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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Ionization of Free and Confined Atoms and Ions (Final Technical Report)

Over the almost 22-year duration of this theoretical research project, a great deal of progress has been made in the understanding of the ionization of free and confined atomic and molecular systems along with a recent particular emphasis upon studies of the ultrafast time-delay phenomenon in the photoemission process. Our methods have emphasized the role of many-body interactions and relativistic effects in the various processes we have studied. The advances that we have made over the course of this project have been in the areas nondipole effects in the photoionization process, inner-shell photoemission from negative atomic ions, excited state photoionization of neutral atoms and positive atomic ions, Cooper minima in quadrupole photoionization, relativistic interaction in the photoionization of atoms and negative atomic ions, photoionization of the fullerene molecule C 60 along with the photoionization of atomic and molecular systems confined in the C 60 molecule, confinement resonances in endohedral atom photoemission, interchannel coupling effects in photoionization, Wigner time delay in free and confined atoms and ions, angular effects in Wigner time delay in atomic photoionization, photoionization of superheavy atoms and the influence of core relaxation on the photoionization of atomic systems, among others. In the course of this project, we have had extensive collaboration with experimentalists where we have explained the physics of numerous measurements and pointed the way to new laboratory investigations with our calculations. We have also collaborated with a number of theorists as well. Thus, owing to these many collaborations with scientists not funded by DOE, we have leveraged the DOE support rather considerably. To illustrate the depth and breadth of the theoretical investigations, listed below are the 138 refereed and invited publications that cite DOE support. These publications detail the many contributions made over the period of this project.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Self-consistent Modeling of Reionization in Cosmological Hydrodynamical Simulations

The ultraviolet background (UVB) emitted by quasars and galaxies governs the ionization and thermal state of the intergalactic medium (IGM), regulates the formation of high-redshift galaxies, and is thus a key quantity for modeling cosmic reionization. The vast majority of cosmological hydrodynamical simulations implement the UVB via a set of spatially uniform photoionization and photoheating rates derived from UVB synthesis models. We show that simulations using canonical UVB rates reionize and, perhaps more importantly, spuriously heat the IGM, much earlier (z∼15) than they should. This problem arises because at z>6, where observational constraints are nonexistent, the UVB amplitude is far too high. We introduce a new methodology to remedy this issue, and we generate self-consistent photoionization and photoheating rates to model any chosen reionization history. Following this approach, we run a suite of hydrodynamical simulations of different reionization scenarios and explore the impact of the timing of reionization and its concomitant heat injection on the thermal state of the IGM. We present a comprehensive study of the pressure smoothing scale of IGM gas, illustrating its dependence on the details of both hydrogen and helium reionization, and argue that it plays a fundamental role in interpreting Lyα forest statistics and the thermal evolution of the IGM. The premature IGM heating we have uncovered implies that previous work has likely dramatically overestimated the impact of photoionization feedback on galaxy formation, which sets the minimum halo mass able to form stars at high redshifts. We make our new UVB photoionization and photoheating rates publicly available for use in future simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE C({sup 3}P) + NH{sub 3} REACTION IN INTERSTELLAR CHEMISTRY. I. INVESTIGATION OF THE PRODUCT FORMATION CHANNELS

The product formation channels of ground state carbon atoms, C({sup 3}P), reacting with ammonia, NH{sub 3}, have been investigated using two complementary experiments and electronic structure calculations. Reaction products are detected in a gas flow tube experiment (330 K, 4 Torr) using tunable vacuum-ultraviolet (VUV) photoionization coupled with time of flight mass spectrometry. Temporal profiles of the species formed and photoionization spectra are used to identify primary products of the C + NH{sub 3} reaction. In addition, H-atom formation is monitored by VUV laser induced fluorescence (LIF) from room temperature to 50 K in a supersonic gas flow generated by the Laval nozzle technique. Electronic structure calculations are performed to derive intermediates, transition states, and complexes formed along the reaction coordinate. The combination of photoionization and LIF experiments supported by theoretical calculations indicate that in the temperature and pressure range investigated, the H + H{sub 2}CN production channel represents 100% of the product yield for this reaction. Kinetics measurements of the title reaction down to 50 K and the effect of the new rate constants on interstellar nitrogen hydride abundances using a model of dense interstellar clouds are reported in Paper II.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Model for photothermal ionization and molecular recombination during pulsed ablation of polyethylene

