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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.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Double-resonance studies of electronically autoionizing states of molecular nitrogen

Double-resonance excitation of molecular nitrogen, N 2 , via selected rotational levels of the a" 1 Σ g + , v' = 0 state has been used to probe the first two bands in the new Ogawa series [M. Ogawa, Can. J. Phys. 42, 1087 (1964)], which lie above the first ionization threshold of the molecule. These bands are rotationally unresolved in room-temperature and jet-cooled absorption spectra, but the double-resonance technique allows a rotational analysis and first confirmation of the assignment as transitions to n = 4 members of the (A 2 II u )ndπ, v Rydberg series. In principle, the probe transition from the a" 1 Σ g + , v= 0 level to these Rydberg states corresponds to a two-electron transition, and this aspect of the results is explored. Lastly, velocity map imaging allows a determination of the photoelectron branching fractions and angular distributions, providing insight into the electronic autoionization dynamics.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Oxygen Opacity Measurements at High-Energy-Density Conditions

We present the first benchmark measurements of oxygen opacity at high-energy-density, crucial for understanding stellar interiors. A 3–5−μm thick SiO 2 sample was heated using a Z facility x-ray source, and opacity was measured in the 5–19.5 Å range with multiple spectrometers. The electron temperature and density inferred from Si 𝐾-shell spectra were 148 ± 4 eV and 8.6 ±1.4 × 10 21 cm −3 , respectively. Six experiments showed opacity reproducibility within ±15% and 5%–25% opacity uncertainty. Measured oxygen opacity and seven different opacity models exhibit extraordinary overall agreement. Discrepancies near the H-like photon ionization threshold and in certain spectral lines suggest the need for model refinements.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Analysis of THz generation by multicolor laser pulses with various frequency ratios

Here, terahertz (THz) generation in a gas medium is simulated by quantum calculations with multicolor intense laser pulses of various frequency ratios. By correlating THz radiation with the above-threshold-ionization photoelectron spectrum, we confirm that our previously proposed free-free transition model for the generation of THz radiation is also applicable to multicolor pulses. THz photon emissions can proceed through transitions between continuous states with similar energies that have opposite parities. This mechanism predicts that THz waves can be efficiently generated at special two-color laser frequency ratios when the multiphoton mixing condition is satisfied. Applying this model to multicolor fields (sawtooth wave shape), we provide a quantum-mechanical interpretation for the reason for THz radiation enhancement. A scheme combining multiple lasers to raise THz radiation satisfying multiphoton mixing conditions is also proposed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Observation of attosecond electron dynamics in the photoelectron momentum distribution of atoms using few-cycle laser pulses

The method of time-resolved measurement with ultrashort laser pulses is vital to the development of attosecond science. Pump-probe measurements using a train of attosecond pulses in combination with a near-infrared (NIR) multicycle driving laser have been successful in capturing the intercycle electron dynamics which repeats every optical cycle and leads to above-threshold ionization (ATI) spectra in the frequency domain. Here in this work, we study the effect of a carrier-envelope phase (CEP) in a few-cycle (< 6 fs) NIR laser pulse on the photoelectron momentum distribution (PMD) of a hydrogen atom and show that interference patterns in a PMD change dramatically with CEPs in the few-cycle regime. When the few-cycle driving laser pulse has a sine shape, the double-slit interference with characteristic modulation of ATI peaks dominates the PMD. On the other hand, when the driving pulse has a cosine shape, the holographical interference featured by a spider-like pattern is isolated. Our results suggest that the CEP-stable few-cycle laser pulses can be used to identify different types of intracycle interference structures in a PMD which reveal the underlying subcycle electron dynamics on an attosecond timescale.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Manifestation of postcollision interaction in Krypton LMN Auger spectra following K-shell photoionization

In this work, we report on an experimental and theoretical study of post-collision interaction (PCI) effects on L 2 -M 4,5 N 2,3 Auger electrons measured above the Kr K-edge in which L 2 vacancies are primarily generated by KL 2 x-ray emission. Such cascade processes, in which a deep inner-shell vacancy decays first by x-ray emission followed by Auger electron emission, is a strong decay mode in heavy atoms. The L 2 -M 4,5 N 2,3 Auger electron peak is observed to become increasingly asymmetric with a shifting peak maximum as the absorbed x-ray energy approaches the K-shell ionization threshold. This is attributed to PCI energy exchanges of the Auger electron with the 1s photoelectron. To model the PCI effects, we have applied a semiclassical approach modified to account for the combined lifetimes of the K and L 2 hole states. In addition, our analysis treats several closely spaced Auger transitions with final ionic states having different terms and angular momenta.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Even lighter particle dark matter

We report on recent progress in the search for dark matter particles with masses from 1 Mev c ^{-2} − 2 to 1 Gev c ^{-2} − 2 . Several dark matter candidates in this mass range are expected to generate measurable electronic-recoil signals in direct-detection experiments. We focus on dark matter particles scattering with electrons in semiconductor detectors since they have fundamentally the highest sensitivity due to their low ionization threshold. Charge-coupled device (CCD) silicon detectors are the leading technology, with significant progress expected in the coming years. We present the status of the CCD program and briefly report on other efforts.

