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Electron ionization, photoionization and photoelectron/photoion coincidence spectroscopy in mass-spectrometric investigations of a low-pressure ethylene/oxygen flame
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Probing sulphur clusters in a microfluidic electrochemical cell with synchrotron-based photoionization mass spectrometry
We present synchrotron-based mass spectrometry to probe products formed in the lithium sulphide electrolyte. In operando analysis was obtained at two different potentials in a vacuum compatible microfluidic electrochemical cell. Mass spectral observations show that the charged electrolyte formed sulfur clusters under dynamic conditions, demonstrating electrolyte electron shuttling.
Photoionization Cross Section of 1s Orthoexcitons in Cuprous Oxide
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Flame-Sampling Photoionization Mass Spectroscopy: Studies of Flame Chemistry with Sampling Photoionization Mass Spectrometry
Research focused on detailed studies of the complex combustion chemistry of oxygenated, bio-derived fuels. In particular, studies were done of the flame chemistry of simple methyl and ethyl esters chosen as surrogates for the long-chain esters that are primary constituents of biodiesel fuels. The principal goals of these studies were: (1) show how fuel-specific structural differences including degree of unsaturation, linear vs. branched chain structures, and methoxy vs. ethoxy functions affect fuel-destruction pathways, (2) understand the chemistry leading to potential increases in the emissions of hazardous air pollutants including aldehydes and ketones inherent in the use of biodiesel fuels, and (3) define the key chemical reaction mechanisms responsible for observed reductions in polycyclic aromatic hydrocarbons and particulate matter when oxygenated fuels are used as replacements for conventional fuels.
Calculation of photoionization cross section near auto-ionizing lines and magnesium photoionization cross section near threshold
The research performed is briefly reviewed. A simple method was developed for the calculation of continuum states of atoms when autoionization is present. The method was employed to give the first theoretical cross section for beryllium and magnesium; the results indicate that the values used previously at threshold were sometimes seriously in error. These threshold values have potential applications in astrophysical abundance estimates.
The evolution of curvature in planar, photoionization-driven heat fronts
Photoionized plasmas are common in astrophysics and cosmology, especially in space near compact objects, and there are effects from photoionization in high-energy-density plasmas due to the large radiation fields present. Photoionized plasmas are an active area of laboratory research and there are currently experiments to study photoionization-supported heat fronts. These photoionization fronts differ from the physics of diffusive radiation waves, commonly called Marshak waves, that are also an active area of research. This work uses a geometric argument to describe the expected evolution of the photoionization front curvature, in a planar geometry. It then compares this curvature to that of a Marshak wave as a method of diagnosing a heat front experiment. It is found that while the curvature of a planar Marshak wave increases in time, it decreases for a photoionization front. A comparison of radiation energy and electron heat fluxes through the container for the heat front propagating medium demonstrates that the geometric argument for the photoionization front curvature is sufficient. Furthermore, this comparison also demonstrates that wall losses are not significant in a photoionization front because the post-front region is very optically thin. A discussion of the implication this work has on material choice in the targets for an experiment follows.
Solar photoionization as a loss mechanism of neutral interstellar hydrogen in interplanetary space
Two primary loss mechanisms of interstellar neutral hydrogen in interplanetary space are resonance charge exchange ionization with solar wind protons and photoionization by solar EUV radiation. The later process has often been neglected since the average photoionization rate has been estimated to be as much as 5 to 10 times smaller than the charge exchange rate. These factors are based on ionization rates from early measurements of solar EUV and solar wind fluxes. Using revised solar EUV and solar wind fluxes measured near the ecliptic plane we have reinvestigated the ionization rates of interplanetary hydrogen. The result of our analysis indicates that indeed the photoionization rate during solar minimum can be smaller than charge exchange by a factor of 5; however, during solar maximum conditions when solar EUV fluxes are high, and solar wind fluxes are low, photoionization can be over 60% of the charge exchange rate at Earth orbit. To obtain an accurate estimate of the importance of photoionization relative to charge exchange, we have included photoionization from both the ground and metastable states of hydrogen. We find, however, that the photoionization from the metastable state does not contribute significantly to the overall photoionization rate.
