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DOE OSTI2025

Ionization of Free and Confined Atoms and Ions (Final Technical Report)

Over the almost 22-year duration of this theoretical research project, a great deal of progress has been made in the understanding of the ionization of free and confined atomic and molecular systems along with a recent particular emphasis upon studies of the ultrafast time-delay phenomenon in the photoemission process. Our methods have emphasized the role of many-body interactions and relativistic effects in the various processes we have studied. The advances that we have made over the course of this project have been in the areas nondipole effects in the photoionization process, inner-shell photoemission from negative atomic ions, excited state photoionization of neutral atoms and positive atomic ions, Cooper minima in quadrupole photoionization, relativistic interaction in the photoionization of atoms and negative atomic ions, photoionization of the fullerene molecule C 60 along with the photoionization of atomic and molecular systems confined in the C 60 molecule, confinement resonances in endohedral atom photoemission, interchannel coupling effects in photoionization, Wigner time delay in free and confined atoms and ions, angular effects in Wigner time delay in atomic photoionization, photoionization of superheavy atoms and the influence of core relaxation on the photoionization of atomic systems, among others. In the course of this project, we have had extensive collaboration with experimentalists where we have explained the physics of numerous measurements and pointed the way to new laboratory investigations with our calculations. We have also collaborated with a number of theorists as well. Thus, owing to these many collaborations with scientists not funded by DOE, we have leveraged the DOE support rather considerably. To illustrate the depth and breadth of the theoretical investigations, listed below are the 138 refereed and invited publications that cite DOE support. These publications detail the many contributions made over the period of this project.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Attosecond delays in X-ray molecular ionization

The photoelectric effect is not truly instantaneous, but exhibits attosecond delays that can reveal complex molecular dynamics. Sub-femtosecond duration light pulses provide the requisite tools to resolve the dynamics of photoionization. Accordingly, the past decade has produced a large volume of work on photoionization delays following single photon absorption of an extreme ultraviolet (XUV) photon. However, the measurement of time-resolved core-level photoionization remained out of reach. The required x-ray photon energies needed for core-level photoionization were not available with attosecond tabletop sources. We have now measured the x-ray photoemission delay of core-level electrons, and here report unexpectedly large delays, ranging up to 700 attoseconds in NO near the oxygen K-shell threshold. These measurements exploit attosecond soft x-ray pulses from a free-electron laser (XFEL) to scan across the entire region near the K-shell threshold. Furthermore, we find the delay spectrum is richly modulated, suggesting several contributions including transient trapping of the photoelectron due to shape resonances, collisions with the Auger-Meitner electron that is emitted in the rapid non-radiative relaxation of the molecule, and multi-electron scattering effects. Finally, the results demonstrate how x-ray attosecond experiments, supported by comprehensive theoretical modelling, can unravel the complex correlated dynamics of core-level photoionization.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Overset-Grid Method with Smooth Orbital Partitioning for Molecular Scattering Calculations

To solve molecular photoionization and electron scattering problems, we use an overset-grid representation of electronic continuum functions, which has an extended central spherical grid that overlaps small spherical grids (subgrids) centered on each atom of a polyatomic molecule. Here, in this work, we present an improved algorithm that smoothly partitions the total wave function between the central grid and the atomic subgrids. The smooth partitioning allows one to use approximately one-fourth the number of partial waves on the central grid compared to our previous implementation with switching functions. The resulting numerical method for treating electron scattering and photoionization of polyatomic molecules combines the accuracy and flexibility of pure numerical grid representations with the rapid convergence of hybrid combinations of atom-centered basis-set expansions and grid methods. The overset-grid representation is implemented using the complex Kohn variational principle for scattering and photoionization amplitudes. The faster convergence with respect to the number of central grid partial waves is demonstrated and accuracy is verified by comparisons with the previous implementation and with far more computationally demanding single-center numerical expansions in electron-molecule scattering and photoionization calculations on the neon dimer (Ne 2 ) system, carbon tetrafluoride (CF 4 ) molecule, and the pyridine (C 5 H 5 N) molecule in the static-exchange approximation.

