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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Single photoionization of the Kr-like Rb ii ion in the photon energy range 22–46.0 eV

Abstract Single photoionization cross-sections for Kr-like Rb+ ions are reported in the energy (wavelength) range from 22 (564 Å) to 46 eV (270 Å). Theoretical cross-section calculations for this trans-Fe element are compared with measurements from the ASTRID radiation facility in Aarhus, Denmark and from the dual laser plasma technique, at respectively 40 and 35 meV FWHM energy resolution. In the photon energy region 22–32 eV the spectrum is dominated by excitation auto-ionizing resonance states. Above 32 eV the cross-section exhibits classic Fano window resonances features, which are analysed and discussed. Large-scale theoretical photoionization cross-section calculations, performed using a Dirac Coulomb R-matrix approximation are benchmarked against these high-resolution experimental results. Comparison of the theoretical work with the experimental studies allowed the identification of resonance features and their parameters in the spectra in addition to contributions from excited metastable states of the Rb+ ions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Attosecond time delays at Cooper minima in valence-shell photoionization of alkali-metal and alkaline-earth-metal atoms

Ji et al. [New J. Phys. 26, 093014 (2024)] established a direct link between the photoionization cross section and the attosecond time delay near Cooper minima (CM) in the valence shells of noble-gas atoms. This link is based on the analytic properties of the ionization amplitude in the complex plane of the photoelectron energy, and is particularly sensitive to the winding number of the amplitude around the origin of the complex energy plane. Here, in this study, we demonstrate an analogous relation for photoionization of the valence 𝑛⁢𝑠 shells of alkali-metal atoms (AMA), from Na (𝑛 = 3) to Cs (𝑛 = 6), as well as alkaline-earth-metal atoms (AEMA), from Mg (𝑛 = 3) to Ba (𝑛 = 6). To this end, we employ a fully relativistic formalism that separates the two complementary 𝑛⁢𝑠 1/2 → 𝐸⁢𝑝 1/2 and 𝐸⁢𝑝 3/2 ionization channels. Each of these channels exhibits a phase variation close to 𝜋, but in opposite directions, near their respective Cooper minima. This phase variation vanishes in a nonrelativistic formulation, where the two channels become degenerate. For AMA, due to the threshold proximity of the CM, the universal Coulomb contribution to the time delay must be subtracted. The remaining component of the time delay is target-specific, angular-dependent, and accessible through comparative measurements.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Comment on “Test of the Stark-effect theory using photoionization microscopy”

An article by Zhao et al. [Phys. Rev. A 86, 053413 (2012)] tests the local frame transformation (LFT) theory by comparing it with benchmark coupled-channel calculations. The system under consideration is an alkali-metal atom that is two-photon ionized in the presence of a static external electric field. Zhao et al. state that the differential cross sections computed in the LFT theory disagree with their supposedly more accurate coupled-channel calculations. They went on to diagnose the discrepancy and claimed that it originates in an inaccurate correspondence between the irregular functions in spherical and parabolic-cylindrical coordinates, a correspondence that lies at the heart of LFT theory. We have repeated the same tests and find that our calculations rule out the discrepancies that were claimed in Zhao et al. [Phys. Rev. A 86, 053413 (2012)] to exist between the LFT approximation and the exact calculations. Lastly, this Comment thus helps to clarify the accuracy of the Harmin-Fano theory and demonstrates that it is in fact remarkably accurate not only for the total photoionization cross section in the Stark effect, but also for the differential cross section in photoionization microscopy.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Effects of molecular rotation after ionization and prior to fragmentation on observed recoil-frame photoelectron angular distributions in the dissociative photoionization of nonlinear molecules

Experimental angle-resolved photoelectron-photoion coincidence experiments measure photoelectron angular distributions (PADs) in dissociative photoionization (DPI) in the reference frame provided by the momenta of the emitted heavy fragments. By extension of the nomenclature used with DPI of diatomic molecules, we refer to such a PAD as a recoil-frame PAD (RFPAD). When the dissociation is fast compared to molecular rotational and bending motions, the emission directions of the heavy fragments can be used to determine the orientation of the bonds that are broken in the DPI at the time of the ionization, which is known as the axial-recoil approximation (ARA). When the ARA is valid, the RFPADs correspond to molecular-frame photoelectron angular distributions (MFPADs) when the momenta of a sufficient number of the heavy fragments are determined. When only two fragments are formed, the experiment cannot measure the orientation of the fragments about the recoil axes so that the resulting measured PAD is an azimuthally averaged RFPAD (AA-RFPAD). In this paper we consider how the breakdown of the ARA due to rotation will modify the observed RFPADs for DPI processes in nonlinear molecules for ionization by light of arbitrary polarization. This model is applied to the core C $1s$ DPI of ${\mathrm{CH}}_{4}$, with the results compared to experimental measurements and previous theoretical calculations done within the ARA. The published results indicate that there is a breakdown in the ARA for two-fragment events where the heavy-fragment kinetic energy release was less than 9 eV. Finally, including the breakdown of the ARA due to rotation in our calculations gives very good agreement with the experimental AA-RFPAD, leading to an estimate of upper bounds on the predissociative lifetimes as a function of the kinetic energy release of the intermediate ion states formed in the DPI process.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2016

