Search NASA⌕ Search

SEARCH · Search NASA

Results for “below- threshold ionization”

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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Effects of screening and pressure ionization on the electron broadening of spectral lines in dense plasmas

Collisions between electrons and radiating atoms broaden spectral absorption and emission lines in dense plasmas. High densities also introduce screening and pressure ionization effects that distort the wave functions of both bound and free electrons. In order to study how dense plasma effects influence the electron broadening of spectral lines, this paper incorporates electron wave functions from an average-atom (AA) model to calculate the linewidth of the B III 2⁢𝑝−2⁢𝑠 transition at 𝑇 = 10 eV for mass densities ranging from 𝜌 = 10 −4 to 0.4⁢ g⁡/cc. The calculation method uses the impact approximation, allowing the linewidth to be written in terms of electron-collision cross sections and an interference term. Compared to an otherwise identical calculation that uses Coulomb free wave functions, the AA method is found to modify both the cross sections and the resulting linewidth at sufficiently high density by introducing screening and pressure-ionized bound states. Screening lowers the cross sections at low energies and near electron excitation thresholds, while pressure-ionized bound states introduce resonances into the continuum. Thus, as the density increases, the relative linewidth between the AA and Coulomb calculations follows a general decrease because of screening, with sharp increases at various intervals due to pressure ionization. Finally, the AA results are also compared with a common approach to introduce screening through the interaction potential and reduced models that use the Bethe formula for the inelastic electron-collision cross sections.

electronic excitation & ionization↗
NASA NTRS1972

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.

Moore, E. N.↗
NASA NTRS1989

Lowest ionization potentials of Al2

Potential curves for the lowest two electronic states, X 2Sigma(+)g and A 2Pi(u), of Al2(+) were computed using complete active space SCF/multireference CI wave functions and large Gaussian basis sets. The lowest observable vertical ionization potential, to Al2(+) X 2Sigma(+)g, of the Al2 X 3 Pi(u) ground state is calculated to occur around 6.1 eV, in excellent agreement with the experimental range of 6.0 to 6.42 eV obtained in recent cluster ionization studies by Cox and co-workers. The second vertical ionization potential, to Al2(+) A 2Pi(u), occurs near 6.4 eV, also within the experimental range. The adiabatic IP of 5.90 eV is in good agreement with the value of 5.8 to 6.1 eV deduced by Hanley and co-workers from the difference in thresholds between collision induced dissociation processes of Al3(+). The computed IP values are somewhat larger than those deduced from branching ratios in cluster fragmentation experiments by Jarrold and co-workers. The observation of an ionization threshold below 6.42 eV is shown to be incompatible with an Al2 ground electronic state assignment of 3Sigma(-)g, but the separation between the two lowest states of Al2 is so small that it is likely that both are populated in the experiments, so that this does not provide unambiguous support for the recent theoretical assignment of the ground state as 3Pi(u).

Bauschlicher, Charles W., Jr.↗
NASA NTRS1988

The lowest ionization potentials of Al2

Potential curves for the lowest two electronic states (X 2 sigma g + and A 2 pi u) of Al2(+) were computed using complete active space SCF/multireference CI wave functions and large Gaussian basis sets. The lowest observable vertical ionization potential (to Al2(+) X 2 sigma g +) of the Al2 X 3 pi u ground state is calculated to occur around 6.1 eV, in excellent agreement with the experimental range of 6.0 to 6.42 eV obtained in recent cluster ionization studies by Cox and co-workers. The second vertical ionization potential (to Al2(+) A 2 pi u) occurs near 6.4 eV, also within the experimental range. The adiabatic IP of 5.90 eV is in good agreement with the value of 5.8 to 6.1 eV deduced by Hanley and co-workers from the difference in thresholds between collision induced dissociation processes of Al3(+). The computed IP values are somewhat larger than those deduced from branching ratios in cluster fragmentation experiments by Jarrold and co-workers. The observation of an ionization threshold below 6.42 eV is shown to be incompatible with an Al2 ground electronic state assignment of 3 sigma g -, but the separation between the two lowest states of Al2 is so small that it is likely that both are populated in the experiments, so that this does not provide unambiguous support for the recent theoretical assignment of the ground state as 3 pi u.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1991

High-resolution threshold photoionization of N2O

Pulsed field ionization (PFI) has been used in conjunction with a coherent VUV source to obtain high-resolution threshold photoelectron spectra for the (000), (010), (020), and (100) vibrational states of the N2O(+) cation. Simulations for the rotational profiles of each vibronic level were obtained by fitting the Buckingham-Orr-Sichel equations using accurate spectroscopic constants for the ground states of the neutral and the ion. The relative branch intensities are interpreted in terms of the partial waves of the outgoing photoelectron to which the ionic core is coupled and in terms of the angular momentum transferred to the core.