Molecules↗
DOE OSTI2026

Strong field ionization and dissociation dynamics of vinyl bromide (C 2 H 3 Br) initiated by few-cycle pulses

Photoelectron-photoion (PEPICO) and photoion-photoion (PIPICO) coincidence measurements, coupled with a few-cycle intense laser field, were employed to investigate strong field ionization/dissociation dynamics of vinyl bromide (C₂H₃Br). The angular streaking technique was used to map the recoil-frame angle-dependent ionization rates to identify ionizing orbitals. Dissociative single ionization was mainly attributed to ionizing the lower lying molecular orbitals, suggesting that direct ionization dominates with few-cycle pulses. Three dissociative double ionization channels, both prompt and delayed fragmentation, were identified. Theoretical analysis using time-dependent configuration interaction with complex absorbing potential (TDCI-CAP) and high-level electronic structure methods was performed to elucidate the underlying ionization and dissociation mechanisms.

Olowolafe, Temitayo A. [Wayne State University, De↗
DOE OSTI2025

Attosecond X-Ray Core-Level Chronoscopy of Aromatic Molecules

Attosecond photoemission or photoionization delays are a unique probe of the structure and the electronic dynamics of matter. However, the spectral congestion of valence photoelectron spectra sets fundamental limits to the complexity of systems that can be studied, and the delocalization of valence electron wave functions blurs the spatial origin of the photoelectron wave packet. Using attosecond x-ray pulses from LCLS, we demonstrate the key advantages of measuring core-level delays: The photoelectron spectra remain atomlike, the measurements become element specific, and the observed scattering dynamics originate from a pointlike source when multicenter interference effects are negligible. We exploit these unique features to reveal the effects of changing functional groups (C-H vs N) and symmetry on attosecond scattering dynamics by measuring and calculating the photoionization delays between N−1⁢𝑠 and C−1⁢𝑠 core shells of a series of aromatic azabenzene molecules. Remarkably, the delays increase with the number of nitrogen atoms in the molecule and reveal multiple resonances. We identify two previously unknown mechanisms regulating the associated attosecond dynamics, namely the enhanced confinement of the trapped wave function with the replacement of C-H groups by N atoms and the decrease of the coupling strength among the photoemitted partial waves with increasing symmetry. This study demonstrates the unique opportunities opened by measurements of core-level photoionization delays for unraveling attosecond electron dynamics in complex matter.

Ji, Jia-Bao [Eidgenoessische Technische Hochschule↗
DOE OSTI2025

On the minimum number of radiation field parameters to specify gas cooling and heating functions

Fast and accurate approximations of gas cooling and heating functions are needed for hydrodynamic galaxy simulations. We use machine learning to analyze atomic gas cooling and heating functions in the presence of a generalized incident local radiation field computed by Cloudy. We characterize the radiation field through binned radiation field intensities instead of the photoionization rates used in our previous work. We find a set of 6 energy bins whose intensities exhibit relatively low correlation. We use these bins as features to train machine learning models to predict Cloudy cooling and heating functions at fixed metallicity. We compare the relative SHapley Additive exPlanation (SHAP) value importance of the features. From the SHAP analysis, we identify a feature subset of 3 energy bins (0.5-1, 1-4, and 13-16Ry) with the largest importance and train additional models on this subset. We compare the mean squared errors and distribution of errors on both the entire training data table and a randomly selected 20% test set withheld from model training. The machine learning models trained with 3 and 6 bins, as well as 3 and 4 photoionization rates, have comparable accuracy everywhere, with errors ≳10 times smaller than for the interpolation table of Gnedin and Hollon (2012). We conclude that 3 energy bins (or 3 analogous photoionization rates: molecular hydrogen photodissociation, neutral hydrogen HI, and fully ionized carbon CVI) are sufficient to characterize the dependence of the gas cooling and heating functions on our assumed incident radiation field model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

