Photoionization Cross Sections Research Articles

Validation of E‐Region Model Electron Density Profiles With AURIC Utilizing High‐Resolution Cross Sections

AbstractE‐region models have traditionally underestimated the ionospheric electron density. We believe that this deficiency can be remedied by using high‐resolution photoabsorption and photoionization cross sections in the models. Deep dips in the cross sections allow solar radiation to penetrate deeper into the E‐region producing additional ionization. To validate our concept, we perform a study of model electron density profiles (EDPs) calculated using the Atmospheric Ultraviolet Radiance Integrated Code (AURIC; D. Strickland et al., 1999, https://doi.org/10.1016/s0022-4073(98)00098-3) in the E‐region of the terrestrial ionosphere. We compare AURIC model outputs using new high‐resolution photoionization and photoabsorption cross sections, and solar spectral irradiances during low solar activity with incoherent scatter radar (ISR) measurements from the Arecibo and Millstone Hills observatories, Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC‐1) observations, and outputs from empirical models (IRI‐2016 and FIRI‐2018). AURIC results utilizing the new high‐resolution cross sections reveal a significant difference to model outputs calculated with the low‐resolution cross sections currently used. Analysis of AURIC EDPs using the new high‐resolution data indicate fair agreement with ISR measurements obtained at various times at Arecibo but very good agreement with Millstone Hills ISR observations from ∼96–140 km. However, discrepancies in the altitude of the E‐region peak persist. High‐resolution AURIC calculations are in agreement with COSMIC‐1 observations and IRI‐2016 model outputs between ∼105 and 140 km while FIRI‐2018 outputs underestimate the EDP in this region. Overall, AURIC modeling shows increased E‐region electron densities when utilizing high‐resolution cross sections and high‐resolution solar irradiances, and are likely to be the key to resolving the long standing data‐model discrepancies.

New O Partial Photoionization Cross Sections Resolve Ionospheric EUV Remote Sensing Issues

AbstractThe ionospheric O+ number density can be measured remotely during the day by observing its optically thick 83.4 nm radiance. Some ambiguity is present in the process of retrieving the density due to uncertainties in the initial excitation rate. This can be removed by observing a companion optically thin emission at 61.7 nm originating from the O+(3s 2P) state, providing that the ratio of the initial excitation rates is known. Analyses of ICON EUV data using an 83.4/61.7 emission ratio of order 10 result in O+ densities lower by ∼2 than other measurements. Key to relating the two emissions is accurate knowledge of the partial photoionization cross sections and the spectroscopy of O+—the topic of this paper. Up to now, no independent evaluation of the ratio of the 83.4/61.6 emission ratio exists. The recent availability of state‐of‐the‐art calculations of O partial photoionization cross sections into a variety of O+ states presents an opportunity to evaluate the O+(2p4 4P)/O+(3s 2P) ionization rate ratio. We calculate excitation of these parent states of the emissions including both direct and cascade excitation from higher lying O+ energy states. The resulting theoretical prediction gives ratios that range from 13.5 to 12 from solar minimum to maximum, larger than the value of 10 used by the ICON 83.4 and 61.7 nm algorithm. The higher theoretical values for the ratio reconcile the ∼2 discrepancy between simultaneous ICON and other electron density measurements.

