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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Double photoionization of atomic oxygen: Feshbach resonances in the two-electron continuum

In this work, we describe a joint experimental and theoretical investigation on oxygen double photoionization—the emission of two electrons from atomic oxygen following single photon absorption. High-resolution experimental measurements were performed at the Advanced Light Source, revealing sharp resonance structure superimposed on the more familiar Wannier-like, nearly-linear background. These resonance features are attributed to ionization-plus-excitation Feshbach resonances embedded in the double ionization continuum, doubly-excited states that lie above the double-ionization threshold. Such features are absent in the double photoionization cross section of He, or other quasi-two-electron systems, for which the doubly-ionized atomic core remains inert. For a corresponding theoretical analysis, the R-matrix with pseudostates (RMPS) method was invoked by calculating final-state, two-electron resonances-plus-continua wavefunctions and corresponding single-photon absorption cross sections. Overall agreement is found in the direct, background double photoionization cross section. However, the RMPS method, using a small basis due to practical computational limitations, was unable to reproduce quantitatively the smooth background or the sharper resonance features observed in the measurements, showing instead large-scale oscillations about the experimental background, and characteristic pseudoresonance jitter, associated with an insufficient convergence of the pseudostate representation to the true two-electron infinite series of Feshbach resonances embedded in the two-electron continuum. The prominent resonance structure observed highlights the need to consider multiple excitation processes in atoms more complex than He or quasi-two-electron systems.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Ratios of double to single ionization of He and Ne by strong 400-nm laser pulses using the quantitative rescattering theory

Here, we present numerical simulations of the ratio between double and single ionization of He and Ne by intense laser pulses at wavelengths of 390 and 400 nm, respectively. The yields of doubly charged ions due to nonsequential double ionization (NSDI) are obtained by employing the quantitative rescattering (QRS) model. In this model, the NSDI ionization probability is expressed as a product of the returning electron wave packet (RWP) and the total scattering cross sections for laser-free electron impact excitation and electron impact ionization of the parent ion. According to the QRS theory, the same RWP is also responsible for the emission of high-energy above-threshold ionization photoelectrons. To obtain absolute double-ionization yields, the RWP is generated by solving the time-dependent Schrödinger equation (TDSE) within a one-electron model. The same TDSE results can also be taken to obtain single-ionization yields. By using the TDSE results to calibrate single ionization and the RWP obtained from the strong-field approximation, we further simplify the calculation such that the nonuniform laser intensity distribution in the focused laser beam can be accounted for. In addition, laser-free electron impact excitation and ionization cross sections are calculated using the state-of-the-art many-electron R-matrix theory. The simulation results for double-to-single-ionization ratios are found to compare well with experimental data and support the validity of the nonsequential double-ionization mechanism for the covered intensity region.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Sensitive Mass Spectrometer for Time-Resolved Gas-Phase Chemistry Studies at High Pressures

Here, we report the construction of a new experimental apparatus for direct time-resolved probing of high-pressure gas-phase chemical reactions by photoionization mass spectrometry. The apparatus uses a laser photolysis slow-flow reactor, capable of operating at P = 0.3 — 100 bar and T = 300 — 1000 K. In this report, we initiate reactions in homogeneous gas mixtures by the photolysis of appropriate radical precursor using laser pulses at repetition rates of 1 — 10 Hz. The reacting mixture is continuously sampled into a vacuum chamber, ionized by VUV photons from laboratory-based discharge lamps or from a synchrotron beamline, and analyzed by a custom-designed mass spectrometer. Soft near-threshold ionization by tunable synchrotron radiation enables spectroscopic quantification of many key intermediates and products of chemical reactions. A novel ionization scheme in the high-density region of the sample gas jet increases the experimental sensitivity 100-fold, compared with existing instruments, without compromising mass resolution. A 40-kHz pulsed reflectron time-of-flight mass spectrometer in the orthogonal acceleration geometry achieves simultaneous detection of all ionized species with 25-μs time resolution. We show the power of this apparatus by investigating the ethyl radical oxidation reaction using very dilute (<10 12 molecules • cm -3 ) ethyl concentrations at pressures up to 25 bar.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Rovibrationally resolved Rayleigh and Raman scattering cross sections for molecular hydrogen