On the minimum number of radiation field parameters to specify gas cooling and heating functions

Fast and accurate approximations of gas cooling and heating functions are needed for hydrodynamic galaxy simulations. We use machine learning to analyze atomic gas cooling and heating functions computed by Cloudy in the presence of a generalized incident local radiation field. We characterize the radiation field through binned radiation field intensities instead of the photoionization rates used in our previous work. We find a set of 6 energy bins whose intensities exhibit relatively low correlation. We use these bins as features to train machine learning models to predict Cloudy cooling and heating functions at fixed metallicity. We compare the relative SHapley Additive exPlanation (SHAP) value importance of the features. From the SHAP analysis, we identify a feature subset of 3 energy bins ($0.5-1, 1-4$, and $13-16 \, \mathrm{Ry}$) with the largest importance and train additional models on this subset. We compare the mean squared errors and distribution of errors on both the entire training data table and a randomly selected 20% test set withheld from model training. The machine learning models trained with 3 and 6 bins, as well as 3 and 4 photoionization rates, have comparable accuracy everywhere, with errors $\gtrsim 10$ times smaller than for the interpolation table of Gnedin and Hollon (2012). We conclude that 3 energy bins (or 3 analogous photoionization rates: molecular hydrogen photodissociation, neutral hydrogen HI, and fully ionized carbon CVI) are sufficient to characterize the dependence of the gas cooling and heating functions on our assumed incident radiation field model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

O 2 -Dependence of reactions of 1,2-dimethoxyethanyl and 1,2-dimethoxyethanylperoxy isomers

Reaction mechanisms of Ṙ and ROȮ radicals derived from low-temperature oxidation of 1,2-dimethoxyethane (CH 3 O(CH 2 ) 2 OCH 3 ) were investigated using speciation from multiplexed photoionization mass spectrometry (MPIMS) measurements via Cl-initiated oxidation, in conjunction with electronic structure calculations. The experiments were conducted at 5 bar, from 450 K – 650 K, and O 2 concentrations from 1 · 10 14 cm –3 – 6 · 10 18 cm –3 to probe the effects on competing reaction channels of 1,2-dimethoxyethanyl (Ṙ) and 1,2-dimethoxyethanylperoxy (ROȮ) isomers. Several species were detected with photoionization spectral fitting – ethene, formaldehyde, methyl vinyl ether, and 2-methoxyacetaldehyde – and, as determined by electronic structure calculations, may form via unimolecular decomposition of 1,2-dimethoxyethanyl or 1,2-dimethoxyethanylperoxy. O 2 -dependent yield ratios show that the formation pathways for all species undergo a competition between O 2 -addition and unimolecular decomposition. Here, adiabatic ionization energies were also calculated and utilized along with exact mass determinations to infer contributions for other species derived exclusively from first- and second-O 2 -addition, including 1,2-dimethoxyethene, cyclic ethers, and dicarbonyls.

1,2-dimethoxyethane↗
DOE OSTI2025

Do We Know How to Model Reionization?

I compare the power spectra of the radiation fields from two recent sets of fully-coupled simulations that model cosmic reionization: “Cosmic Reionization On Computers” (CROC) and “Thesan”. While both simulations have similar power spectra of the radiation sources, the power spectra of the photoionization rate are significantly different at the same values of cosmic time or the same values of the mean neutral hydrogen fraction. However, the power spectra of the photoionization rate can be matched at large scales for the two simulations when the matching snapshots are allowed to vary independently. I.e., on large scales, the clustering of the radiation field in two simulations evolves similarly, but the exact timing of this evolution is different in different simulations and is not parameterized by an easily interpretable physical quantity like the mean neutral fraction or the mean free path. On small scales, large differences are present and remain partially unexplained. Both CROC and Thesan use the Variable Eddington Tensor approximation for modeling radiative transfer, but adopt different closure relations (optically thin OTVET versus M1). The role of this key difference is tested by using smaller simulations with a new cosmological simulation code that implements both closure relations in a controlled environment (the same hydro, cooling, and gravity solvers and the star formation recipe). In these controlled tests, both the M1 closure and the OTVET ansatz follow the expected behavior from a simple analytical approximation, demonstrating that the differences in the 2-point function of the radiation field induced by the choice of the Eddington tensor are not dominant.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Attosecond time delays in the body frame for photodetachment from molecular anions