A combined theoretical and experimental study of the pyrolysis of pyrrolidine

Pyrrolidine is a suitable model substance featuring a five-membered N-heterocycle representing the typical structure of the N-containing compounds in biomass. Previous studies have provided ambiguous arguments on the reaction mechanism of pyrrolidine thermal decomposition. Knowledge on the fate of the most dominant decomposition product, the unstable diradical ·CH2NHCH2·, is lacking. In this work, a high-level potential energy surface of the unimolecular reactions of ·CH2NHCH2·, including isomerization and decomposition channels, was explored. Then, the rate coefficients of various channels were obtained by the RRKM/master equation method over 500–2000 K and 0.001–100 atm. The results show that the thermal stabilization of cyc-C2H5N is highly favored over other isomerization and decomposition channels. The channels isomerizing to CH3NCH2, cis-HNCHCH3 and trans-HNCHCH3 compete with each other, and the rate constants are at least two orders of magnitude lower than the formation of cyc-C2H5N. Being thermodynamically unstable, cyc-C2H5N will mainly isomerize back into the diradical at temperatures ≤ 1200 K at 1 atm or isomerize to cis-HNCHCH3 when the temperature is higher. To validate the postulated reaction pathways, a pyrolysis experiment of pyrrolidine was conducted in a SiC reactor with a short residence time (40–60 μs) at 1050 K and 0.263 atm. The experimental result confirms the collisional stabilization of H2NCHCH2 and cyc-C2H5N + CH3NCH2. The diradical ·CH2NHCH2· was not readily detectable due to its low concentration, which falls below the detection limit of current analytical techniques, while the stabilization of cis-HNCHCH3 and trans-HNCHCH3 was not sure because of their extremely low photoionization cross section under the studied energy range. The rate constants of the isomerization and decomposition reactions of diradical ·CH2NHCH2· and cyc-C2H5N are provided, which are valuable for developing the mechanism for pyrrolidine and deepening our understanding of the mechanism of N-heterocyclic compounds pyrolysis/combustion.

Ionization of aluminum clusters

Near-threshold photoionization cross sections for several aluminum clusters have been estimated from the ionization efficiency curves. Their magnitude raises the possibility that the edge of a collective electron resonance (surface plasmon) may contribute to the photoemission channel. On the other hand, electron impact ionization efficiently produces multiply charged metal cluster ions and is unaccompanied by noticeable fragmentation. Consequently it does not appear to involve collective excitations.

Total and orbital density-based analyses of molecules revealing long-range interaction regions.

Total and orbital electron densities of molecules are explored for the effect of the long-range correction (LC) for density functional theory (DFT) exchange functionals by comparing to the effect of the ab initio coupled cluster singles and doubles (CCSD) method. Calculating the LC effect on the total electron densities shows that the LC stabilizes the electrons around the long-range interaction regions of kinetic energy density, which are assumed to be electrons other than free electrons and self-interacting electrons, while the CCSD method stabilizes the electrons in the long-range interaction regions in the vertical molecular planes. As a more precise test, the LC effect on orbital densities are compared to the CCSD effect on Dyson orbital densities. Surprisingly, these effects are similar for the unoccupied orbitals, indicating that the LC covers the effects required to reproduce the CCSD Dyson unoccupied orbitals. For exploring the discrepancies between these effects on the occupied orbitals, the photoionization cross sections are calculated as a direct test for the shapes of the HOMOs to investigate the differences between these effects on the occupied orbitals. Consequently, the LC clearly produces the canonical HOMOs close to the CCSD Dyson and experimental ones, except for the HOMO of benzene molecule that mixes with the HOMO 1 for the CCSD Dyson orbitals. This indicates that the orbital analyses using the photoionization cross sections are available as a direct test for the quality of DFT functionals.

Tuning of binding energy, polarizability and photoionization cross-section of a hydrogenic impurity in CdTe/ZnTe core-shell nanostructure by the electric field and capped matrix

The current work describes in detail the effect of polarization charges on the electronic properties of core/shell spherical quantum dots (CSQDs) structures composed of CdTe/ZnTe semiconductors. In the calculations, energy levels and wave functions matching these levels were calculated by using the effective mass approach (EMA). Furthermore, the binding energy of a hydrogenic impurity was calculated under the variational approach. The matrix effects surrounding the CSQDs structure, the quantum dot size, the impurity, and how the applied external electric field affects the impurity binding energy (Eb), polarizability (Pa) and photoionization cross-section (PICS) are calculated and discussed in detail. The obtained results revealed that the Eb, the PICS and Pa are not only affected by the quantum parameter size but also by the capped matrix, the impurity and external electric field EF actions.