Accurate Rayleigh and Raman scattering cross sections, tensor components, depolarization ratios, and reversal coefficients for all rovibrational transitions within the X1Σg+ ground electronic state of H2 have been calculated. Raman spectra have been generated using these data. A method for calculating Raman scattering cross sections is formulated that is valid below the ionization threshold and in the region containing resonances, which explicitly accounts for all bound and dissociative vibrational levels of the bound intermediate electronic states and approximately accounts for the ionization continuum. A representative set of cross sections is presented for incident photon energies below 15 eV and compared with existing results in the literature where possible. Convergence of our results with an increasing number of bound intermediate electronic states is demonstrated. The accuracy of the Placzek–Teller approximation is discussed. The effect of accounting for the intermediate ionization continuum is investigated. Local thermal equilibrium cross sections are calculated for Rayleigh and Raman scattering. This work represents the most accurate and complete treatment of Raman scattering for molecular hydrogen to date. A total of 9582 Rayleigh and Raman scattering cross sections have been generated and are openly available on Zenodo under an open-source Creative Commons Attribution license at https://zenodo.org/doi/10.5281/zenodo.13441471.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Resonant Auger decay of dissociating CH 3 I near the I 4d threshold

Resonant Auger processes provide a unique perspective on electronic interactions and excited vibrational and electronic states of molecular ions. Here, new data are presented on the resonant Auger decay of excited CH 3 I in the region just below the I 4d -1 ionization threshold. The resonances include the Rydberg series converging to the five spin–orbit and ligand-field split CH 3 I (I 4d -1 ) thresholds, as well as resonances corresponding to excitation from the I 4d 5/2,3/2 orbitals into the σ* lowest unoccupied molecular orbital. This study focuses on participator decay that populates the lowest lying states of CH 3 I + , in particular, the X̃ 2 E 3/2 and 2 E 1/2 states, and on spectator decay that populates the lowest-lying (CH 3 I 2+ )σ* states of CH 3 I + . The CH 3 I (I 4d -1 )σ* resonances are broad, and dissociation to CH 3 + I competes with the autoionization of the core-excited states. Auger decay as the molecule dissociates produces a photoelectron spectrum with a long progression (up to v 3 + ~ 25) in the C–I stretching mode of the X̃ 2 E 3/2 and 2 E 1/2 states, providing insights into the shape of the dissociative core-excited surface. Further, the observed spectator decay processes indicate that CH 3 I + is formed on the repulsive wall of the lower-lying (CH 3 I 2+ )σ* potentials, and the photon-energy dependence of the processes provides insights into the relative slopes of the (4d -1 )σ* and (CH 3 I 2+ )σ* potential surfaces. Data are also presented for the spectator decay of higher lying CH 3 I (I 4d -1 )nl Rydberg resonances. Photoelectron angular distributions for the resonant Auger processes provide additional information that helps distinguish these processes from the direct ionization signal.

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

Dissecting subcycle interference in photoelectron holography

Multipath holographic interference in strong-field quantum tunnel ionization is key to revealing subangstrom attosecond dynamics for molecular movies. This critical subcycle motion is often obscured by longer timescale effects such as ring-shaped patterns that appear in above-threshold ionization (ATI). In the present work, we overcome this problem by combining two techniques in theory and experimental analysis: unit-cell averaging and time-filtering data and simulations. Together these suppress ATI rings and enable an unprecedented highly detailed quantitative match between strong-field ionization experiments in argon and the Coulomb-quantum orbit strong-field approximation (CQSFA) theory. Velocity map images reveal fine modulations on the holographic spiderlike interference fringes that form near the polarization axis. CQSFA theory traces this to the interference of three types of electron pathways. The level of agreement between experiment and theory allows sensitive determination of quantum phase differences and symmetries, providing an important tool for quantitative dynamical imaging in quantum systems.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Experimental assignment of many-electron excitations in the photoionization of NiO

The absorption of a photon and the emission of an electron is not a simple, two-particle process. The complicated many-electron features observed during core photoionization can therefore reveal many of the hidden secrets about the ground and excited-state electronic structures of a material. Careful analysis of the photon-energy dependence of the Ni KLL Auger deexcitation spectra at and above the Ni 1s photoionization threshold has identified the satellite structure that appears in both the photoelectron emission and the x-ray absorption spectra of NiO as Ni metal 3d e g → Ni metal 3d e g and O ligand 2p e g → Ni metal 3d e g charge-transfer excitations, respectively. These assignments clarify the conflicting theoretical predictions of the last five decades in addition to other anomalous effects in the spectroscopy of this unique material.

