arXiv subjects

A. L. Harris

Publications and source records attributed to A. L. Harris.

16 recordsLinked to original sources

arXiv2026

Spectral Phase Pulse Shaping Alters Photoionization Time

Photoionization is a key step in many attosecond processes. Accurately determining the photoionization time delay is critical to understanding electron dynamics during and after ionization and can guide future efforts to manipulate electron motion. Prior studies have shown that the photoionization time delay is non-zero and that pulse shaping through alteration of the spectral phase may change the number and timing of ionization events. In order to more quantitatively assess whether and how the spectral phase modifies the photoionization time delay, we use attosecond streaking simulations to extract the streaking time delay for ionizing pulses with identical power spectra, but different spectral phases. We compare streaking time delays for Gaussian, Airy, and fifth order phase extreme ultraviolet (XUV) pulses. We find that the streaking delay depends on the XUV spectral phase and that the sign of the delay is determined by the sign of the phase for large phases. Pulses with non-zero spectral phase show an asymmetry in the streaking spectrogram that is associated with phase-dependent spectral compression or broadening. Comparison of the streaking delays for short- and long-range potentials indicates that Coulomb-laser coupling contributions to the streaking shift are independent of spectral phase, confirming that the observed phase dependence arises from intrinsic photoionization dynamics. Overall, our work suggests that the spectral phase may open the door to new opportunities for controlling ionization timing and provide new avenues for coherent control of ultrafast electron

physics.atom-ph↗

arXiv2025

Twisted Electron Collisions Enhance the Production of Circular Rydberg States

Circular Rydberg states offer advantages for quantum information and quantum simulation platforms due to their long lifetimes and strong dipole-dipole interactions. Unfortunately, current techniques for the production of these states remain technically challenging. Here we investigate the ability of twisted electron collisions to produce circular Rydberg states. Twisted electrons carry quantized orbital angular momentum that can be transferred to the electronic state of the atom, potentially providing an efficient means to generate circular Rydberg states. Using a fully quantum mechanical framework, we compute total excitation cross sections for circular Rydberg states of hydrogen, rubidium, and cesium targets using Bessel electron beams. Our models account for the full Bessel-beam structure of the incident electron and incorporate macroscopic target effects to model experimentally-relevant conditions. Our results show that twisted electrons with large opening angles produce significant enhancements in the excitation probability relative to plane-wave electrons, particularly for large opening angles and low energies. We trace this enhancement to contributions from projectiles with large values of orbital angular momentum. These findings demonstrate that twisted-electron excitation may provide a feasible and potentially advantageous pathway for generating circular Rydberg states.

physics.atom-ph↗

arXiv2024

A distorted-wave approach to the elastic scattering of twisted electrons

The elastic scattering of spinless vortex electrons on realistic target atoms has been investigated. In particular, expressions are derived in different approximations for the elastic angular-differential cross sections. We develop a distorted wave formalism that includes the effect of the atomic potential on the impinging vortex electron and compare this to a plane-wave Born approximation without such a distortion. Detailed computations have been performed for elastic scattering of vortex electrons on helium, neon, and argon targets by varying the energy, topological charge, and opening angle. Our results show that the overall magnitude of the cross section increases when the distortion by the bound-state electrons is taken into account. We also show that under certain conditions, such as high-Z targets or projectiles with low values of topological charge, significant differences in cross section shape and magnitude are observed between the distorted-wave and plane-wave Born models. Thus, the plane-wave Born approximation must be used with caution when describing vortex electron collisions.

physics.atom-ph↗

arXiv2023

Controlling Atom-Surface Scattering with Laser Assisted Quantum Reflection

In low energy atom-surface scattering, it is possible for the atom to be reflected in a region of attractive potential with no classical turning point. This phenomenon has come to be known as quantum reflection and it can reduce the sticking probability of atoms to surfaces, as well be used for atom trapping. We simulate the quantum reflection process in a one-dimensional model with a slow-moving atom moving in a Morse potential in the presence of an applied laser field. We show that in the case of laser-assisted quantum reflection, the laser field imparts additional momentum and kinetic energy to the atom. This results in a decreased distance of closest approach between the atom and surface. Our results show that the distance of closest approach and can be controlled through the timing and intensity of the laser pulse, which may result in enhanced sticking probability and/or reduced quantum reflection probability.

physics.atom-ph↗

arXiv2023

A Data-Driven Machine Learning Approach for Electron-Molecule Ionization Cross Sections