The surface erosion of polyethylene is studied under conditions characteristic of extremely high-rate heating expected in laser-driven and high energy density experiments. A coupled model for photoionization and volumetric photothermal degradation of polyethylene is developed that includes finite rate chemistry and molecular recombination of radicals. First, the model is used to explore the evolution of radicals and hydrocarbons during isochoric heating, for which it is found that polyethylene is not in chemical equilibrium for rates >108K/s. Then, the model is used to explore the cooperativity between photoionization and photothermal ionization during one-dimensional ablation from a pulsed heating source, accounting for coupled energy deposition, thermokinetics, thermochemistry, hydrodynamics, mechanics, and thermal conduction in the ALEGRA multi-physics code. The ablation depth per energy pulse is found to agree with an analytical model for instantaneous energy deposition and absent molecular recombination or photoionization. Otherwise, molecular recombination of radicals reduces the depth of ablation per pulse, owing to thermal stabilization on forming branched molecules. Radical formation through photoionization can be compensated for by recombination, but it accelerates ablation at high enough photoionization rates. Finally, the influence of molecular recombination on the ablation depth is found to decrease for long pulses, owing to decreased recombination reaction rates at lower vapor densities near the ablated surface. These and other findings highlight competition between the processes of scission, recombination, and photoionization during pulsed ablation of polyethylene.

Moore, N. W. (ORCID:0000000272259674)↗
DOE OSTI2014

Radiative recombination data for tungsten ions: III. W{sup 14+}–W{sup 23+}

This paper completes the cycle of our calculations of the radiative recombination and photoionization data for tungsten ions. Presented here are the photoionization and radiative recombination cross sections, radiative recombination rate coefficients, and radiated power loss rate coefficients for ten tungsten impurity ions from W{sup 14+} to W{sup 23+}. These data are required in diagnostics and modeling fusion plasmas studied in such devices as ITER, ASDEX Upgrade, and EBIT. Partial photoionization cross sections have been fitted by an analytical expression with five fit parameters tabulated here. Total radiative recombination cross sections are presented in the electron energy range from 1 eV to ∼80 keV. Radiative recombination rates and radiated power loss rates are given in the temperature range from 10{sup 4} K to 10{sup 9} K. Calculations have been performed on the basis of the fully relativistic treatment of photoionization and radiative recombination taking into account all significant multipoles of the radiative field. Electron wave functions have been obtained by the Dirac–Fock method with the proper consideration of the electron exchange. The relativistic Maxwell–Jüttner distribution of continuum electrons has been used in calculations of radiative recombination rates and radiated power loss rates. This decreases values of the rates noticeably at a high temperature as compared to the usual non-relativistic Maxwell–Boltzmann distribution. -- Highlights: •Radiative recombination data for ten tungsten ions W{sup 14+}–W{sup 23+} are presented. •Photoionization cross sections are also given. •Calculations are fully relativistic including all multipoles of the radiative field. •We use the Dirac–Fock method to obtain the electron wave functions. •The data are required for diagnostics and modeling fusion plasmas studied in ITER.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Attosecond delays in X-ray molecular ionization