Dirac–Fock photoionization parameters for HAXPES applications, Part II: Inner atomic shells
Highlights: • Photoionization parameters are given for inner atomic shells with binding energies ≳ 1.5 keV. • The data are extension of our calculations for use in the HAXPES spectroscopy studies. • Relativistic calculations were performed by the Dirac–Fock method. • Photoionization cross sections were calculated including all multipoles. • The angular distribution parameters were obtained in the quadrupole approximation. - Abstract: Presented here are the photoionization cross sections and parameters of the photoelectron angular distribution for inner atomic subshells with binding energies beyond 1.5 keV in the photon energy range from 2 keV to 12 keV. The calculations are an extension of our previous paper containing the photoionization parameters for comparatively outer shells with binding energies lower than 1.5 keV (Trzhaskovskaya and Yarzhemsky, 2018). The calculations are intended for use in the experimental HAXPES spectroscopy investigations. The up-to-date HAXPES activity tends to increase the photon energy to investigate the deep core levels resulting in a necessity of theoretical consideration of photoionization parameters for inner atomic shells. Our relativistic calculations have been performed by the Dirac–Fock method. The photoionization cross sections have been found including all multipoles of the radiative field while the photoelectron angular distribution parameters have been obtained in the quadrupole approximation. The effect of the hole resulting in the atomic subshell after photoionization has been taken into account using the frozen orbital model.
Atomic modeling of photoionization fronts in nitrogen gas
Photoionization fronts play a dominant role in many astrophysical environments but remain difficult to achieve in a laboratory experiment. Recent papers have suggested that experiments using a nitrogen medium held at ten atmospheres of pressure which is irradiated by a source with a radiation temperature of T R ~ 100 eV can produce viable photoionization fronts. We present a suite of one-dimensional numerical simulations using the Helios multimaterial radiation hydrodynamics code that models these conditions and the formation of a photoionization front. We study the effects of varying the atomic kinetics and radiative transfer model on the hydrodynamics and ionization state of the nitrogen gas, finding that more sophisticated physics, in particular, a multi-angle long characteristic radiative transfer model and a collisional-radiative atomics model, dramatically changes the atomic kinetic evolution of the gas. A photoionization front is identified by computing the ratios between the photoionization rate, the electron impact ionization rate, and the total recombination rate. We find that due to the increased electron temperatures found using more advanced physics that photoionization fronts are likely to form in our nominal model. We report the results of several parameter studies. In one of these, the nitrogen pressure is fixed at ten atmospheres and the source radiation temperature is varied, while in another, the temperature is fixed at 100 eV and the nitrogen pressure is varied. In conclusion, lower nitrogen pressures increase the likelihood of generating a photoionization front while varying the peak source temperature has little effect.
Experimental methods to test photoionized plasma models in emission
The expanding foil photoionized plasma platform was developed on the Z-machine at Sandia National Laboratories to produce terrestrial photoionized plasmas at some of the same conditions as those observed in astrophysical black hole accretion disks. The specific scientific goals of the platform are motivated by long-standing astrophysical puzzles related to accreting black hole systems. Ultimately, the experimental goal is to inform the supersolar Fe abundance problem by testing photoionized plasma models in emission with at-parameter laboratory data. High quality high resolution emission data with absolute intensity calibration from a laboratory photoionized plasma have never been collected prior to the experiments described here, providing broader motivation. A robust test of the model predictions requires the successful production of a laboratory photoionized plasma along with measurements of the supporting model inputs including the radiation drive spectrum used to create the photoionized plasma, the electron density, and the temperature. The measured absolute spectral radiance in emission collected from the independently diagnosed plasma can be compared against model calculations performed using the measured inputs. In this paper, we focus on the experimental platform and the model inputs, specifically detailing the methodology used to measure the plasma conditions. The moderate electron temperature of ∼41 eV ± 15 eV and electron density of ∼1e19 ± 1.6e18 e−/cm3 and the ∼3e12 W/cm2 x-ray irradiance at the sample confirm that photoionization dominates the plasma ionization and spectra.