Chavarria, Alvaro E.↗
DOE OSTI2019

A boron-coated CCD camera for direct detection of Ultracold Neutrons (UCN)

A new boron-coated CCD camera is described for direct detection of ultracold neutrons (UCN) through the capture reactions 10 B (n,α0γ) 7 Li (6%) and 10 B(n,α1γ) 7 Li (94%). The experiments, which extend earlier works using a boron-coated ZnS:Ag scintillator, are based on direct detections of the neutron-capture byproducts in silicon. The high position resolution, energy resolution and particle ID performance of a scientific CCD allows for observation and identification of all the byproducts α, 7 Li and γ (electron recoils). A signal-to-noise improvement on the order of 10 4 over the indirect method has been achieved. Sub-pixel position resolution of a few microns is demonstrated. The technology can also be used to build UCN detectors with an area on the order of 1 m 2 . The combination of micrometer scale spatial resolution, few electrons ionization thresholds and large area paves the way to new research avenues including quantum physics of UCN and high-resolution neutron imaging and spectroscopy.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Ionization-Based Optical Structures for the Control of Short-Pulse Lasers (Abbreviated LDRD Report)

The intensity of light from the highest-power lasers is limited by the size of the optics required to amplify, compress, transport, and focus a laser beam. Indeed, optics damage puts strict constraints on the laser energy density that solid optics can sustain; this leads to enormous optics sizes and limits the maximum intensity that these lasers can achieve. In this project we investigated new types of diffractive optics based on mixtures of neutral gas and plasmas. The basic idea is to interfere laser beams in a gas near the ionization threshold, so that plasma is created in the constructive interference regions (i.e., the “bright” fringes of the interference pattern) while the destructive interference regions remain a neutral gas. The resulting refractive index modulation, due to the different indices in gas vs. plasma, effectively turns such structures into diffractive optics elements. We proposed several new types of optics based on this general concept: i) reflection gratings, which can act as mirrors of “pulse cleaners” to eliminate laser pre-pulse; ii) transmission gratings, as part of a new type of pulse compression scheme with the potential to achieve laser intensities orders of magnitude above what is currently feasible using solid-state optics; and iii) holographic lenses, to focus high-power lasers. We conducted laboratory experiments which demonstrated high efficiency reflection gratings, paving the way for practical applications of our new concepts.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Photoemission Induced Plasma Breakdown (Update)

Laser-induced photoemission of electrons offers opportunities to trigger and control plasmas and discharges. However, the underlying mechanisms are not sufficiently characterized to be fully utilized. Photoemission is highly nonlinear, achieved through multiphoton absorption, above threshold ionization, photo-assisted tunneling, etc., where the dominant process depends on the work function of the material, photon energy and associated fields, surface heating, background fields, etc. To characterize the effects of photoemission on breakdown, breakdown experiments were performed and interpreted using a 0D plasma discharge circuit model and quantum model of photoemission.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1977

Double ionization of CO2 by photon impact

Double ionization of CO2 by photon impact from the double-ionization threshold at energies of 36.2 eV (342 A) to 110 eV (113 A) has been observed. The ratio of doubly to singly charged ions shows a linear rise with photon energy for approximately 15 eV above the threshold and reaches a constant value of about 2% between 67 and 110 eV.

Samson, J. A. R.↗
NASA NTRS1981

The production rate of C/+/ from the photoionization of CO and CO2

The dissociative photoionization cross sections for the production of C(+) ions are presented for the spectral region from the dissociative ionization threshold to 90 A. The cross sections are considerably larger than previously published data and should therefore influence the calculations of the C(+) concentrations in the Mars and Venus ionospheres. It is suggested that the released energy in the C(+) fragment ion could reduce the loss rate of the ions in collisional processes, thereby accounting for the apparent discrepancies between the observed and calculated C(+) ion density in the Venus ionosphere.

Samson, J. A. R.↗
NASA NTRS1982

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.

Samson, J. A. R.↗
NASA NTRS1989

Photodissociation of CO in the thermosphere of Venus

Recent investigations of CO photoabsorption demonstrate that photodissociation longward of the ionization threshold at 88.5 nm occurs primarily through line absorptions rather than continuous processes. High-resolution photoabsorption cross sections for CO at rotational temperatures near 250 K have been constructed from the improved data on dissociating transitions. The effects of the new cross sections on the rate of solar photodissociation of CO in the thermosphere of Venus are examined, and the results are compared to values obtained with the lower resolution cross sections available previously. It is found that the photodissociation profile peaks slightly higher in the atmosphere and the peak value and integrated total rate both decrease by about a factor of two.