The photoelectric effect is not truly instantaneous, but exhibits attosecond delays that can reveal complex molecular dynamics. Sub-femtosecond duration light pulses provide the requisite tools to resolve the dynamics of photoionization. Accordingly, the past decade has produced a large volume of work on photoionization delays following single photon absorption of an extreme ultraviolet (XUV) photon. However, the measurement of time-resolved core-level photoionization remained out of reach. The required x-ray photon energies needed for core-level photoionization were not available with attosecond tabletop sources. We have now measured the x-ray photoemission delay of core-level electrons, and here report unexpectedly large delays, ranging up to 700 attoseconds in NO near the oxygen K-shell threshold. These measurements exploit attosecond soft x-ray pulses from a free-electron laser (XFEL) to scan across the entire region near the K-shell threshold. Furthermore, we find the delay spectrum is richly modulated, suggesting several contributions including transient trapping of the photoelectron due to shape resonances, collisions with the Auger-Meitner electron that is emitted in the rapid non-radiative relaxation of the molecule, and multi-electron scattering effects. Finally, the results demonstrate how x-ray attosecond experiments, supported by comprehensive theoretical modelling, can unravel the complex correlated dynamics of core-level photoionization.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Dynamics of Product Branching from Radical Intermediates in Elementary Combustion Reactions (Final Technical Report)

While the total rate constant for many elementary reactions is well-characterized, understanding the product branching in complex reactions presents a formidable challenge. To gain an incisive probe of such reactions, our experiments investigate the dynamics of the product channels that arise from transient radical intermediates along the bimolecular reaction coordinates. Our recent published work uses the methodology developed in my group in the last fifteen years, using both imaging and scattering apparatuses. The experiments generate a particular isomeric form of an unstable radical intermediate along a bimolecular reaction coordinate and study the branching between the ensuing product channels of the energized radical as a function of its internal rotational and vibrational energy under collision-less conditions. The experiments use a combination of: 1) measurement of product velocity and angular distributions in a crossed laser-molecular beam apparatus, with electron bombardment detection in my lab in Chicago or 2) with tunable vacuum ultraviolet photoionization detection at Taiwan's National Synchrotron Radiation Research Center (NSRRC), and 3) velocity map imaging using state-selective REMPI and single photon VUV ionization of radical intermediates and reaction products. We also employ tunable VUV photoionization detection in our imaging apparatus, using difference frequency four-wave mixing to produce photoionization light tunable from 8 to 10.8 eV. This year of support funded our work to photolytically generate the radical intermediate important in the OH + propene reaction when OH adds to an end carbon. Our other major result this past year was on the photoionization detection of vinoxy radicals. Our experiments at the NSRRC definitively show that vinoxy dissociatively ionizes to both CH 3 + and HCO + , and establishes the absolute partial photoionization cross section of vinoxy to CH 3 + at 10.5 and 11.44 eV. This should allow experimentalists detecting vinoxy radicals produced in bimolecular reactions such as OH + propene to get good branching fractions to that product channel. Finally we initiated studies of the photodissociation of 1-bromo-2-chloroethane and 1,1-bromochloroethane at 193 nm at the NSRRC.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Study of vacuum ultraviolet emission in helium and helium/nitrogen mixtures

Helium is frequently used as a working medium for the generation of plasmas and is capable of energetic photon emissions. These energetic photon emissions are often attributed to the formation of helium excimer and subsequent photon emission. When the plasma device is exposed to another gas, such as nitrogen, this energetic photon emission can cause photoionization and further ionization wave penetration into the additional gas. Often ignored are the helium resonance emissions that are assumed to be radiation trapped and therefore not pertinent to photoionization. Here, experimental evidence for the presence of helium atomic emission in a pulsed discharge at ten’s of Torr is shown. Simulations of a discharge in similar conditions agree with the experimental measurements. In this context, the role of atomic and molecular helium light emission on photoionization of molecular nitrogen in an ionization wave is studied using a kinetic modeling approach that accounts for radiation dynamics in a developing low-temperature plasma. Three different mixtures of helium at a total pressure of 250 Torr are studied in simulation. Photoionization of the nitrogen molecule by vacuum ultraviolet helium emission is used as the only seed source ahead of the ionization front. It is found that even though radiation trapped, the atomic helium emission lines are the significant source of photoionization of nitrogen. The significant effect of radiation trapped photon emission on ionization wave dynamics demonstrates the need to consider these radiation dynamics in plasma reactors where self-absorbed radiation is ignored.