Photoionization microscopy of the lithium atom: Wave-function imaging of quasibound and continuum Stark states

Photoionization of an atom in the presence of a uniform static electric field provides the unique opportunity to expand and visualize the atomic wave function at a macroscopic scale. In a number of seminal publications dating back to the 1980s, Fabrikant, Demkov, Kondratovich, and Ostrovsky showed that this goal could be achieved by projecting slow (meV) photoionized electrons onto a position-sensitive detector and underlined the distinction between continuum and resonant contributions. Here, the uncovering of resonant signatures was achieved fairly recently in experiments on the nonhydrogenic lithium atoms [Cohen et al., Phys. Rev. Lett. 110, 183001 (2013)]. The purpose of the present article is the general description of these findings, with emphasis on the various manifestations of resonant character. From this point of view, lithium has been chosen as an illustrative example between the two limiting cases of hydrogen, where resonance effects are more easily identified, and heavy atoms like xenon, where resonant effects were not observed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Cross sections for photoionization of fullerene molecular ions C n + with n = 40, 50, 70, 76, 78, and 84

Absolute cross-section measurements are reported for single photoionization of C n + fullerene molecular ions (n=40, 50, 70, 76, 78, and 84) in the photon-energy range 18-70 eV. The experiments were performed by merging a mass and charge selected beam of C n + molecular ions with a beam of monochromatized synchrotron radiation and measuring the yield of C n 2+ product ions as a function of the photon energy. Oscillator strengths determined by integrating the measured cross sections over this energy range exhibit a linear dependence on n. The cross sections are parametrized by fits to three Lorentzian functions to represent plasmon excitations and a linear function for direct ionization. Finally, the highest-energy resonance in the data near 46 eV is similar to that previously observed in single photoionization of C 60 and may be attributable to a harmonic of the dominant surface-plasmon resonance near 23 eV.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Photoionization of metastable heliumlike C 4 + ( 1 s 2 s S 1 3 ) ions: Precision study of intermediate doubly excited states

In a joint experimental and theoretical endeavor, photoionization of metastable C 4+ (1s2s 3 S 1 ) ions via intermediate levels with hollow, double-K-vacancy configurations 2s2p, 2s3p, 2p3s, 2p3d, 2s4p, 2p4s, and 2p4d has been investigated. High-resolution photon-ion merged-beams measurements were carried out with the resolving power reaching up to 25 800 which is sufficient to separate the leading fine-structure components of the 2s2p 3 P term. Many-body perturbation theory was employed to determine level-to-level cross sections for K-shell excitation with subsequent autoionization. The resonance energies were calculated with inclusion of electron correlation and radiative contributions. Their uncertainties are estimated to be below ±1 meV. Detailed balance confirms the present photoionization cross-section results by comparison with previous dielectronic-recombination measurements. Finally, the high accuracy of the theoretical transition energies together with the present experimental results qualify photoabsorption resonances in heliumlike ions as new, greatly improved energy-reference standards at synchrotron radiation facilities.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Photoionization and photofragmentation of singly charged positive and negative Sc 3 N @ C 80 endohedral fullerene ions

Photoprocesses of the endohedral fullerene ions $\mathrm{Sc_3N@C_{80}^{+}}$ and $\mathrm{Sc_3N@C_{80}-}$ in the gas phase have been investigated in the photon energy ranges 30-50 eV and 280-420 eV. Single and double ionization as well as single ionization accompanied by the release of a C 2 dimer were observed as a function of the photon energy for the positive parent ion and double detachment was measured for the negative parent ion. The emphasis of the experiments was on the specific effects of the encapsulated trimetallic nitride cluster Sc 3 N on the observed reactions. Clear evidence of photoexcitation near the Sc L edge is obtained with the dominating contributions visible in the one- and two-electron-removal channels. K-vacancy production in the encapsulated central nitrogen atom is seen in the single ionization of $\mathrm{Sc_3N@C_{80}^{+}}$ but is much less pronounced in the photoionization-with-fragmentation channel. Comparison of the cross sections near the carbon K edge with the corresponding channels measured previously in the photoionization of $\mathrm{Lu_3N@C_{80}^+}$ reveal strong similarities. Previously predicted sharp resonance features in the ionization of $\mathrm{Sc_3N@C_{80}^{+}}$ ions below the Sc M edge are not confirmed. Finally, the experiments are accompanied by quantum-chemistry calculations in the Hartree-Fock approximation and by model calculations employing density functional theory (DFT).