Luminescence characterization of BioGlass undoped and doped with europium and silver ions

The objective of this study was to investigate the properties of BioGlass, with and without doping with europium and silver, with a specific focus on its potential application in thermoluminescent (TL) and optically stimulated luminescent (OSL) dosimetry. The structural and optical characteristics of the samples were also analyzed using techniques such as X-ray diffraction (XRD), optical absorption (OA), and fluorescence spectroscopy (FL). An XRD analysis confirmed the amorphous phase of the BioGlass. OA and FL spectra were obtained at room temperature, and characteristic bands of dopant ions were observed which confirmed the incorporation of the Eu3+ ions and silver nanoparticles Ag(NP) ion into the BioGlass. The OSL decay curves exhibited a characteristic exponential behavior, with a notable presence of fast and medium decay components; this suggests that the charge traps within the BioGlass samples possess a high photoionization cross section when exposed to blue LEDs, which are commonly used as the light source in OSL readers. Different TL glow peaks with varying shapes of the glow curve were observed when the dopant, the co-dopant, and the concentration of silver were altered in the samples. The TL kinetic parameters were determined, such as the order value, activation energy, and frequency factor, and the OSL parameters for the compound were also analyzed, including an exponential fit to the curves. Based on these initial results, we conclude that BioGlass has the potential for use in radiation dosimetry.

Electric and Magnetic Fields Effects in Vertically Coupled GaAs/AlxGa1−xAs Conical Quantum Dots

Vertically coupled quantum dots have emerged as promising structures for various applications such as single photon sources, entangled quantum pairs, quantum computation, and quantum cryptography. We start with a structure composed of two vertically coupled GaAs conical quantum dots surrounded by AlxGa1−x, and the effects of the applied electric and magnetic fields on the energies are evaluated using the finite element method. In addition, the effects are evaluated by including the presence of a shallow-donor impurity. The electron binding energy behavior is analyzed, and the effects on the photoionization cross-section are studied. Calculations are carried out in the effective mass and parabolic conduction band approximations. Our results show a notable dependence on the electric and magnetic fields applied to the photoionization cross-section. In general, it has been observed that both the electric and magnetic fields are useful parameters for inducing blueshifts of the resonant photoionization cross-section structure, which is accompanied by a drop in its magnitude.

The effect of boron co-doping on thermoluminescence and optically stimulated luminescence properties of LiMgPO4:Tb,Sm

The effect of boron co-doping on optically stimulated luminescence (OSL) and thermoluminescence (TL) properties of LiMgPO4:Tb,Sm,B prepared by solid-state reaction was studied. The structure of the prepared materials was confirmed by X-ray diffraction. The TL glow curves of samples irradiated with different doses of β rays were tested. No matter how much B is doped, the peaks at 190 or 280 °C are for a first-order kinetic glow curve. The activation energy and frequency factor remained almost unchanged with the increase of boron doping concentration, which indicated the trap centers of LiMgPO4:Tb,Sm,B were independent of boron doping concentration. The E at 190 and 280 °C are around 1.16 eV and 1.55 eV. Photo-ionization cross-section showed significant changes and the sensitivity of OSL reached a maximum for a concentration of 4 mol% with increasing of boron doping concentration. B-doping enhanced the OSL-signal with a factor 2.6 when B concentration is 4 mol%. The repeatability of the powders for OSL were also studied and the relative standard deviation was less than 1.5%. The minimal detectable dose (MDD) of LiMgPO4:Tb,Sm,B with a boron doping concentration of 4 mol% was 8.2 μGy. This material has a fast decay time, good repeatability, low MDD and has further research value.