Attosecond time delays in molecular photoionization at low energies are dominated by the contribution of the Coulomb phase, which diverges as the energy of the photoelectron decreases towards zero. With calculations on core-electron ejection from CN – and C 2 – we demonstrate that in photodetachment of molecular anions the absence of the long-range Coulomb interaction in the final state can reveal the delays of narrow low-energy shape resonances that do not generally occur in photoionization. Delays of several femtoseconds can be associated with such resonances. Finally, the angular dependence of photodetachment time delays in the body frame is highly sensitive to short-range anisotropies in the electron-molecule interaction as well as to the initial state.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Gas-phase synthesis of anthracene and phenanthrene via radical-radical reaction induced ring expansions

Unraveling reaction mechanisms of aromatic and resonance-stabilized radicals is critical to understanding molecular mass growth processes to polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanoparticles in distinct astrophysical environments (molecular clouds, circumstellar envelopes) and combustion systems. Using photoelectron photoion coincidence spectroscopy (PEPICO), we explored the gas-phase reaction of the methyl radical (CH 3 • ) with the aromatic and resonance-stabilized fluorenyl radical (C 13 H 9 • ) under high-temperature conditions in a chemical microreactor. Anthracene and phenanthrene were detected isomer-selectively using photoionization efficiency (PIE) curves and mass-selected threshold photoelectron (ms-TPE) spectra. While phenanthrene is produced through a radical-radical recombination of the carbon-centered radicals, anthracene may plausibly be formed through an unconventional radical addition to a low spin-density fluorenyl carbon. These pathways result in five-membered ring expansion—a critical mechanism crucial to PAH mass growth converting bent PAHs into planar nanostructures.

Goettl, Shane J. [University of Hawai‘i at Mānoa, ↗
DOE OSTI2025

Damping Wing-Like Features in the Spectra of High Redshift Quasars: a Challenge for Fully-Coupled Simulations

Recently, several observational detections of damping-wing-like features at the edges of “dark gaps” in the spectra of distant quasars (the “Malloy-Lidz effect”) have been reported, rendering strong support for the existence of “neutral islands” in the universe at redshifts as low as z<5.5. We apply the procedure from one of these works, Zhu et al 2024, to the outputs of fully coupled cosmological simulations from two recent large projects, “Cosmic Reionization On Computers” (CROC) and “Thesan”. Synthetic spectra in both simulations have statistics of dark gaps similar to observations, but do not exhibit the damping wing features. Moreover, a toy model with neutral islands added “by hand” only reproduces the observational results when the fraction of neutral islands among all dark gaps approaches 90%. I.e., simulations and observations appear to produce two distinct “populations” of dark gaps. In addition, in the simulations, the neutral islands at z=5.9 should be short-lived and should not extend to z<5.5. A plausible explanation for this discrepancy is that both simulations underestimate the fluctuations in the photoionization rate and, hence, are missing a population of long-lived neutral islands, located in the large downward fluctuations of the photoionization rate.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