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Two-Center Interference in the Photoionization Delays of Kr 2

Herein we present the experimental observation of two-center interference in the ionization time delays of Kr 2 . Using attosecond electron-ion-coincidence spectroscopy, we simultaneously measure the photoionization delays of krypton monomer and dimer. The relative time delay is found to oscillate as a function of the electron kinetic energy, an effect that is traced back to constructive and destructive interference of the photoelectron wave packets that are emitted or scattered from the two atomic centers. Our interpretation of the experimental results is supported by solving the time-independent Schrödinger equation of a 1D double-well potential, as well as coupled-channel multiconfigurational quantum-scattering calculations of Kr 2 . This work opens the door to the study of a broad class of quantum-interference effects in photoionization delays and demonstrates the potential of attosecond coincidence spectroscopy for studying weakly bound systems.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Angularly resolved photoionization dynamics in atoms and molecules combining temporally and spectrally resolved experiments at ATTOLab and Synchrotron SOLEIL

We report results for XUV-IR two-photon ionization of Ar, Ne, NO, and O2, where an XUV attosecond pulse train is superimposed with a synchronized IR pulse, obtained at the ATTOLab laser facility using electron–ion coincidence 3D momentum spectroscopy. Temporally resolved photoelectron angular distributions providing angle-resolved time-delays for np ionization of Ar and Ne, achieved by reconstruction of attosecond beating by interference of two-photon transitions through a unified formalism (Joseph et al. in J Phys B At Mol Opt Phys 53:184007, 2020), are summarized. For inner valence XUV-IR dissociative photoionization of NO and O2 molecules, we report electron–ion kinetic energy correlation diagrams and disentangle the dissociative photoionization processes relying on parallel XUV experiments at Synchrotron SOLEIL. For ionization into the NO+(c3Π) ionic state, extending the formalism developed for single-photon ionization, we focus on photoelectron angular distributions averaged on the delay between the XUV and the IR field in the field frame, molecular frame, and electron frame of reference.

Joseph, J↗
DOE OSTI2016

High-Resolution Photoionization, Photoelectron and Photodissociation Studies. Determination of Accurate Energetic and Spectroscopic Database for Combustion Radicals and Molecules

The main goal of this research program was to obtain accurate thermochemical and spectroscopic data, such as ionization energies (IEs), 0 K bond dissociation energies, 0 K heats of formation, and spectroscopic constants for radicals and molecules and their ions of relevance to combustion chemistry. Two unique, generally applicable vacuum ultraviolet (VUV) laser photoion-photoelectron apparatuses have been developed in our group, which have used for high-resolution photoionization, photoelectron, and photodissociation studies for many small molecules of combustion relevance.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2025

Molecular theory capability: LANL-PEM-Atomic implements universal descriptions of molecular photoionization

Photoionization of atomic ions, the process through which atoms in bound states absorb radiation by losing electrons, is a vital part of describing radiative transfer in mission-relevant dynamics. The photoionization of molecules dominates radiative transfer of ultraviolet (UV) and extreme UV (EUV) radiation in colder atmospheres (temperature < 30,000 degrees Kelvin). Consequently, air, which is quite transparent to our eyes (i.e., in the visible frequency regime) is remarkably opaque to radiation in much of the UV and EUV frequency regimes. This opacity makes air and other gas systems quite efficient at absorbing UV and EUV radiation, which heats the gas until ionization makes it transparent. This effect is important in describing a host of relevant phenomena, ranging from charge separation in high-altitude nuclear events and the dynamics of a nuclear fireball to the electrostatic discharges (sparks) that complicate weapons disassembly at Pantex.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

State-Selective Double Photoionization of Atomic Carbon and Neon

Double photoionization (DPI) allows for a sensitive and direct probe of electron correlation, which governs the structure of all matter. For atoms, much of the work in theory and experiment that informs our fullest understanding of this process has been conducted on helium, and efforts continue to explore many-electron targets with the same level of detail to understand the angular distributions of the ejected electrons in full dimensionality. Expanding on previous results, we consider here the double photoionization of two 2p valence electrons of atomic carbon and neon and explore the possible continuum states that are connected by dipole selection rules to the coupling of the outgoing electrons in 3P, 1D, and 1S initial states of the target atoms. Carbon and neon share these possible symmetries for the coupling of their valence electrons. Results are presented for the energy-sharing single differential cross section (SDCS) and triple differential cross section (TDCS), further elucidating the impact of the initial state symmetry in determining the angular distributions that are impacted by the correlation that drives the DPI process.

Yip, Frank L. (ORCID:000000025409495X)↗
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.↗