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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Pulse shaping in strong-field ionization: Theory and experiments

Intense ultrafast pulses cause dissociative ionization and shaping the pulses may allow control of both electronic and nuclear dynamics that determine ion yields. We report on a combined experimental and theoretical effort to determine how shaped laser pulses affect tunnel ionization, the process that precedes many strong-field phenomena. We carried out experiments on Ar, N 2 , H 2 O, and O 2 using a phase-step function of amplitude 3/4π that is scanned across the spectrum of the pulse. In addition, we changed the amount of chirp in the pulses. Semiclassical as well as fully quantum mechanical time-dependent Schrödinger equation calculations are found to be in excellent agreement with experimental results. We find that precise knowledge of the field parameters in the time and frequency domains is essential to afford reproducible results and quantitative theory and experiment comparisons.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Higher-order harmonic generation and strong field ionization with Bessel–Gauss beams in a thin jet geometry

A promising alternative to Gaussian beams for use in strong field science is Bessel–Gauss (BG or Bessel-like) laser beams, as they are easily produced with readily available optics and provide more flexibility of the spot size and working distances. Here we use BG beams produced with a lens-axicon optical system for higher-order harmonic generation (HHG) in a thin gas jet. The finite size of the interaction region allows for scans of the HHG yield along the propagation axis. Further, by measuring the ionization yield in unison with the extreme ultraviolet (XUV), we are able to distinguish regions of maximum ionization from regions of optimum XUV generation. This distinction is of great importance for BG fields, as the generation of BG beams with axicons often leads to oscillations of the on-axis intensity, which can be exploited for extended phase-matching conditions. We observed such oscillations in the ionization and XUV flux along the propagation axis for the first time. As is the case for Gaussian modes, the harmonic yield is not maximum at the point of highest ionization. Finally, despite Bessel beams having a hole in the center in the far field, the XUV beam is well collimated, making BG modes a great alternative when spatial filtering of the fundamental is desired.

Davino, Michael↗
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 OSTI2020

Angle-dependent strong-field ionization and fragmentation of carbon dioxide measured using rotational wave packets

In this work, we experimentally study the angle-dependent single ionization of carbon dioxide (CO 2 ) by linearly and circularly polarized pulses. The angle dependence of the ionization probability by linearly polarized pulses extracted from time-domain measurements on an impulsively excited rotational wave packet is compared with data obtained from a direct angle-scan measurement. The results from the measurement with linear and circular polarization are consistent with the adiabatic ionization approximation. Further, we extend the time-domain method to extract the dependence of the asymptotic momentum distribution of fragment ions on the orientation of the molecular axis, and apply it to investigate dissociative double ionization of CO 2 . We show that such measurements can directly test the validity of the axial recoil approximation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Isotope-selective strong-field ionization of semiheavy water

Semiheavy water (HOD) is one of the simplest molecules in which the bonds are labeled by isotope. We demonstrate that a pair of intense few-femtosecond infrared laser pulses can be used to selectively tunnel ionize along one of the two bonds. The first pulse doubly ionizes HOD, inducing rapid bond stretching and unbending. Femtoseconds later, the second pulse arrives and further ionization is selectively enhanced along the OH bond. These conclusions arise from 3D time-resolved measurements of H + , D + , and O + momenta following triple ionization. Published by the American Physical Society 2025

Howard, A. J. (ORCID:0000000303033222)↗
DOE OSTI2020

Coherent electronic-vibrational dynamics in deuterium bromide probed via attosecond transient-absorption spectroscopy

Ultrafast laser excitation can trigger multiplex coherent dynamics in molecules. Here, we report attosecond transient-absorption experiments addressing simultaneous probing of electronic and vibrational dynamics in a prototype molecule, deuterium bromide (DBr), following its strong-field ionization. Electronic and vibrational coherences in the ionic X$^2\Pi_{3/2}$ and X$^2\Pi_{1/2}$ states are characterized in the Br-$3d$ core-level absorption spectra via quantum beats with 12.6-fs and 19.9-fs periodicities, respectively. Polarization scans reveal that the phase of the electronic quantum beats depends on the probe direction, experimentally showing that the coherent electronic motion corresponds to the oscillation of the hole density along the ionization-field direction. The vibrational quantum beats are found to maintain a relatively constant amplitude, whereas the electronic quantum beats exhibit a partial decrease in time. Quantum wave-packet simulations show that the decoherence effect from the vibrational motion is insignificant because of the parallel relation between the X$^2\Pi_{3/2}$ and X$^2\Pi_{1/2}$ potentials. Finally, a comparison between the DBr and HBr results suggests that rotation motion is responsible for the decoherence since it leads to initial alignment prepared by the strong-field ionization.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Ionization of HCCI Neutral and Cations by Strong Laser Fields Simulated With Time Dependent Configuration Interaction

Strong field ionization of neutral iodoacetylene (HCCI) can produce a coherent superposition of the X and A cations. This superposition results in charge migration between the CC π orbital and the iodine π-type lone pair which can be monitored by strong field ionization with short, intense probe pulses. Strong field ionization of the X and A states of HCCI cation was simulated with time-dependent configuration interaction using singly ionized configurations and singly excited, singly ionized configurations (TD-CISD-IP) and an absorbing boundary. Studies with static fields were used to obtain the 3-dimensional angular dependence of instantaneous ionization rates by strong fields and the orbitals involved in producing the cations and dications. The frequency of charge oscillation is determined by the energy separation of the X and A states; this separation can change depending on the direction and strength of the field. Furthermore, fields along the molecular axis can cause extensive mixing between the field-free X and A configurations. For coherent superpositions of the X and A states, the charge oscillations are characterized by two frequencies–the driving frequency of the laser field of the probe pulse and the intrinsic frequency due to the energy separation between the X and A states. For linear and circularly polarized pulses, the ionization rates show marked differences that depend on the polarization direction of the pulse, the carrier envelope phase and initial phase of the superposition. Varying the initial phase of the superposition at the beginning of the probe pulse is analogous to changing the delay between the pump and probe pulses. The charge oscillation in the coherent superposition of the X and A states results in maxima and minima in the ionization yield as a function of the superposition phase