Vibrationally Resolved Absorption and Fluorescence Cross Sections of Adamantane in the Far-ultraviolet Spectral Range on an Absolute Scale

High-resolution absorption, dispersed fluorescence emission, and photoionization cross sections are presented for gas-phase adamantane excited by synchrotron radiation in the exciting-photon energy range of 6–30 eV. Relative and absolute absorption cross sections of so-far unmatched resolution of down to 0.27 cm−1 line width in the region from 6.4–28 eV are shown along with newly discovered vibronic substructures around the HOMO–LUMO transition. Absorption line positions are provided with very high accuracy and listed in tabular form to be used as spectral fingerprints for the detection of adamantane in interstellar media, where its column density may be determined via the absolute cross sections. The fluorescence emission lies in the ultraviolet range from 190–250 nm and is excited starting at the HOMO–LUMO transition at 6.49 eV, which corresponds to the highest fluorescence emission energy. Hitherto unreported fluorescence in the same spectral range and relative photoionization cross sections in the exciting-photon energy range up to 30 eV are also presented along with lifetime measurements for differentiation of the involved electronic states.

Photoionization Cross Section for H@Cn${\text{H@C}}_{n}$ Implanted in Nonideal Classical Plasmas

AbstractIn this study, photoionization cross sections of guest hydrogen atom in endohedral fullerene () modeled by the Woods–Saxon confinement potential implanted in the nonideal classical plasma (NICP) under spherical confinement are reported for the first time in the related literature. The relevant wave equation is solved numerically via the tridiagonal matrix method and then the energy levels, bound and continuum wave functions are interpreted. Plasma effect is examined by considering plasma temperature and density and is evaluated in the photoionization process. Since the plasma modifies the discrete and continuum spectra by changing the potential energy of hydrogen atom, it closely affects the overlapping of the wave functions of ground state and continuum state. This effect has a distinct response on photoionization resonances. Using different values of endohedral confinement parameters, which means regarding different types of fullerenes, detailed analysis of energy levels, bound and continuum wave functions, and photoionization cross sections are provided by evaluating confinement width, depth, smoothing effect and distance from the spherical encompassement center. The photoionization process of implanted in the NICP (nonideal classical plasma) is highly sensitive to both plasma and endohedral confinement. At this sensitivity, Cooper resonance character is like an encoder for fullerene structure.

A novel and holistic approach for experimental x-ray fundamental parameter determination—the Ru L-shell

The present work aims to improve both the experimental and the data evaluation procedures necessary for a determination of L-subshell atomic fundamental parameters (FPs). The main motivation lies in the fact that the established approach, consisting of a sequential evaluation of recorded transmission and fluorescence spectra in the vicinity of the three L-absorption edges, provides relatively large uncertainties for the Coster–Kronig (CK) factors. Up to now, reliable uncertainty budgets were in the order of 15%–100% and more. The novel holistic evaluation procedure addresses these large uncertainties, by employing a much larger experimental dataset extending far above the L1 absorption edge and a combined evaluation approach for a more reliable determination of the CK factors. Here, using the Ru-L shells as an example, their uncertainties are reduced to under 15%. In addition, this enables also the determination of other relevant FP data, such as the L-subshell Auger yields, the L-subshell photoionization cross sections as well as the L-subshell fluorescence production cross sections with significantly lower uncertainties as compared to the established approach.

Photodissociation and photoionization of molecules of astronomical interest

Context. Vacuum-ultraviolet (VUV) photons are important drivers of chemical processes in space. Thus, it is important to accurately characterize and constrain photorates in different radiation fields, via the photodissociation and photoionization cross sections of individual atoms and molecules. These have been available in the Leiden VUV photodissocation and photoionization cross section database. Aims. Experimental and theoretical advances in the past decade or so have allowed multiple new cross sections to be obtained, particularly photoionization cross sections of radicals. The database is hereby updated by including these more recent cross sections and is also expanded with several astronomically relevant species. Methods. The cross sections have been used to calculate photodissociation and photoionization rates in several different radiation fields as well as from cosmic-ray-induced VUV fluxes. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic carbon, and self-shielding column densities. The relative importance of these shielding types is molecule and atom dependent, as well as the assumed dust absorbance. All the data are publicly available from the Leiden VUV cross section database. Results. The Leiden VUV cross section database has been updated with 14 new astrophysically relevant molecular species and 16 updates to previous entries. The database update is accompanied by a brief review of the basic physical processes, particularly photoionization processes which have not been reviewed in the context of previous database updates.