The XRISM/Resolve View of the Fe K Region of Cyg X-3

The X-ray binary system Cygnus X-3 (4U 2030+40, V1521 Cyg) is luminous but enigmatic owing to the high intervening absorption. High-resolution X-ray spectroscopy uniquely probes the dynamics of the photoionized gas in the system. In this Letter, we report on an observation of Cyg X-3 with the XRISM/Resolve spectrometer, which provides unprecedented spectral resolution and sensitivity in the 2–10 keV band. We detect multiple kinematic and ionization components in absorption and emission whose superposition leads to complex line profiles, including strong P Cygni profiles on resonance lines. The prominent Fe xxv Heα and Fe xxvi Lyα emission complexes are clearly resolved into their characteristic fine-structure transitions. Self-consistent photoionization modeling allows us to disentangle the absorption and emission components and measure the Doppler velocity of these components as a function of binary orbital phase. We find a significantly higher velocity amplitude for the emission lines than for the absorption lines. The absorption lines generally appear blueshifted by ∼−500–600 km s −1 . We show that the wind decomposes naturally into a relatively smooth and large-scale component, perhaps associated with the background wind itself, plus a turbulent, denser structure located close to the compact object in its orbit.

Audard, Marc (ORCID:000000034721034X)↗
DOE OSTI2024

Air quality impacts from the development of unconventional oil and gas well pads: Air toxics and other volatile organic compounds

Unconventional oil and natural gas development (UOGD) has expanded rapidly across the United States in recent decades and raised concerns about associated air quality impacts. While significant effort has been made to quantify methane emissions, relatively few observations have been made of Volatile Organic Compounds (VOCs), especially during drilling and completion of new wells. Extensive air monitoring during development of several large, multi-well pads in Broomfield, Colorado, in the Denver-Julesburg Basin, provides a novel opportunity to examine changes in local air toxics and other VOC concentrations during well drilling and completions and production. These operations offer an especially useful case to study as several management practices were implemented to reduce emissions (e.g., electrified, grid-powered drill rigs and closed loop fluid handling systems to reduce truck traffic and limit fluid handling on the pad). With simultaneous measurements of methane and 50 VOCs from October 2018 to December 2022 at as many as 19 sites near well pads, in adjacent neighborhoods, and at a more distant reference location, we identify impacts from each phase of well development and production. Use of weekly, time-integrated canisters, a Proton Transfer Reaction Mass Spectrometer (PTR-MS), continuous photoionization detectors (PID) to trigger canister collection upon detection of VOC-rich plumes, and an instrumented vehicle, provided a powerful suite of measurements to characterize both transient plumes and longer-term changes in air quality. Prior to the start of well development, VOC gradients were small across Broomfield. Once drilling commenced, concentrations of oil and gas (O&G) related VOCs, including alkanes and aromatics, increased around active well pads. Concentration increases were clearly apparent during certain operations, including drilling, coil tubing/millout operations, and production tubing installation. Emissions of C 8 –C 10 n-alkanes during drilling operations highlighted the importance of VOC emissions from synthetic drilling mud chosen to reduce odor impacts. More than 90 samples were collected of transient plumes. Using composition measurements, meteorological data, and information about well pad activities, these plumes were connected with specific UOGD operations including drilling, flowback, and production equipment maintenance. The chemical signatures of these plumes differed by operation type (e.g., C 8 –C 10 n-alkanes constituted a larger fraction of measured VOCs in drilling-related plumes). Concentrations of individual, oil and gas-related VOCs in these plumes were often several orders of magnitude higher than in background air, with maximum ethane and benzene concentrations of 79,600 and 819 ppbv, respectively. Because these plumes typically impact a monitoring site for just several minutes, they are easily missed by slower-responding instruments. Study measurements highlight future emission mitigation opportunities during UOGD operations, including better control of emissions from shakers that separate drill cuttings from drilling mud, production separator maintenance operations, and periodic emptying of sand cans during flowback operations.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Investigation of warm dense matter by combining static and dynamic compression

Shock-compressing material samples that are precompressed with diamond-anvil cells (DACs) enables benchmarking equation of state measurements over a wide range of high-pressure conditions, in particular, warm dense matter relevant to planetary science, astrophysics, and inertial confinement fusion. These targets provide optical access to the shocked sample, so optical velocity interferometry and pyrometry are typical diagnostics. We present a scaling for the coupling of the laser drive to the shock pressure in DACs based on over 150 prior experiments that can be used to design future experiments. For initial static pressures ≤ 5 GPa, using a sapphire window mitigates the effect of photoionization compared to diamond windows and allows the measurement of shock pressures up to 1 TPa with optical diagnostics.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Ionization dynamics of intense laser-produced argon plasmas revealed by NLTE modeling