42 ENGINEERING↗
DOE OSTI2021

Electron energy distributions in the extended gas nebulae associated with high-z AGN: Maxwell–Boltzmann versus κ distributions

ABSTRACT Emission line observations together with photoionization models provide important information about the ionization mechanisms, densities, temperatures, and metallicities in active galactic nuclei (AGN)-ionized gas. Photoionization models usually assume Maxwell–Boltzmann (M–B) electron energy distributions (EED), but it has been suggested that using κ distributions may be more appropriate and could potentially solve the discrepancies in temperatures and abundances found in HII regions and planetary nebulae. We consider the impact of the presence of κ distributions in photoionized nebulae associated with AGN and study how this might affect spectral modelling and abundance analyses for such regions. Using the photoionization code MAPPINGS 1e, we compute models adopting M–B and κ distributions of electron energies and compare the behaviour of emission line ratios for different values of κ, gas metallicity, density, ionization parameter, and SED slope. We find that the choice of EED can have a large impact on some UV and optical emission lines emitted by photoionized nebulae associated with AGN, and that the impact of adopting a κ distribution is strongly dependent on gas metallicity and ionization parameter. We compile a sample of line ratios for 143 type 2 AGN and compare our models against the observed line ratios. We find that for 98 objects, κ distributions provide a better fit to the observed line ratios than M–B distributions. In addition, we find that adopting κ-distributed electron energies results in significant changes in the inferred gas metallicity and ionization parameter in a significant fraction of objects.

Morais, S. G. (ORCID:0000000200927813)↗
DOE OSTI2016

Wigner photoemission time delay from endohedral anions

Characteristic features of Wigner photoemission time delay from endohedral anions A@C 60 q along with their dependence on the anion charge q are unraveled. Specifically, significant enhancement of the time delay in the innermost dipole photoionization channels near threshold is found, owing to the presence of the Coulomb confined resonances (CRs). Moreover, it is shown that interchannel coupling of the inner-shell Coulomb CRs with outer-shell photoionization channels results in resonantly enhanced time delay in the release of the outer-shell photoelectron well above, several hundreds eV, the outer-shell thresholds. It is also demonstrated that, and explained why, photoionization cross sections of the innermost subshells as well as outer subshells (near the inner-subshell threshold) depends only very weakly on the anion charge q, but the dependence of the corresponding time delays on q can be significant. Furthermore, Coulomb CRs are found to emerge in the innermost quadrupole photoionization channels as well, thereby causing considerable time delay in the quadrupole photoemission. These findings are illustrated in calculations of the photoionization of inner and outer subshells of the endohedral anions Ne@C 60 -1 and Ne@C 60 -5 that were chosen as case studies.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Calculations of photo-induced X-ray production cross-sections in the energy range 1–150 keV and average fluorescence yields for Zn, Cd and Hg

In this paper, we calculate the K-, L- and M-shells X-ray production, and X-ray fluorescence cross-sections after photo-induced ionization, for Zn, Cd, and Hg, and for incident photon energy range from 1 to 150 keV. For this purpose, the corresponding average fluorescence yields for Zn, Cd, and Hg as well as the photoionization cross-sections were calculated using the Dirac–Fock method. Subshell fluorescence, intrashell and intershell yields are obtained consistently from radiative and radiationless transitions calculated in the exact same method. A comprehensive account of the relations between the X-ray production, X-ray fluorescence cross-sections and the photoionization cross-sections and these yields is presented. Comparisons are made with results from other authors. The obtained values for the photoionization cross-sections are in good agreement with the widely used data of Scofield in the studied energy range. However our results for the X-ray fluorescence cross sections seem to favor some data relatively to others. The energy dependence of the average fluorescence yields is discussed, in particular, the reliability of extrapolated data for lighter elements from measurements and calculations in heavier elements above the inner shell absorption edges is questioned. Tabulated data on photoionization and X-ray production cross-sections are presented for the incident photon energy range 1–150 keV in steps of 1 keV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