36 MATERIALS SCIENCE↗
DOE OSTI2012

On the delayed gas breakdown in a ringing theta-pinch with bias magnetic field

A single particle model and particle-in-cell simulations are used to elucidate the breakdown physics in a ringing theta-pinch with a bias magnetic field. Previous experimental results show that gas breakdown occurs when the bias magnetic field is nullified by the theta-pinch magnetic field. The analyses presented here agree with the experimental results and show that electron kinetic energy does not exceed the ionization threshold of deuterium until the net magnetic field is approximately zero. Despite the presence of a strong electric field, the gyromotion of electrons within the bias magnetic field prevents them from gaining energy necessary to ionize the gas. Parametric analysis of the peak electron energy as a function of the bias and pre-ionization magnetic fields reveals that: (1) when the bias magnetic field is Almost-Equal-To 97% of the pre-ionization magnetic field, peak electron energies are highly erratic resulting in poor overall ionization, and (2) full ionization with repeatable behavior requires a pre-ionization to bias magnetic field ratio of approximately 2 to 1 or higher.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Photoelectron spectra in circularly and elliptically polarized laser pulses

We present results of numerical simulations of the time-dependent Schrödinger equation and theoretical analysis concerning the interaction of a rare gas atom with circularly and elliptically polarized laser pulses. In agreement with recent observation in circularly polarized fields, the photoelectron energy spectra for counterrotating electrons are peaked at lower kinetic energy than those for corotating electrons. We show that this difference can be interpreted as being due to the additional pathways to ionization that are available for the counterrotating electrons only. Furthermore, our results show, in agreement with earlier work, that the offset angle by which the emission of electrons is rotated in an elliptically polarized field increases with each successive above-threshold ionization (ATI) order and is larger for the emission from counterrotating states as compared to that from corotating states. As a result, a simple model based on the interference and relative phase difference between just three continuum states provides remarkable agreement and once again emphasizes the importance of the additional ionization pathways available for the counterrotating electrons.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Laser information encoded in atomic asymmetrical ionization in few-cycle laser fields

The pulse duration and carry-envelope phase (CEP) are two important parameters to characterize a few-cycle intense laser. We systematically study asymmetrical ionization of Ar in few-cycle laser fields with different pulse durations (3-7 fs) and laser intensities (10{sup 13}-10{sup 14} W/cm{sup 2}) by solving the time-dependent Schroedinger equation. Analyzing the CEP-dependent asymmetry of above-threshold ionization as a function of the photoelectron energy, we find that the qualitative global features of CEP-dependent asymmetry are insensitive to the pulse duration. But the fine structures of the asymmetry are sensitive to the laser pulse duration. Changing Ar to H atoms, we find that the information encoded in asymmetry is insensitive to target atoms at a moderate laser intensity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Following electron impact excitations of Rn, Ra, Th, U and Pu single atom L sub-shells ionization cross section calculations by using Lotz’s equation

L shell ionization cross section and L{sub i} subshells ionization cross sections of Rn, Ra, Th, U, Pu atoms calculated. For each of atoms, ten different electron impact energy values (E{sub o}) are used. Calculations carried out by using Lotz equation in Matlab. First, calculations done for non-relativistic case by using non-relativistic Lotz equation then repeated with relativistic Lotz equation. σ{sub L} total and σ{sub Li}(i = 1,2,3) subshells ionisation cross section values obtained for E{sub o} values in the energy range of E{sub Li} <E{sub o}<4E{sub Li} for each atom. Starting almost from E{sub o} = E{sub Li} values of the each sub shell ionization threshold energy, σ{sub L} total and σ{sub Li} (i = 1,2,3) are increasing rapidly with E{sub o}. For a fixed E{sub o} = 3.E{sub Li}), while Z increases from 86<Z<94, σL and σLi (i = 1, 2,3) are decrease. These results will also help to understand some σ{sub Li}(i = 1,2,3) subshells results obtained from other electron-atom impact studies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Dynamics of the reaction of C{sub 3}(a{sup 3}Π{sub u}) radicals with C{sub 2}H{sub 2}: A new source for the formation of C{sub 5}H