The ionization dynamics and transient behavior of under-dense plasma irradiated by an intense laser are investigated. We report two significant effects in the ionization behavior: (1) a surprisingly large delay in ionization response and (2) a stepwise ionization process which involves collisional and laser-driven photoionization (LDP) processes. Ionization induced by intense lasers can exhibit delayed responses due to rapid changes in conditions, particularly when atomic transition processes occur more slowly than the relevant time scales. Furthermore, modeling reveals that the two-step ionization process—collisional excitation followed by LDP—plays an important role in this ionization delay, with collisional excitation acting as the bottleneck. Even low-energy photons (∼3.5 eV) can predominantly ionize plasmas, challenging the conventional belief that such energies are insufficient to overcome the binding energy of bound electrons. These findings underscore the necessity of including such processes into plasma simulations for various laser-plasma experiments.

Collisional excitation↗
DOE OSTI2025

2D Modeling of Plasma Streamer and Glow Phases at Ammonia-Air Flame Conditions

Streamer and glow plasma phases have been modeled at two thermochemical states of an ammonia-air flame: fresh reactants and burnt products. A new AMReX-based solver has been verified against benchmarks in the literature and has been used to perform these simulations. A Helmholtz-equation based photoionization model with parameters accounting for the presence of NH3 in air has been coupled with the solver to accurately model the streamer propagation phase. A detailed plasma kinetics mechanism has been compiled and used to predict the evolution of electrons, excited states, ions, and radicals during streamer propagation and glow formation. The propagation velocity of streamers was found increase by almost two-fold when the mixture was changed from the fresh reactants to the burnt products. Moreover, vibrational excitation was found to be limited to the streamer body, whereas ionization predominantly occurred at the streamer head, as is expected. Finally, the differences in the pathways of O and H radical production during the streamer propagation and glow phases have been briefly discussed.

ammonia-air flame↗
DOE OSTI2026

A lumped particle direct simulation Monte-Carlo method combined with the collisional-radiative model for simulations of non-equilibrium laser-induced plasma plumes

Collisional plasma plumes induced by laser irradiation of material targets exhibit large variations in local density as well as ionization and excitation states, making purely hydrodynamic or kinetic simulations inaccurate or infeasible. To address this challenge and capture non-equilibrium effects in laser-induced plasma plumes at arbitrary degrees of ionization, we develop a hybrid computational approach that combines the kinetic direct simulation Monte Carlo (DSMC) method with a collisional-radiative model (CRM). This ℓDSMC-CRM approach utilizes a lumped particle method to represent minor fractions of excited ions in particle-based simulations and a special coarse-graining technique for atomic spectra and photoionization rates, ensuring numerical convergence at reduced computational cost. The hybrid approach is applied to simulate spatially homogeneous relaxation as well as one- and two-dimensional expansions of plasma plumes induced by irradiation of a copper target by a nanosecond laser pulse in a vacuum or background gas. The comparison with an equilibrium model, where local Saha-Boltzmann equilibrium is enforced, shows that the non-equilibrium effects play a dominant role. The equilibrium model can fail to predict the flow structure and strongly underestimate the degree of absorption of laser radiation by the plume. The ℓDSMC-CRM approach is validated against experimental data demonstrating reasonable agreement with the experimental electron density and temperature, while the equilibrium model is found to dramatically underestimate electron density and temperature. The flexibility of the ℓDSMC-CRM approach allows for its seamless integration into existing DSMC frameworks, making it a valuable tool for high-fidelity plasma modeling in laser-material interactions, laser-based manufacturing, and beyond.

97 MATHEMATICS AND COMPUTING↗