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Femtosecond pulse parameter estimation from photoelectron momenta using machine learning

Abstract Deep learning models have provided huge interpretation power for image-like data. Specifically, convolutional neural networks (CNNs) have demonstrated incredible acuity for tasks such as feature extraction or parameter estimation. Here we test CNNs on strong-field ionization photoelectron spectra, training on theoretical data sets to ‘invert’ experimental data. Pulse characterization is used as a ‘testing ground’, specifically we retrieve the laser intensity, where ‘traditional’ measurements typically lead to 20% uncertainty. We report on crucial data augmentation techniques required to successfully train on theoretical data and return consistent results from experiments, including accounting for detector saturation. The same procedure can be repeated to apply CNNs in a range of scenarios for strong-field ionization. Using a predictive uncertainty estimation, reliable laser intensity uncertainties of a few percent can be extracted, which are consistently lower than those given by traditional techniques. Using interpretability methods can reveal parts of the distribution that are most sensitive to laser intensity, which can be directly associated with holographic interferences. The CNNs employed provide an accurate and convenient ways to extract parameters, and represent a novel interpretational tool for strong-field ionization spectra.

Szołdra, Tomasz (ORCID:0000000228970506)↗
DOE OSTI2025

Light control of intramolecular nuclear dynamics by vortex electron localization

In strong-field ionization of molecules, intense light pulses are thought to have a negligible direct influence on atomic nuclei. Molecular dissociation is thus expected to be determined by the geometrical configuration of the molecular ion at the ionization instant. Contrary to this picture, we observe a counterintuitive electron-proton angular correlation and the formation of proton vortices following strong-field ionization of H 2 molecules by bicircularly polarized two-color laser fields. We explain this phenomenon by the pathway interference and localization of the residual H 2 ⁡+ electron in different angular-momentum states formed in the tail of the driving laser pulse. We validate this interpretation by combining a quantum-mechanical numerical simulation of the field-driven coupled electronic-nuclear dynamics and a semiclassical-trajectory model for the phase accumulation of the laser-driven electronic-nuclear wave packet. Our joint experimental and theoretical study reveals a general picture of vortex electron localization which can be used for controlling molecular-bond breaking with circularly polarized laser fields.

Atomic & molecular processes in external fields↗
DOE OSTI2022

Femtosecond intramolecular rearrangement of the CH 3 NCS radical cation

Strong-field ionization, involving tunnel ionization and electron rescattering, enables femtosecond time-resolved dynamics measurements of chemical reactions involving radical cations. Here, we compare the formation of CH 3 S + following the strong-field ionization of the isomers CH 3 SCN and CH 3 NCS. The former involves the release of neutral CN, while the latter involves an intramolecular rearrangement. Here, we find the intramolecular rearrangement takes place on a single picosecond timescale and exhibits vibrational coherence. Density functional theory and coupled-cluster calculations on the neutral and singly ionized species help us determine the driving force responsible for intramolecular rearrangement in CH 3 NCS. Our findings illustrate the complexity that accompanies radical cation chemistry following electron ionization and demonstrate a useful tool for understanding cation dynamics after ionization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Size-dependent femtosecond proton transfer in protonated methanol clusters

Methanol clusters (CH 3 OH) n are prototypical hydrogen-bonded systems that bridge isolated molecules and bulk liquid methanol. Strong-field ionization of these clusters can trigger ultrafast charge migration, proton transfer, and isomerization; however, the onset timescales of these pathways remain largely unexplored despite extensive steady-state mass-spectrometric evidence for protonation and chemical rearrangement. Here we use femtosecond time-resolved strong-field ionization and disruptive probing to directly track the early-time dynamics of methanol clusters. Transient mass spectra yield size-dependent timescales for the formation of protonated clusters H + (CH 3 OH) n (n = 1–3) and an associated CHO˙ + radical cation. We find that both proton-transfer and subsequent stabilization become markedly faster with increasing cluster size. These measurements establish methanol clusters as a complementary benchmark to water for understanding ionization-initiated proton motion and chemical reorganization in hydrogen-bonded liquids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Nonadiabatic Strong-Field Photoionization Revisited

We measure strong field ionization of cesium atoms, observing a robust feature near 2⁢𝑈p in the photoelectron spectrum, which we call the intermediate energy structure (IES). Using a Coulomb-corrected strong-field approximation, we show it arises from electrons born with large inward velocities that rapidly undergo forward scattering off the Coulomb potential. The IES is similar to the previously identified low energy structure in that they are both due to forward scattering. However, it is different in that IES requires both nonadiabatic ionization conditions (as defined by the Keldysh parameter) and a more weakly bound initial state. Furthermore, our joint experimental and theoretical study supports the need for an additional parameter in characterization of strong field ionization, which depends both on nonadiabaticity of the process and the principal quantum number of the target. Together with Reiss and Keldysh parameters, this dimensionless parameter expands the description of strong field ionization across a broad range of atomic targets and incorporates ionization from excited states.

74 ATOMIC AND MOLECULAR PHYSICS↗