Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Photoionization”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Photoionization in the halo of the Galaxy

The ionizing radiation field in the halo is calculated and found to be dominated in the 13.6-45 eV range by light from O-B stars that escapes the disk, by planetary nebulae at 45-54 eV, by quasars and the Galactic soft X-ray background at 54-2000 eV, and by the extragalactic X-ray background at higher energies. Photoionization models are calculated with this radiation field incident on halo clouds of constant density for a variety of densities, for normal and depleted abundances, and with variations of the incident spectrum. For species at least triply ionized, such as Si IV, C IV, N V, and O VI, the line ratios are determined by intervening gas with the greatest volume, which is not necessarily the greatest mass component. Column densities from doubly ionized species like Si III should be greater than from triply ionized species. The role of photoionized gas in cosmic ray-supported halos and Galactic fountains is discussed. Observational tests of photoionization models are suggested.

Bregman, Joel N.↗
NASA NTRS1989

Flash-photoionized nebulae

Under conditions of high radiation intensity and low gas density, recombination may be neglected in determining the ionization state of a photoionized gas. Calculations of the ionization structure of nebulae in this 'flash-photoionized' regime are reported. Very hard spectra of ionizing ultraviolet radiation may be produced by filtration of the ionizing flux through a neutral hydrogen layer which preferentially absorbs photons just above the hydrogen photoionization threshold. Fluxes with these hard spectra produce gas layer in which helium is largely doubly ionized while hydrogen is largely neutral. Such a layer leads to anomalously high ratios of He II to H I recombination line strengths. These results are applied to the problem of the spectrum of the arc in the cluster of galaxies A370. It is found that the spectrum may possibly be reconciled with the light echo model.

Katz, J. I.↗
NASA NTRS1990

X-ray spectral signatures of photoionized plasmas

Plasma emission codes have become a standard tool for the analysis of spectroscopic data from cosmic X-ray sources. However, the assumption of collisional equilibrium, typically invoked in these codes, renders them inapplicable to many important astrophysical situations, particularly those involving X-ray photoionized nebulae. This point is illustrated by comparing model spectra which have been calculated under conditions appropriate to both coronal plasmas and X-ray photoionized plasmas. It is shown that the (3s-2p)/(3d-2p) line ratios in the Fe L-shell spectrum can be used to effectively discriminate between these two cases. This diagnostic will be especially useful for data analysis associated with AXAF and XMM, which will carry spectroscopic instrumentation with sufficient sensitivity and resolution to identify X-ray photoionized nebulae in a wide range of astrophysical environments.

Liedahl, Duane A.↗
NASA NTRS2004

Photoionization Modeling and the K Lines of Iron

We calculate the efficiency of iron K line emission and iron K absorption in photoionized models using a new set of atomic data. These data are more comprehensive than those previously applied to the modeling of iron K lines from photoionized gases, and allow us to systematically examine the behavior of the properties of line emission and absorption as a function of the ionization parameter, density and column density of model constant density clouds. We show that, for example, the net fluorescence yield for the highly charged ions is sensitive to the level population distribution produced by photoionization, and these yields are generally smaller than those predicted assuming the population is according to statistical weight. We demonstrate that the effects of the many strongly damped resonances below the K ionization thresholds conspire to smear the edge, thereby potentially affecting the astrophysical interpretation of absorption features in the 7-9 keV energy band. We show that the centroid of the ensemble of K(alpha) lines, the K(beta) energy, and the ratio of the K(alpha(sub 1)) to K(alpha(sub 2)) components are all diagnostics of the ionization parameter of our model slabs.