Fox, J. L.↗
NASA NTRS1989

Jet-resolved vibronic structure in the higher excited states of N2O - Ultraviolet three-photon absorption spectroscopy from 80,000 to 90,000/cm

Ionization-detected UV multiphoton absorption spectroscopy of the excited states of N2O is presented, showing Rydberg structure within 20,000/cm of the first ionization threshold. Despite evidence for strong Rydberg-continuum coupling in the form of broadened bands and Fano line-shapes, the Rydberg structure persists, with atomic-like quantum defects and vibration structure well-matched with that of the ion. In the most clearly resolved spectrum, corresponding to the 3p(delta)1Pi state, Renner-Teller and Herzberg-Teller coupling of electronic and vibrational angular momentum are revealed. It is suggested that these mixings are properties of the N2O(+)Pi ion core.

Patsilinakou, E.↗
NASA NTRS1991

Photoabsorption and photoionization cross sections of NH3, PH3, H2S, C2H2, and C2H4 in the VUV region

Using synchrotron radiation as a continuum light source, the photoabsorption and photoionization cross sections of NH3, PH3, H2S, C2H2, and C2H4 have been measured from their respective ionization thresholds to 1060 A. The vibrational constants associated with the nu(2) totally symmetric, out-of-plane bending vibration of the ground electronic state of PH3(+) have been obtained. The cross sections and quantum yields for producing neutral products through photoexcitation of these molecules in the given spectral regions have also been determined. In the present work, autoionization processes were found to be less important than dissociation and predissociation processes in NH3, PH3, and C2H4. Several experimental techniques have been employed in order to examine the various possible systematic errors critically.

Xia, T. J.↗
NASA NTRS1992

Impulsive phase Fe K-alpha emission in a flare of 1989 March

Evidence for enhanced Fe K-alpha line emission that is coincident (to within 15 s) of an intense hard X-ray (greater than 50 keV) burst is presented on the basis of observations of the Fe K-alpha soft X-ray line made with the SMM. Three different models for explaining the K-alpha enhancement are investigated, namely, photoexcitation by soft X-ray thermal bremsstrahlung radiation from an isothermal source, collisional excitation by nonthermal thick-target electrons, and photoexcitation by a nonthermal hard X-ray flux distribution which extends with a power-law spectrum down to the Fe K-alpha ionization threshold at 7.1 keV. It is found that, to within the limits of isothermal temperature and emission measure set by soft X-ray observations, the thermal photoexcitation model cannot reproduce satisfactorily the intensity of enhanced K-alpha emission during the hard X-ray impulsive phase. The impulsive phase K-alpha flux exceeds that predicted by the thermal photoexcitation model by about 3 sigma. The implications of these results for the nonthermal interpretation of impulsive hard X-ray bursts in solar flares are discussed.

Zarro, D. M.↗
DOE OSTI2019

A boron-coated CCD camera for direct detection of Ultracold Neutrons (UCN)

A new boron-coated CCD camera is described for direct detection of ultracold neutrons (UCN) through the capture reactions $^{10}$B (n,$\alpha$0$\gamma$)$^7$Li (6%) and $^{10}$B(n,$\alpha$1$\gamma$)$^7$Li (94%). The experiments, which extend earlier works using a boron-coated ZnS:Ag scintillator, are based on direct detections of the neutron-capture byproducts in silicon. The high position resolution, energy resolution and particle ID performance of a scientific CCD allows for observation and identification of all the byproducts $\alpha$, $^7$Li and $\gamma$ (electron recoils). A signal-to-noise improvement on the order of 10$^4$ over the indirect method has been achieved. Sub-pixel position resolution of a few microns is demonstrated. The technology can also be used to build UCN detectors with an area on the order of 1 m$^2$. The combination of micrometer scale spatial resolution, few electrons ionization thresholds and large area paves the way to new research avenues including quantum physics of UCN and high-resolution neutron imaging and spectroscopy.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Spectroscopic and Computational Studies of Spin-Orbit Coupling of Lanthanide Oxides

Spin-orbit (SO) coupling makes it possible for spin forbidden transitions or reactions within non-relativistic quantum theory feasible. Thus, quantification of such interactions has important implications in photophysics and chemical catalysis. This work aims to quantify SO interactions using lanthanide (Ln) oxides as target molecular systems. Through these systems, we examine the impact on the SO coupling by electron configurations and 4f orbital occupancies of Ln elements and sizes of the metal oxides. By examining these factors, we explore how SO coupling is affected by the number of Ln 4f electrons for a given size of molecules and if the Ln 4f orbitals remain atomic in nature in these small clusters. Ln oxides are produced in laser ablation molecular beams, identified with time-of-fight mass spectrometry, and characterized with laser spectroscopy and relativistic quantum chemical computations. Spectroscopic measurements include mass-analyzed threshold ionization (MATI), zero electron kinetic energy, and slow electron velocity-map imaging spectroscopies. Relativistic computations treat scalar relativistic corrections, electron correlations, and SO interactions. The main results are SO terms and energies of the neutral molecules and singly charged cations, ionization energies of the neutral species, metal-metal and metal-oxygen vibrational frequencies of the ions and, in some cases, neutrals as well, and charge effects on the bonding and structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