Autoionization from the plasmon resonance in isolated 1-cyanonaphthalene.

Polycyclic aromatic hydrocarbons have widely been conjectured to be ubiquitous in space, as supported by the recent discovery of two isomers of cyanonaphthalene, indene, and 2-cyanoindene in the Taurus molecular cloud-1 using radioastronomy. Here, the photoionization dynamics of 1-cyanonaphthalene (1-CNN) are investigated using synchrotron radiation over the hν = 9.0-19.5eV range, revealing that prompt autoionization from the plasmon resonance dominates the photophysics for hν = 11.5-16.0eV. Minimal photo-induced dissociation, whether originating from an excited state impulsive bond rupture or through internal conversion followed by a statistical bond cleavage process, occurs over the microsecond timescale (as limited by the experimental setup). The direct photoionization cross section and photoelectron angular distributions are simulated using an ezDyson model combining Dyson orbitals with Coulomb wave photoejection. When considering these data in conjunction with recent radiative cooling measurements on 1-CNN+, which showed that cations formed with up to 5eV of internal energy efficiently stabilize through recurrent fluorescence, we conclude that the organic backbone of 1-CNN is resilient to photodestruction by VUV and soft XUV radiation. These dynamics may prove to be a common feature for the survival of small polycyclic aromatic hydrocarbons in space, provided that the cations have a suitable electronic structure to support recurrent fluorescence.

Studies on luminescence properties of self-activated SrMoO4 phosphor: Kinetic analysis

The structure, photoluminescence, thermoluminescence and optically stimulated luminescence of SrMoO4 phosphor and its kinetic analysis have been investigated. The SrMoO4 sample was prepared by the solid-state route. Rietveld refinement of XRD pattern confirms that the SrMoO4 is in a tetragonal structure with space group I41/a. The particles are agglomerated and their size is around 5.8 ± 1.2 µm confirmed by scanning electron microscopy results. The observed seven internal modes of vibration in the Raman spectrum indicate the synthesized SrMoO4 is in scheelite type. Photoluminescence (PL) shows a broad peak with maxima at 490 nm when excited at 288 nm. An electron paramagnetic resonance spectrum shows a signal at a g-factor of 2.17 and is assigned to Mo5+ ions. Thermoluminescence (TL) of SrMoO4 shows two peaks, a prominent one at 365 K and a secondary peak at 472 K measured at a heat rate of 1 K s−1. Thermal cleaning and Tm-Tstop analysis reveal five peaks associated with the glow curve. The kinetic parameters analyzed using various methods show 365 K peak follows the non-first order of kinetics and the activation energy of this peak is about 0.78 ± 0.04 eV. The CW-OSL response increases linearly with a dose from 10 to 100 Gy. The CW-OSL and LM-OSL of SrMoO4 phosphor irradiated with 100 Gy fitted for three decay components. The photoionization cross sections values are (8.25 ± 0.24) × 10–18, (9.47 ± 0.61) × 10–19 and (8.14 ± 0.82) × 10–19 cm2 for fast medium and slow components respectively. A comparison of TL and residual TL (TL recorded after CW-OSL) confirms the traps of TL and OSL are the same and may possibly be shallow traps.

Intense Laser Field Effect on the Photo-Ionization Cross-Section of the First Exciton Transition in a Core/Shell Quantum Dot Submitted to an Applied Electric Field

In the current work, we study the intense laser pulse influences on the behaviors of the first excitonic transition in a core/shell quantum dot submitted to an electric field. Therefore, the exciton binding energy and the mean distance between the correlated electron–hole pair are discussed, considering the electric field and laser strength. Our calculations show that both external fields play significant repulsive effects. Through their effects, they oppose the attractive nature of the Coulomb potential between the correlated pair, which decreases the excitonic binding energy. We also analyze the dissociation process by determining the photo-ionization cross-section (PICS). Our findings show that the peaks of the PICS redshift when the shell thickness b−a increases. For a given core radius, the laser and electric field induce a shift toward the low-energy region for the PICS; this displacement is more pronounced for the laser case. Our study also compares simple quantum dots and core/shell quantum dots to show the effect of the inner radius on the obtained results. Our theoretical results can lead to promising applications of exciton-based devices controlled by sizes and external fields.