The reaction C{sub 3}(a{sup 3}Π{sub u}) + C{sub 2}H{sub 2} → C{sub 5}H + H was investigated at collision energy 10.9 kcal mol{sup −1} that is less than the enthalpy of ground-state reaction C{sub 3}(X{sup 1}Σ{sub g}{sup +}) + C{sub 2}H{sub 2} → C{sub 5}H + H. C{sub 3}(a{sup 3}Π{sub u}) radicals were synthesized from 1% C{sub 4}F{sub 6}/He by pulsed high-voltage discharge. The title reaction was conducted in a crossed molecular-beam apparatus equipped with a quadrupole-mass filter. Product C{sub 5}H was interrogated with time-of-flight spectroscopy and synchrotron vacuum-ultraviolet ionization. Reactant C{sub 3}(a{sup 3}Π{sub u}) and product C{sub 5}H were identified using photoionization spectroscopy. The ionization thresholds of C{sub 3}(X{sup 1}Σ{sub g}{sup +}) and C{sub 3}(a{sup 3}Π{sub u}) are determined as 11.6 ± 0.2 eV and 10.0 ± 0.2 eV, respectively. The C{sub 5}H product is identified as linear pentynylidyne that has an ionization energy 8.4 ± 0.2 eV. The title reaction releases translational energy 10.6 kcal mol{sup −1} in average and has an isotropic product angular distribution. The quantum-chemical calculation indicates that the C{sub 3}(a{sup 3}Π{sub u}) radical attacks one of the carbon atoms of C{sub 2}H{sub 2} and subsequently a hydrogen atom is ejected to form C{sub 5}H + H, in good agreement with the experimental observation. As far as we are aware, the C{sub 3}(a{sup 3}Π{sub u}) + C{sub 2}H{sub 2} reaction is investigated for the first time. This work gives an implication for the formation of C{sub 5}H from the C{sub 3}(a{sup 3}Π{sub u}) + C{sub 2}H{sub 2} reaction occurring in a combustion or discharge process of C{sub 2}H{sub 2}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Time-dependent Cooling in Photoionized Plasma

I explore the thermal evolution and ionization states in gas cooling from an initially hot state in the presence of external photoionizing radiation. I compute the equilibrium and nonequilibrium cooling efficiencies, heating rates, and ion fractions for low-density gas cooling while exposed to the ionizing metagalactic background radiation at various redshifts ( z = 0 − 3), for a range of temperatures (10{sup 8}–10{sup 4} K), densities (10{sup −7}–10{sup 3} cm{sup −3}), and metallicities (10{sup −3}–2 times solar). The results indicate the existence of a threshold ionization parameter, above which the cooling efficiencies are very close to those in photoionization equilibrium (so that departures from equilibrium may be neglected), and below which the cooling efficiencies resemble those in collisional time-dependent gas cooling with no external radiation (and are thus independent of density).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Detection Identification and Quantification of Keto-Hydroperoxides in Low-Temperature Oxidation.

Keto-hydroperoxides are reactive partially oxidized intermediates that play a central role in chain-branching reactions during the low-temperature oxidation of hydrocarbons. In this Perspective, we outline how these short lived species can be detected, identified, and quantified using integrated experimental and theoretical approaches. The procedures are based on direct molecular-beam sampling from reactive environments, followed by mass spectrometry with single-photon ionization, identification of fragmentation patterns, and theoretical calculations of ionization thresholds, fragment appearance energies, and photoionization cross sections. Using the oxidation of neo-pentane and tetrahydrofuran as examples, the individual steps of the experimental approaches are described in depth together with a detailed description of the theoretical efforts. For neo-pentane, the experimental data are consistent with the calculated ionization and fragment appearance energies of the keto-hydroperoxide, thus adding confidence to the analysis routines and the employed levels of theory. For tetrahydrofuran, multiple keto-hydroperoxide isomers are possible due to the presence of nonequivalent O 2 addition sites. Despite this additional complexity, the experimental data allow for the identification of two to four keto-hydroperoxides. Mole fraction profiles of the keto-hydroperoxides, which are quantified using calculated photoionization cross sections, are provided together with estimated uncertainties as function of the temperature of the reactive mixture and can serve as validation targets for chemically detailed mechanisms.