Laboratory Photoionization Fronts in Nitrogen Gas: A Numerical Feasibility and Parameter Study
Photoionization fronts play a dominant role in many astrophysical situations but remain difficult to achieve in a laboratory experiment. We present the results from a computational parameter study evaluating the feasibility of the photoionization experiment presented in the design paper by Drake et al. in which a photoionization front is generated in a nitrogen medium. The nitrogen gas density and the Planckian radiation temperature of the X-ray source define each simulation. Simulations modeled experiments in which the X-ray flux is generated by a laser-heated gold foil, suitable for experiments using many kJ of laser energy, and experiments in which the flux is generated by a "z-pinch" device, which implodes a cylindrical shell of conducting wires. The models are run using crash, our block-adaptive-mesh code for multimaterial radiation hydrodynamics. The radiative transfer model uses multigroup, flux-limited diffusion with 30 radiation groups. In addition, electron heat conduction is modeled using a single-group, flux-limited diffusion. In the theory, a photoionization front can exist only when the ratios of the electron recombination rate to the photoionization rate and the electron-impact ionization rate to the recombination rate lie in certain ranges. These ratios are computed for several ionization states of nitrogen. Photoionization fronts are found to exist for laser-driven models with moderate nitrogen densities (~10 21 cm –3 ) and radiation temperatures above 90 eV. For "z-pinch"-driven models, lower nitrogen densities are preferred (<10 21 cm –3 ). Here, we conclude that the proposed experiments are likely to generate photoionization fronts.
Photoionization from the Ground and Excited Vibrational States of H$^+_2$ and Its Deuterated Isotopologues
Photoionization cross sections and rate coefficients have been calculated for all bound vibrational levels of the 1sσ g state of H$^+_2$, HD+, and D$^+_2$. The Born–Oppenheimer approximation is employed in our calculation of vibrationally resolved photoionization cross sections. Vibrationally resolved and local thermal equilibrium photoionization rate coefficients are presented for photon temperatures less than 50,000 K and are found to be several orders of magnitude larger than previous results in the literature. Analytic fits for the vibrationally resolved and local thermal equilibrium photoionization rate coefficients are provided. Near-threshold oscillations in the vibrationally resolved photoionization are observed. A benchmark set of photoionization cross sections are presented. Fixed-nuclei photoionization cross sections are calculated using two-center true continuum wave functions and are verified by comparison with previous calculations and are found to be in excellent agreement in all cases. Data files for our set of benchmark cross sections, rate coefficients, and fitting parameters for H$^+_2$, HD + , and D$^+_2$ are available on Zenodo under an open-source Creative Commons Attribution license at doi:10.5281/zenodo.8304061.
Enhancing photoionization rate calculations in low-temperature plasmas using spectral methods
Photoionization plays a central role in the development of streamer discharges and other non-equilibrium plasma phenomena. It creates seed electrons, which are essential for positive streamer propagation, allowing the ionization front to move forward. Because of this, accurate modeling of photoionization is very important for predicting streamer behavior and plasma evolution. The photoionization process in air (N 2 – O 2 mixture) is often described by the Zheleznyak model (1982). This model is usually solved through Helmholtz-type equations that approximate the Zheleznyak photoionization model (Zheleznyak et al. 1982) as Partial Differential Equations (PDEs). Conventional numerical methods, such as the Finite Difference Method (FDM) or Finite Volume Method (FVM), are widely used to solve these equations. Although they are prevalent, the computational cost of these methods due to their need for matrix operations and iterative solver is demanding. To address this challenge, this work develops a spectral solver based on the Fast Fourier Transform (FFT) combined with Discrete Cosine Transform (DCT) and Discrete Sine Transform (DST) to calculate the photoionization rate efficiently in an axisymmetric cylindrical domain. This method naturally satisfies the boundary conditions used in the model and converts the PDE into algebraic ones in spectral space. Thus, avoids the need for iterative matrix solvers. When compared with FDM results, it is demonstrated that the new solver not only maintains accuracy, but also reduces the computational cost, showing a performance increase of approximately 100 compared to FDM over a wide range of problem sizes. The method is parallelized using Message Passing Interface (MPI) and has been integrated into a fluid plasma model for streamer simulation. Here, this FFT-based approach provides a fast and reliable alternative for calculating photoionization in fluid models, helping large-scale plasma simulations run faster and efficiently, and allows higher-resolution simulation without extra computational cost.
VUV Photoionization Cross Sections of HO 2 , H 2 O 2 , and H 2 CO
The absolute vacuum ultraviolet (VUV) photoionization spectra of the hydroperoxyl radical (HO2), hydrogen peroxide (H 2 O 2 ), and formaldehyde (H2CO) have been measured from their first ionization thresholds to 12.008 eV. HO 2 , H 2 O 2 , and H 2 CO were generated from the oxidation of methanol initiated by pulsed-laser-photolysis of Cl 2 in a low-pressure slow flow reactor. Reactants, intermediates, and products were detected by time-resolved multiplexed synchrotron photoionization mass spectrometry. Absolute concentrations were obtained from the time-dependent photoion signals by modeling the kinetics of the methanol oxidation chemistry. Photoionization cross sections were determined at several photon energies relative to the cross section of methanol, which was in turn determined relative to that of propene. These measurements were used to place relative photoionization spectra of HO 2 , H 2 O 2 , and H 2 CO on an absolute scale, resulting in absolute photoionization spectra.