Photoionization of C60 at high energies

Calculations of the photoionization cross section of C60 including all valence and core subshells have been carried out for photon energies up to 1 keV using density functional theory (DFT) and time-dependent DFT (TDDFT) within the framework of a fully molecular model. The high-energy valence subshell cross sections behave rather differently than the results from model potentials where the cross section falls far too rapidly with energy. This unphysical behavior of the model calculations is traced to smearing out of the carbon nuclei in the model potentials which makes it difficult for momentum to be conserved at the higher energies. Comparison with 60 times carbon atomic cross section sheds light on similarities and differences. The high energy behavior of individual valence cross sections reflects the amount of C 2s/2p mixing in the molecular orbitals.

Semiclassical approximations of photoabsorption cross sections beyond the continuum threshold

We develop semiclassical approximations for calculating photoabsorption cross sections beyond the continuum threshold in quantum many-body systems. These approximations use the fully quantum-mechanical Wigner function of the ground state and semiclassical expansions only for the part of the cross section depending on the continuum states, thus avoiding the difficult explicit calculation of the continuum states. Even though the approach is general, we test it in electronic-structure theory for the photoionization cross sections of the hydrogen and helium atoms. The results suggest that these semiclassical approximations can be used to obtain good estimates of cross sections at high energy.

High-temperature oxidation and a comprehensive kinetic mechanism of HFO-1234yf

To cope with the environmental problems caused by the greenhouse effect, the world has focused on the issue of refrigerant replacement. The potential alternative 2,3,3,3-tetrafluoropropene (CH2CFCF3, HFO-1234yf) has excellent thermal properties, system performances and environmental characteristics, while has the risk of combustion. In this study, the oxidation process of HFO-1234yf was first studied by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The important oxidation products of HFO-1234yf were detected, which included fluorine-containing compounds CH2F2, CHF3, CH2FCF3 and CF3CF3 except for CO, CO2, H2O. The molar fraction distributions of these specific fluorine-containing species were derived by the photoionization cross section measurements, which provided new test data to construct the combustion mechanism of HFO-1234yf. Furthermore, eleven newly oxidation paths of HFO-1234yf were proposed by the quantum chemistry calculation method. A modify kinetic model for HFO-1234yf combustion was established, which included 1,557 reactions and 193 species. In addition, rates of production (ROP) analysis of intermediates C2H2, CH2F2, CHF3, CF3CF3 were performed to probe the microscopic paths of HFO-1234yf oxidation. The results showed that most of HFO-1234yf were consumed by reacting with free radicals of O, H, OH, F and CF3. The sensitivity analysis indicated that HFO-1234yf was most sensitive to chemical reactions H + O2 = O + OH and CF3 + O2 = CF3O + O, and these reactions provided plenty of free radicals to promote the decomposition of HFO-1234yf.

Photoionization cross-section and polarizability of impurity in CdS/ZnS core/shell quantum dots capped in a dielectric matrix

Using the effective mass approximation (E.M.A), we have performed theoretically the influence of the capped matrix, the geometrical parameters and the applied electric intensity on the Polarizability and photoionization cross-section (PCS) of CdS/ZnS core/shell quantum dots CSQDs. The energy states of the binding energy (BE) and their corresponding wave functions are computed by solving Schrödinger equation in three-dimensional based on the variational approach. Our numerical results show that the EF and the nature of dielectric medium have an obvious influence on the BE, PCS and Polarizability of the structure. The present study suggests that the c/s nanostructure dispersed in a dielectric matrix may improve the performance of optoelectronic and photonic devices.