Wiedmann, R. T.↗
DOE OSTI2022

Microwave breakdown of sub-atmospheric argon gas in the vicinity of a microgap

The microwave electric field at breakdown for argon gas in a microgap (generally, gap size <1 mm) over a wide range of pressures and gap sizes was found to exhibit two behavioral pressure regimes. A high microwave electric field was established in the gap of a re-entrant mode resonator. By slowly raising the microwave electric field level, a breakdown level is reached where impact ionization induces low pressure argon gas to breakdown preferentially outside of the gap. At high pressure, breakdown is found to occur preferentially inside the gap, but only after the pressure is high enough for impact ionization to produce breakdown there. Impact ionization has a lower threshold electric field than the boundary-controlled processes, such as secondary electron emission, that would eventually produce breakdown inside the microgap if impact ionization did not occur first. Thus, the lower pressure regime is characterized by breakdown outside of the gap, while in the high-pressure regime, breakdown occurs inside the gap, in both cases by impact ionization. The transition pressure P T between these two pressure regimes depends on the gap size d gap , roughly as P T ∝ d gap –2 . Consistent with the model developed here, this corresponds to the full-amplitude of electron oscillation in the gap field being commensurate with the gap size.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

The effect of autoionization on the HBr + X 2 Π 3/2,1/2 state photoelectron angular distributions

A double-imaging photoelectron-photoion spectrometer and synchrotron radiation have been used to measure the HBr + X 2 Π 3/2 v + = 0, 1, 2 and the 2 Π 1/2 v + = 0 state photoelectron angular distributions, as characterized by the anisotropy parameter β , the total ion yield, and the threshold photoelectron spectrum. Particular attention has been focussed on the photon energy range between the 2 Π 3/2 and the 2 Π 1/2 spin–orbit components of the ground ionic state. This region encompasses Rydberg states, belonging to series converging onto the upper 2 Π 1/2 ionization limit, which may decay by autoionization into the 2 Π 3/2 ionization continuum. A detailed study has been performed on the effects of autoionization on the 2 Π 3/2 v + = 0 state photoelectron angular distributions. Furthermore, the observed energy dependent variations in the β -values exhibit a regular pattern that correlates with excitation into members of a very broad d-type Rydberg series. Additional rapid variations in the β -parameters, which occur over a narrow energy range, appear to coincide with sharp autoionizing Rydberg states belonging to s, p and d series. The present experimental results for the HBr + X 2 Π 3/2 v + = 0 state photoelectron anisotropy parameter are compared to previously reported theoretical predictions and to earlier studies of the Kr + 4p 5 2 P 3/2 state β -parameter. The threshold photoelectron spectrum of the X 2 Π 3/2 v + = 0 band exhibits partially resolved rotational structure. A simulation of this structure yields an ionization threshold of 11.6673 ± 0.0010 eV, which is consistent with previous measurements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Atomic and Molecular Physics in Strong Fields

In this project, we have investigated a broad range of atomic, molecular, and optical (AMO) processes of current significance in strong-field and ultrafast science and technology. The main goals are to develop new theoretical formalisms and accurate computational methods for ab initio and in-depth non-perturbative study of time-resolved multiphoton quantum dynamics and spectroscopy, very-high-order harmonic generation, optimal control of ultrafast attosecond pulse generation, and resonance-enhanced below threshold harmonics in intense ultrashort laser fields. We have worked on the following projects and we have published 20 top 15 % SCI journal articles during the last 3.5 years (Jan. 1, 2016 to June 30, 2019), including one article in top raking Physical Review Letters, and 3 articles in top ranking (Nature) Scientific Reports, and one Physical Review A article was chosen as Editors’ suggestion paper, etc. (a) Development of new time-dependent methods for accurate and efficient solution of multi-dimensional time-dependent Schrödinger equation and time-dependent density functional theory. (b) Non-perturbative treatment of very-high order (1000th – 5000th order) harmonic generation and broadband supercontinuum spectra from XUV to soft x-ray regime. (c) Development of synchrosqueezing transform (SST) for precision time-frequency analysis of electron multi-rescattering dynamics. (d) Exploration of the dynamical origin of resonance-enhanced near- and below-threshold harmonics. (e) Probing the high-order harmonics generation in disordered semiconductor. (f) Multiphoton ionization of one-electron relativistic diatomic quasi-molecules in strong laser fields. and, (g) New development of non-adiabatic density functional theory. The theoretical and simulation projects we have pursued are closely related to the ongoing experimental researches in several leading laboratories in the US and Europe, etc. It is expected that the outcome of this project will provide timely and significant new information and insight for the advancement of strong-field atomic and molecular physics and ultrafast science and technology.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1974