Despite their importance in a wide variety of applications, the estimation of ionization cross sections for large molecules continues to present challenges for both experiment and theory. Machine learning algorithms have been shown to be an effective mechanism for estimating cross section data for atomic targets and a select number of molecular targets. We present an efficient machine learning model for predicting ionization cross sections for a broad array of molecular targets. Our model is a 3-layer neural network that is trained using published experimental datasets. There is minimal input to the network, making it widely applicable. We show that with training on as few as 10 molecular datasets, the network is able to predict the experimental cross sections of additional molecules with an accuracy similar to experimental uncertainties in existing data. As the number of training molecular datasets increased, the network's predictions became more accurate and, in the worst case, were within 30% of accepted experimental values. In many cases, predictions were within 10% of accepted values. Using a network trained on datasets for 25 different molecules, we present predictions for an additional 27 molecules, including alkanes, alkenes, molecules with ring structures, and DNA nucleotide bases.

physics.atom-ph↗

arXiv2023

Controlling electron projectile coherence effects using twisted electrons

In traditional scattering theory, the incident projectile is assumed to have an infinite coherence length. However, over the last decade, experimental and theoretical studies of collisions using heavy ion projectiles have shown that this assumption is not always valid. This has led to a growing number of studies that specifically examined the effects of the projectile's coherence length on collision cross sections. These studies have used heavy ion projectiles because they offer a straight-forward method to control the projectile's coherence length through its momentum, and using these techniques, it has been demonstrated that the projectile's coherence length alters the cross sections. In contrast, it is widely presumed that the coherence length of an electron projectile is always sufficiently large that any effects on the cross sections can be safely neglected. We show that, contrary to this prevailing opinion, coherence effects are observable for electron projectiles and they can be controlled. We calculate triple differential cross sections (TDCSs) for ionization of H2+ using twisted electron projectiles in the form of Laguerre-Gauss and Bessel electrons. Effects of the projectile's coherence length are observed through the presence or absence of two-slit interference features in the TDCSs. When the electron projectile's coherence length is large, ionization occurs from either nuclear center of the molecule, and two-slit interference features are visible in the TDCSs. In contrast, when the projectile's coherence length is small, ionization occurs from only one nuclear center and the TDCSs resemble those for ionization of atomic hydrogen. We demonstrate that the intrinsic parameters of the vortex projectiles, such as beam waist and opening angle, can be used to control the coherence length of electron projectiles.

physics.atom-ph↗

arXiv2023

Spectral Phase Effects in High-Order Above Threshold Ionization of Noble Gas Atoms

We present theoretical studies of above threshold ionization (ATI) using sculpted laser pulses for noble gas atoms. The time-dependent Schroedinger equation is solved to calculate the ATI energy and momentum spectra, and a qualitative understanding of the electron motion after ionization is explored using a classical model that solves Newton's equation of motion. Results are presented for Gaussian and Airy laser pulses with identical power spectra, but differing spectral phases. The simulations show that the third order spectral phase of the Airy pulse, which can alter the temporal envelope of the electric field, causes changes to the timing of ionization and the dynamics of the rescattering process. Specifically, the use of Airy pulses in the ATI process results in a shift of the Keldysh plateau cutoff to lower energy due to a decreased pondermotive energy of the electron in the laser field. Additionally, the side lobes of the Airy laser pulse change the number and timing of rescattering events, which results in changes to the high-order ATI plateau. Our results also show that laser pulses with identical carrier envelope phases and nearly identical envelopes yield different photoelectron momentum densities, which are a direct result of the pulse's spectral phase.

physics.atom-ph↗

arXiv2021

Electron Spectra for Twisted Electron Collisions

Ionization collisions have important consequences in many physical phenomena, and the mechanism that leads to ionization is not universal. Double differential cross sections (DDCSs) are often used to identify ionization mechanisms because they exhibit features that distinguish close collisions from grazing collisions. In the angular DDCS, a sharp peak indicates ionization through a close binary collision, while a broad angular distribution points to a grazing collision. In the DDCS energy spectrum, electrons ejected through a binary encounter collision result in peak at an energy predicted from momentum conservation. These insights into ionization processes are well-established for plane wave projectiles. However, the recent development of sculpted particle wave packets reopens the question of how ionization occurs for these new particle wave forms. We present theoretical DDCSs for (e,2e) ionization of atomic hydrogen for electron vortex projectiles. Our results predict that the ionization mechanism for vortex projectiles is similar to that of non-vortex projectiles, but that features in the DDCS that distinguish ionization mechanisms are obscured by the projectile's momentum uncertainty. Additionally, the projectile's non-zero transverse momentum increases the cross section for high energy ejected electrons.

