spectra is obtained on the absolute photon energy scale. This result enables one to treat the experimental XAFS without using adjustable parameter E0. The neutral-atoms approximation is shown to be adequate for the phase-shift calculations in XAS theory for both metals and insulators. A comparison of Ti K XAFS of Ti metal calculated by means of various exchange-correlation potentials for the photoelectrons inside MT spheres is carried out. The best results are shown to be provided by Hartree-Fock and Dirac-Hara exchange potentials.

Electronic valence band structure of high-T c superconductors : X-ray emission spectroscopy study

The distribution of the electronic density of O 2p and Cu 3d states in a series of bismuthates and cuprates has been studied by X-ray emission spectroscopy (XES). It has been found that in BaPb 1- x Bi x O 3, La 2- x Sr x CuO 4 and YBa 2Cu 3O 7- x , a high-energy shift of the O Kα spectrum in both photon energy and Fermi level-related scales occurs during the transition from semiconducting to superconducting phase. This shift is explained by the redistribution of 2p states of apical oxygen atoms. The binding energy of Cu 3d states for YBa 2Cu 3O 6.2 is rather low in contrast to that for YBa 2Cu 3O 6.9 because of the presence of monovalent copper atoms in the semiconductor as a result of oxygen depletion of Cu-O chains. A correlation between T c and the energy localization of the O 2p band relative to E F has been observed in a series of cuprates, the superconducting transition temperature of which varies from 35 to 125 K. On increasing T c, O 2p states shift towards the Fermi level while the binding energy of Cu 3d states insignificantly rises.

The problem of potential construction and phaseshift calculation in X-ray-absorption spectra theory of molecules and complexes containing low-Z atoms

K X-ray absorption spectra of molecules and complexes containing low-Z atoms (CO, CO2, OCS, ferrocene) are studied. Agreement of theoretical and experimental spectra on the photon energy scale is obtained if the electron scattering phaseshifts and amplitudes are calculated with the help of the spherical wave formalism and the non-local Hartree-Fock muffin-tin (MT) potential, constructed so as to provide the potential continuity on MT sphere surfaces. In the case of molecules and complexes the approach proposed enables one to determine MT zero position in relation to the absorption threshold without any fitting procedure.

A study of post-collision interaction effects in Kr 3d and Xe 4d near-threshold photoionization

Near-threshold photoionization of the Kr 3d and Xe 4d subshells has been studied by measuring the emitted electrons with various fixed energies (0-10 eV) against photon energy. These spectra show peaks corresponding to the Kr(3d)-1 and Xe(4d)-1 ionic states as well as peaks due to the decay of the Kr(3d)-1nl and Xe(4d)-1nl core excited Rydberg states. The peaks due to the ionic states are observed to shift to higher photon energy, while their profiles are asymmetrically broadened. Both the effects are attributed to post-collision interaction (PCI). The Rydberg peaks enable accurate calibration of the photon and electron energy scales. The observed energy shifts of the photoelectron peaks and their lineshapes are compared with theoretical predictions of the semiclassical model of van der Straten et al. (1988).