Atomic photoionization
After a brief historical review, this work discusses in detail the experimental techniques available for the investigation of photoionization processes, presents results on typical atoms, and examines some of the problems yet unsolved; in general, the discussion is concerned primarily with the photoionization of atoms in the spectral range from ionization threshold to about 100 A. The examination of experimental techniques treats such topics as light sources, monochromators and spectrographs, absorption and photoionization cells, branching-ratio studies, and electron-impact 'photoionization'. Photoionization measurements are discussed with reference to total and partial photoionization cross sections.
EMISSION SPECTRUM OF HELIUM-LIKE IONS IN PHOTOIONIZED PLASMAS
The aim of the present paper is to investigate the influence of inner-shell photoionization and photoexcitation on He{sub {alpha}} and its satellite's spectra in photoionized plasmas. An analysis is carried out on the relative importance of the various atomic processes in photoionized plasmas as a function of the electron temperature and irradiation conditions. In particular, we investigate the influence of K-shell photoionization of Li-like ions on the He{sub {alpha}} spectrum and of Be-like ions on the He{sub {alpha}} satellites. It is found that in photoionized plasmas these inner-shell processes contribute significantly under low radiation temperature and/or intensity, when Li- and Be-like ions are highly abundant but highly ionized H-like ions are rare. A short discussion is presented about the parameter space in which the excited 1s2p state has statistical or non-statistical distributions, and how such distributions affect the emission spectrum.
A high-resolution vacuum ultraviolet laser photoionization and photoelectron study of the Co atom
We have measured the vacuum ultraviolet–photoionization efficiency (VUV–PIE) spectrum of Co in the energy range of 63,500–67,000 cm{sup −1}, which covers the photoionization transitions of Co(3d{sup 7}4s{sup 2 4}F{sub 9/2}) → Co{sup +}(3d{sup 8 3}F{sub 4}), Co(3d{sup 7}4s{sup 2 4}F{sub 7/2}) → Co{sup +}(3d{sup 8 3}F{sub 3}), Co(3d{sup 7}4s{sup 2 4}F{sub 9/2}) → Co{sup +}(3d{sup 8 3}F{sub 3}), Co(3d{sup 7}4s{sup 2 4}F{sub 9/2}) → Co{sup +}(3d{sup 8 3}F{sub 2}), and Co(3d{sup 7}4s{sup 2 4}F{sub 9/2}) → Co{sup +}(3d{sup 7}4s{sup 1 5}F{sub 5}). We have also recorded the pulsed field ionization photoelectron spectrum of Co in the same energy range, allowing accurate determinations of ionization energies (IEs) for the photoionization transitions from the Co(3d{sup 7}4s{sup 2 4}F{sub 9/2}) ground neutral state to the Co{sup +}({sup 3}F{sub J}) (J = 4 and 3) and Co{sup +}({sup 5}F{sub 5}) ionic states, as well as from the Co(3d{sup 7}4s{sup 2 4}F{sub 7/2}) excited neural state to the Co{sup +}(3d{sup 8 3}F{sub 3}) ionic state. The high-resolution nature of the VUV laser used has allowed the observation of many well-resolved autoionizing resonances in the VUV–PIE spectrum, among which an autoionizing Rydberg series, 3d{sup 7}4s{sup 1}({sup 5}F{sub 5})np (n = 19–38), converging to the Co{sup +}(3d{sup 7}4s{sup 1 5}F{sub 5}) ionic state from the Co(3d{sup 7}4s{sup 2 4}F{sub 9/2}) ground neutral state is identified. The fact that no discernible step-like structures are present at these ionization thresholds in the VUV–PIE spectrum indicates that direct photoionization of Co is minor compared to autoionization in this energy range. The IE values, the autoionizing Rydberg series, and the photoionization cross sections obtained in this experiment are valuable for understanding the VUV opacity and abundance measurement of the Co atom in stars and solar atmospheres, as well as for benchmarking the theoretical results calculated in the Opacity Project and the IRON Project, and thus are of relevance to astrophysics.