Kallman, T. R.↗
NASA NTRS2004

Photoionization in Ultraviolet Processing of Astrophysical Ice Analogs at Cryogenic Temperatures

Two recent experimental studies have demonstrated that amino acids or amino acid precursors are generated when astrophysical ice analogs are subjected to ultraviolet (UV) irradiation at cryogenic temperatures. Understanding the complete phenomenology of photoprocessing is critical to elucidating chemical reaction mechanisms that can function within an ice matrix under very cold conditions. Pushing beyond the much better characterized study of photolytic dissociation of chemical bonds through electronic excitation, this work explored the ability of UV radiation present in the interstellar medium to ionize small molecules embedded in ices. Quantum chemical calculations, including bulk solvation effects, were used to study the ionization of hydrogen (H2), water, and methanol (CH3OH) bound in small clusters of water. Ionization potentials were found to be much smaller in the condensed phase than in the gas phase; even a small cluster can account for large changes in the ionization potentials in ice, as well as the known formation of an OH--H3O+ pair in the case of H2O photoionization. To gauge the impact of photoionization on subsequent grain chemistry, the reaction between OH and CO in the presence of H3O+ was studied and compared with the potential energy surface without hydronium present, which is relevant to chemistry following photolysis. The differences indicate that the reaction is somewhat more likely to proceed to products (H + CO2) in the case of photoionization.

Woon, David E.↗
NASA NTRS2011

K-Shell Photoionization of Nickel Ions Using R-Matrix

We present R-matrix calculations of photoabsorption and photoionization cross sections across the K edge of the Li-like to Ca-like ions stages of Ni. Level-resolved, Breit-Pauli calculations were performed for the Li-like to Na-like stages. Term-resolved calculations, which include the mass-velocity and Darwin relativistic corrections, were performed for the Mg-like to Ca-like ion stages. This data set is extended up to Fe-like Ni using the distorted wave approximation as implemented by AUTOSTRUCTURE. The R-matrix calculations include the effects of radiative and Auger dampings by means of an optical potential. The damping processes affect the absorption resonances converging to the K thresholds causing them to display symmetric profiles of constant width that smear the otherwise sharp edge at the K-shell photoionization threshold. These data are important for the modeling of features found in photoionized plasmas.

Witthoeft, M. C.↗
NASA NTRS2010

K-shell Photoionization of Na-like to Cl-like Ions of Mg, Si, S, Ar, and Ca

We present R-matrix calculations of photoabsorption and photoionization cross sections across the K edge of Mg, Si, S, Ar, and Ca ions with more than 10 electrons. The calculations include the effects of radiative and Auger damping by means of an optical potential. The wave functions are constructed from single-electron. orbital bases obtained using a Thomas-Fermi-Dirac statistical model potential. Configuration interaction is considered among all states up to n = 3. The damping processes affect the resonances converging to the K-thresholds causing them to display symmetric profiles of constant width that smear the otherwise sharp edge at the photoionization threshold. These data are important for the modeling of features found in photoionized plasmas.

Witthoeft, M. C.↗
NASA NTRS2013

Photoionization Modeling of Oxygen K Absorption in the Interstellar Medium:

We present detailed analyses of oxygen K absorption in the interstellar medium (ISM) using four high-resolution Chandra spectra toward the X-ray low-mass binary XTE J1817-330. The 11-25 Angstrom broadband is described with a simple absorption model that takes into account the pile-up effect and results in an estimate of the hydrogen column density. The oxygen K-edge region (21-25 Angstroms) is fitted with the physical warmabs model, which is based on a photoionization model grid generated with the xstar code with the most up-to-date atomic database. This approach allows a benchmark of the atomic data which involves wavelength shifts of both the K lines and photoionization cross sections in order to fit the observed spectra accurately. As a result we obtain a column density of N(sub H) = 1.38 +/- 0.01 × 10(exp 21) cm(exp −2); an ionization parameter of log xi = −2.70 +/- 0.023; an oxygen abundance of A(sub O) = 0.689 (+0.015/−0.010); and ionization fractions of O(sub I)/O = 0.911, O(sub II)/O = 0.077, and O(sub III)/O = 0.012 that are in good agreement with results from previous studies. Since the oxygen abundance in warmabs is given relative to the solar standard of Grevesse & Sauval, a rescaling with the revision by Asplund et al. yields A(sub O) = 0.952(+0.020/−0.013), a value close to solar that reinforces the new standard.We identify several atomic absorption lines-K(alpha), K(beta), and K(gamma) in O(sub I) and O(sub II) and K(alpha) in O(sub III), O(sub VI), and O(sub VII)-the last two probably residing in the neighborhood of the source rather than in the ISM. This is the first firm detection of oxygen K resonances with principal quantum numbers n greater than 2 associated with ISM cold absorption.