Hansen, Nils↗
DOE OSTI2019

Monochromatic 2D K α Emission Images Revealing Short-Pulse Laser Isochoric Heating Mechanism

The rapid heating of a thin titanium foil by a high intensity, subpicosecond laser is studied by using a 2D narrow-band x-ray imaging and x-ray spectroscopy. Here, a novel monochromatic imaging diagnostic tuned to 4.51 keV Ti Kα was used to successfully visualize a significantly ionized area ( < Z > >17±1) of the solid density plasma to be within a ~35 μm diameter spot in the transverse direction and 2 μm in depth. The measurements and a 2D collisional particle-in-cell simulation reveal that, in the fast isochoric heating of solid foil by an intense laser light, such a high ionization state in solid titanium is achieved by thermal diffusion from the hot preplasma in a few picoseconds after the pulse ends. The shift of Kα and formation of a missing Kα cannot be explained with the present atomic physics model. The measured Kα image is reproduced only when a phenomenological model for the Kα shift with a threshold ionization of < Z > =17 is included. This work reveals how the ionization state and electron temperature of the isochorically heated nonequilibrium plasma are independently increased.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Photoionisation study of Xe.CF{sub 4} and Kr.CF{sub 4} van-der-Waals molecules

We report on photoionization studies of Xe.CF{sub 4} and Kr.CF{sub 4} van-der-Waals complexes produced in a supersonic expansion and detected using synchrotron radiation and photoelectron-photoion coincidence techniques. The ionization potential of CF{sub 4} is larger than those of the Xe and Kr atoms and the ground state of the Rg.CF{sub 4}{sup +} ion correlates with Rg{sup +} ({sup 2}P{sub 3/2}) + CF{sub 4}. The onset of the Rg.CF{sub 4}{sup +} signals was found to be only ∼0.2 eV below the Rg ionization potential. In agreement with experiment, complementary ab initio calculations show that vertical transitions originating from the potential minimum of the ground state of Rg.CF{sub 4} terminate at a part of the potential energy surfaces of Rg.CF{sub 4}{sup +}, which are approximately 0.05 eV below the Rg{sup +} ({sup 2}P{sub 3/2}) + CF{sub 4} dissociation limit. In contrast to the neutral complexes, which are most stable in the face geometry, for the Rg.CF{sub 4}{sup +} ions, the calculations show that the minimum of the potential energy surface is in the vertex geometry. Experiments which have been performed only with Xe.CF{sub 4} revealed no Xe.CF{sub 4}{sup +} signal above the first ionization threshold of Xe, suggesting that the Rg.CF{sub 4}{sup +} ions are not stable above the first dissociation limit.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

VUV and soft x-ray ionization of a plant volatile: Vanillin (C{sub 8}H{sub 8}O{sub 3})

Plant volatiles are emitted by plants in response to several forms of stress, including interaction with energetic photons. In the present work, we discuss the interaction of extreme UV and soft X-ray photons with a plant volatile, vanillin. The single and double (multiple) ionization of the vanillin molecule have been studied for the first time using time-of-flight mass spectrometry and VUV and soft X-ray photons (synchrotron radiation, at 12.0 eV, 21.2 eV, 130 eV, 310 eV, 531 eV, and 550 eV). At 12.0 and 21.2 eV, only singly charged species are observed and the parent ion, C{sub 8}H{sub 8}O{sub 3}{sup +}, is the dominant species. Energy differences for some selected fragments were calculated theoretically in this energy region. At 130 eV, direct double and triple ionization of the valence electrons may occur. The fragmentation increases and CHO{sup +} becomes one of the main cations in the mass spectrum. The molecular ion is still the dominant species, but other fragments, such as C{sub 6}H{sub 5}O{sup +}, begin to present similar intensities. At 310 eV, C 1s electrons may be ionized and Auger processes give rise to dissociative doubly ionized cations. Ionization around the O 1s edge has been studied both at the 531 eV resonance and above the ionization edge. Resonant and normal Auger processes play a significant role in each case and a large fragmentation of the molecule is observed at both photon energies, with intense fragments such as CHO{sup +} and CH{sub 3}{sup +} being clearly observed. A near edge X-ray absorption fine structure spectrum of the vanillin molecule was obtained around the O 1s ionization threshold. In addition, the fragmentation of vanillin has also been studied using a fast beam of electrons (800 eV), for the sake of comparison.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