Laser-induced electric breakdown in solids

A review is given of recent experimental results on laser-induced electric breakdown in transparent optical solid materials. A fundamental breakdown threshold exists characteristic for each material. The threshold is determined by the same physical process as dc breakdown, namely, avalanche ionization. The dependence of the threshold on laser pulse duration and frequency is consistent with this process. The implication of this breakdown mechanism for laser bulk and surface damage to optical components is discussed. It also determines physical properties of self-focused filaments.

Bloembergen, N.↗
NASA NTRS1995

Ensemble Monte Carlo calculation of the hole initiated impact ionization rate in bulk GaAs and silicon using a k-dependent, numerical transition rate formulation

The hole initiated impact ionization rate in bulk silicon and GaAs is calculated using a numerical formulation of the impact ionization transition rate incorporated into an ensemble Monte Carlo simulation. The transition rate is calculated from Fermi's golden rule using a two-body screened Coulomb interaction including a wavevector dependent dielectric function. It is found that the effective threshold for hole initiated ionization is relatively soft in both materials, that the split-off band dominates the ionization process in GaAs. and that no clear dominance by any one band is observed in silicon, though the rate out of the light hole band is greatest.

Oguzman, Ismail H.↗
NASA NTRS2020

Plasma-environment effects on K lines of astrophysical interest III. IPs, K thresholds, radiative rates, and Auger widths in Fe ix – Fe xvi

Aims. In the context of black-hole accretion disks, we aim to compute the plasma-environment e ects on the atomic parameters used to model the decay of K-vacancy states in moderately charged iron ions, namely Fe ix – Fe xvi. Methods. We used the fully relativistic multiconfiguration Dirac–Fock method approximating the plasma electron–nucleus and electron–electron screenings with a time-averaged Debye–Hückel potential. Results. We report modified ionization potentials, K-threshold energies, wavelengths, radiative emission rates, and Auger widths for plasmas characterized by electron temperatures and densities in the ranges 10^(5)-10^(7) K and 10^(18)-10^(22)/cu. cm. Conclusions. This study confirms that the high-resolution X-ray spectrometers onboard the future XRISM and Athena space missions will be capable of detecting the lowering of the K edges of these ions due to the extreme plasma conditions occurring in accretion disks around compact objects.

J. Deprince↗
NASA NTRS2020

Plasma environment effects on K lines of astrophysical interest IV. IPs, K thresholds, radiative rates, and Auger widths in Fe ii–Fe viii

Aims. Within the framework of compact-object accretion disks, we calculate plasma environment e ects on the atomic structure and decay parameters used in the modeling of K lines in lowly charged iron ions, namely Fe ii–Fe viii. Methods. For this study, we used the fully relativistic multiconfiguration Dirac–Fock method approximating the plasma electron–nucleus and electron-electron screenings with a time-averaged Debye-Hückel potential. Results. We report modified ionization potentials, K-threshold energies, wavelengths, radiative emission rates, and Auger widths for plasmas characterized by electron temperatures and densities in the ranges 10(exp 5)-10(exp 7) K and 10(exp 18)-10(exp 22) per cu.cm. In addition, we propose two universal fitting formulae to predict the IP and K-threshold lowerings in any elemental ion. Conclusions. We conclude that the high-resolution X-ray spectrometers onboard the future XRISM and ATHENA space missions will be able to detect the lowering of the K edges of these Fe ions due to the extreme plasma conditions occurring in the accretion disks around compact objects.