oxygen↗
NASA NTRS2022

A Study of Photoionized Gas in Two H װ Regions of the N44 Complex in the LMC Using MUSE Observations

We use the optical integral field observations with Multi-Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, together with CLOUDY photoionization models, to study ionization structure and physical conditions of two luminous H װ regions in the N44 star-forming complex of the Large Magellanic Cloud. The spectral maps of various emission lines reveal a stratified ionization geometry in N44 D1. The spatial distribution of [O ו] λ6300 emission in N44 D1 indicates a partially covered ionization front at the outer boundary of the H װ region. These observations reveal that N44 D1 is a blister H װ region. The [O ו] λ6300 emission in N44 C does not provide a well-defined ionization front at the boundary, while patches of [S װ] λ6717 and [O ו] λ6300 emission bars are found in the interior. The results of spatially resolved MUSE spectra are tested with the photoionization models for the first time in these H װ regions. A spherically symmetric ionization-bounded model with a partial covering factor, which is appropriate for a blister H װ region, can well reproduce the observed geometry and most of the diagnostic line ratios in N44 D1. Similarly, in N44 C we apply a low-density and optically thin model based on the observational signatures. Our modeling results show that the ionization structure and physical conditions of N44 D1 are mainly determined by the radiation from an O5 V star. However, local X-rays, possibly from supernovae or stellar wind, play a key role. In N44 C, the main contribution is from three ionizing stars.

H II regions↗
NASA NTRSDate not supplied

Design of Electrostatic Dust Lofting Suborbital Flight Experiment Examining Photoionization under Lunar Gravity

Dust on the lunar surface electrostatically charges due to the plasma environment surrounding the Moon, causing grains to become lofted and adhere to nearby surfaces including landers and astronauts. Studying the behaviors of these charged particles in the lunar environment is essential to plan around the deleterious effects of dust to future Moon missions. Models attempt to predict the amount of dust loading that can be expected in many of these scenarios, but they require experimental validation to be predictive. This physics cannot be fully studied on Earth due to the six times larger gravitational force obscuring the electrostatic interactions, so it is necessary to run experiments in a more relevant environment, including vacuum and near-lunar gravitational effects. An experiment has been designed to fly on the Lunar Gravity Acceleration (LGA) mission aboard the Blue Origin New Shepard suborbital rocket. This experiment will perform photoionization charging of lunar regolith simulant grains under the illumination of an ultraviolet (UV) source. As a result, the charged grains will then electrostatically repel one another and loft in the reduced gravity environment; their trajectories will be imaged via a high-speed camera. Preliminary laboratory results influencing the design of this experiment will be presented, including characterization of several UV sources, measurements of photoionization currents under various vacuum conditions, and examination of lunar simulant dust lofting under terrestrial gravity. Results from this flight will be compared with ground-based testing and the laboratory results outlined above to examine the dependence on gravity and will be fed into the dust charging and lofting models currently under development.

electrostatics↗
NASA NTRS1973

Photoionization from excited states of helium

The cross sections for photoionization from the 2 1S, 2 3S, 2 1P and 2 3P excited states of helium are calculated for photoelectron energies below the n = 2 threshold of He(+) using Hylleraas bound state wave functions and 1s-2s-2p close coupling final state wave functions. The resonant structures associated with the lowest-lying 1S, 1P, 3P, and 1D autoionizing states of helium are found to be characterized by large values of the line profile parameter q. The cross sections and the photoelectron angular distribution asymmetry parameters for the P-states are calculated for various polarization states of the target atom and the incident photon. Experiments which would lead to the separate determinations of the S- and D- wave partial photoionization cross sections are discussed.