J. Deprince↗
NASA NTRS1999

Ultralow Energy Electron Attachment at Sub-Millielectron Volt Resolution

The technique of rare-gas photoionization has been extended by use of direct laser ionization to electron energies epsilon in the range 0-100 meV, with a resolution Delta(epsilon) of 0.4-0.5 meV (FWHM). Tunable UV light at (Lambda)276 nm is produced using a pulsed Nd:YAG laser and nonlinear mixing techniques. The beam is frequency tripled in a pulsed jet of xenon. The VUV radiation, tunable at (Lambda)92 nm, is then used to photoionize Xe at its 2P(sub 1/2) threshold (single-photon ionization). The photoelectrons produced interact with admixed target gas to generate negative ions through the s-wave capture process. Recent results in electron attachment to SF(sub 6) will be reported which show resonance structure at the opening of the ground-state vibrational channels. This structure corresponds to the process of vibrational excitation + attachment, which is superimposed on the underlying s-wave (direct) capture process. It should be a general phenomenon, present in a wide variety of zero-energy electron attaching molecules.

Chutjian, A.↗
DOE OSTI2022

Transient behavior of drift and ionization in atmospheric pressure nitrogen discharge

The fluid models are frequently used to describe a non-thermal plasma such as a streamer discharge. The required electron transport data and rate coefficients for the fluid model are parametrized using the local field approximation (LFA) in first order models and the local-mean-energy approximation (LMEA) in second order models. We performed Monte Carlo simulations in Nitrogen gas with step changes in the E/N (reduced electric field) to study the behavior of the transport properties in the transient phase. During the transient phase of the simulation, we extract the instantaneous electron mean energy, which is different from the steady state mean electron energy, and the corresponding transport parameters and rate coefficients. Our results indicate that the mean electron energy is not a suitable parameter for mobility/drift of electrons due to big difference in momentum relaxation and energy relaxation. Furthermore, the high energy threshold rates such as ionization show a strong correlation to mean electron energy. In second order models where the energy-balance equation is solved, we suggest that it would rather be appropriate to use the local electric field to find electron drift velocity in gases such as Nitrogen and the local mean electron energy to determine the ionization and excitation rates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Photoionization from the Xe 4d orbitals of XeF 2

Here, we present a comparison of the photoionization dynamics of the 4d shell of XeF 2 from threshold to 250 eV to those of the prototypical 4d shell of atomic Xe. The new experimental data include spin–orbit and ligand-field-resolved partial cross sections, photoelectron angular distributions, branching fractions, and lifetime widths for the 4d-hole states. The spin–orbit branching fractions and angular distributions are remarkably similar to the corresponding distributions from atomic Xe across a broad energy interval that includes both the intense shape resonance in the f continuum and a Cooper minimum in the same channel. The angular distributions and branching fractions are also in reasonably good agreement with our first-principles theoretical calculations on XeF 2 . Data are also presented on the lifetime widths of the substate-resolved 4d-hole states of XeF 2 . While the trends in the widths are similar to those in the earlier experimental and theoretical work, the linewidths are considerably smaller than in the previous measurements, which may require some reinterpretation of the decay mechanism. Finally, we present new data and an analysis of the Auger electron spectra for ionization above the 4d thresholds and resonant Auger spectra for several pre-edge features.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Filamentation of long-wave infrared pulses in the atmosphere

Filamentation of a high-power laser beam in air opens possibility of the diffraction-compensated propagation of a laser beam over long distances and as such is being considered for remote stand-off detection, lightning control, free-space communications, and long-range projection of high-energy pulses. Switching to long-wave infrared (LWIR) range for filamentation, as shown in recent experiments, allows for generation of a single centimeter-diameter channel in air that, in comparison with a short-wavelength laser filament, has four orders of magnitude larger cross-section and guides many Joules of pulse energy over multiple Rayleigh distances at a clamped intensity of ~10 12 W/cm 2 . Self-guiding of LWIR pulses in air arises from the balance between self-focusing, diffraction and defocusing caused by free carriers generated via many-body Coulomb induced ionization which effectively decrease the molecular polarizability during the long-wavelength laser pulse. Understanding where this new ionization regime plays a role, below the threshold of the well adopted single-atom tunnel ionization in gases, could became a new frontier in strong-field interactions. Here, this paper offers an overview of the atmospheric filamentation research at long-wave infrared wavelengths.

43 PARTICLE ACCELERATORS↗
NASA NTRS1988

Ionisation cross sections of rare-gas atoms by electron impact

A pulsed electron beam and ion extraction method is used to measure normalized values of partial ionization cross sections for rare gases from threshold to 1000 eV. Cross sections obtained for singly ionized species are used to calibrate the mass transmission efficiency of the ion extraction/analyzer/detection system by the relative flow technique, and this mass transmission curve is then used to determine the absolute cross sections of the multiply ionized species. Total ion cross sections are found by summation of the individual partial cross sections with proper weighting for charge.

Krishnakumar, E.↗