Jacobs, V. L.↗
NASA NTRS1973

Photoionization of carbon dioxide

Investigation of the mechanism for dissociative ionization of CO2 on the basis of measurements of the yields of photoions from CO2. It is found that at photon energies immediately above the molecular ionization limit of 13.773 plus or minus 0.002 eV the photoion yield curve exhibits a number of closely spaced peaks in a pattern not resembling Rydberg series. Two fragment ions, O(+) and CO(+), are observed as products of dissociative ionization. The yield of O(+) follows a weak, oscillatory curve from the thermochemical threshold at 19.07 eV to 19.39 eV, where a sudden increase in O(+) yield is observed exactly at the onset of the state CO2(+)/C 2 Sigma g (+)/, indicating predissociation of this state of the molecular ion. The magnitude of the jump in O(+) yield at 19.39 eV suggests that this predissociation is the principal mechanism for dissociative ionization.

Mcculloh, K. E.↗
NASA NTRS1975

Near-threshold photoionization of xenon metastable atoms

Photoionization from metastable levels of atomic xenon has been studied in the wavelength range from the threshold at 4622 A to 2700 A. Structure in the photoionization signal, due to autoionization, is analyzed to provide term values, lifetimes, and line-shape parameters of the autoionizing states. These parameters, together with the estimated absolute cross sections, are used to derive discrete and continuum oscillator strengths.

Rundel, R. D.↗
NASA NTRS1975

A photoionization study of the formation of CO2/+/ by reaction of excited O2/+/ ions with CO

The production of CO2(+) by the ion-molecule reaction O2(+) + CO yields CO2(+) + O has been investigated using a photoionization mass spectrometer. The photoionization efficiency for production of CO2(+) by this reaction was measured from threshold at 924 A (13.42 eV) to 650 A (19.07 eV). The appearance potential corresponds to reaction of ground vibronic O2(+) ions formed in the nu-prime = 6 level. The high-vibrational-level ions of the ground ionic state are most likely produced by autoionization of O2. At wavelengths shorter than 760 A, there is a large increase in the reaction cross section associated with formation of ions in the metastable state. The peak reaction cross section occurs at 720 A.

Ajello, J. M.↗
NASA NTRS1976

Absolute measurement of the photoionization cross section of atomic hydrogen with a shock tube for the extreme ultraviolet

The paper reports an experiment which is part of a program to measure the absolute values of the atomic photoionization cross sections of astrophysically abundant elements, particularly in stars and planetary atmospheres. An aerodynamic pressure-driven shock tube constructed from stainless steel with a quadratic cross section was used to measure the photoionization cross section of H I at 19 wavelength points from 910 to 609 A with experimental uncertainties between 7 and 20%. The shock tube was used to produce fully dissociated hydrogen and neon mixtures for the photoabsorption measurements.

Palenius, H. P.↗
NASA NTRS1977

A photoionization study of the formation of NO2/+/ by reaction of excited O2/+/ ions with NO

Photoionization mass spectrometer results are presented for the first observation of the ion-molecule reaction in which O2(+) + NO yields NO2(+) + O. The reaction is energetically possible for ground state O2(+) ions in the lowest vibrational level. Photoionization efficiency curves for NO(+), O2(+), and NO2(+) are presented and compared, with special emphasis on autoionization features. In addition to the production of NO2(+) by the cited reaction, there is also a possibility for NO2(+) formation by the process O2 + NO(+) yielding NO2(+) + O. This reaction is calculated to be exothermic for incident photon energies of 11.73 eV.

Ajello, J. M.↗