physics.atom-ph↗

arXiv2021

Single and Double Scattering Mechanisms in Ionization of Helium by Electron Vortex Projectiles

Triple differential cross sections (TDCSs) for electron vortex projectile ionization of helium into the azimuthal plane are calculated using the distorted wave Born approximation. In this collision geometry, the TDCSs at low and intermediate energies exhibit unique qualitative features that can be used to identify single and double scattering mechanisms. In general, our results predict that the ionization dynamics for vortex projectiles are similar to those of their non-vortex counterparts. However, some key differences are observed. For non-vortex projectiles, a double scattering mechanism is required to emit electrons into the azimuthal plane, and this mechanism becomes more important with increasing energy. Our results demonstrate that for vortex projectiles, emission into the azimuthal plane does not require a double scattering mechanism, although this process still significantly influences the shape of the TDCS at higher energies. At low projectile energies, non-vortex ionization proceeds primarily through single binary collisions. The same is generally true for vortex projectiles, although our results indicate that double scattering is also important, even at low energy. Vortex projectiles have an inherent uncertainty in their incident momentum, which causes a broadening of the binary peak at all energies and results in a splitting of the binary peak at higher energies. The results presented here lead to several predictions that can be experimentally tested.

physics.atom-ph↗

arXiv2020

Control of Arrival Time using Structured Wave Packets

Scattering dynamics are examined for Gaussian and non-Gaussian wave packets with identical momentum densities. Average arrival time delays, dwell times, and phase time delays are calculated for wave packets scattering from a square barrier, and it is shown that the non-Gaussian wave packets exhibit different average arrival time delays than the Gaussian wave packets. These differences result from the non-linear terms in the momentum wave function phase of the non-Gaussian wave packets, which alters the self-interaction times of the wave packets. Control of the average arrival time delay can be achieved through adjustment of the momentum wave function phase, independent of wave packet energy and momentum density.

physics.atom-ph↗

arXiv2020

Projectile Transverse Momentum Controls Emission in Electron Vortex Ionization Collisions

The realization of electron vortex beams in the past decade has led to numerous proposed applications in fields from electron microscopy to control and manipulation of individual molecules. Yet despite the many unique characteristics and promising advantages of electron vortex beams, such as transverse momentum and quantized orbital angular momentum, there remains a limited understanding of their fundamental interactions with matter at the atomic scale. Collisions between electron vortex projectiles and atomic targets can provide some insight into these interactions and we present here fully differential cross sections for ionization of excited state atomic hydrogen targets using electron vortex projectiles. We show that the projectile's transverse momentum causes the ionized electron angular distributions to be altered compared to non-vortex projectiles and that the ionized electron's ejection angle can be controlled by adjustment of the vortex opening angle, a feature unique to vortex projectiles. Additionally, an inherent uncertainty in the projectile's momentum transfer leads to a broadening of the classical binary peak, making signatures of the target electron density more readily observable. Fully differential cross sections for aligned 2p targets exhibit structures that can be used to determine the alignment.

physics.atom-ph↗

arXiv2019

Recovery Time of Matter Airy Beams using the Path Integral Quantum Trajectory Model

The Path Integral Quantum Trajectory (PIQTr) model is introduced as a new computational tool for the study of non-relativistic, quantum mechanical wave packets. We introduce the numerical algorithm and show that one of the primary advantages of the PIQTr model is its ability to efficiently handle heavy mass particles, while still resulting in a numerically exact answer. We then apply the model to the study of matter Airy beam particles and show that the recovery time of a damaged wave packet increases approximately with mass, but is independent of momentum, velocity, and wave packet width.

physics.comp-ph↗

arXiv2019

The Frozen Core Approximation and Nuclear Screening Effects in Single Electron Capture Collisions

Fully Differential Cross Sections (FDCS) for single electron capture from helium by heavy ion impact are calculated using a frozen core 3-Body model and an active electron 4-Body model within the first Born approximation. FDCS are presented for H+, He2+, Li3+, and C6+ projectiles with velocities of 100 keV/amu, 1 MeV/amu, and 10 MeV/amu. In general, the FDCS from the two models are found to differ by about one order of magnitude with the active electron 4-Body model showing better agreement with experiment. Comparison of the models reveals two possible sources of the magnitude difference: the inactive electron's change of state and the projectile-target Coulomb interaction used in the different models. Detailed analysis indicates that the uncaptured electron's change of state can safely be neglected in the frozen core approximation, but that care must be used in modeling the projectile-target interaction.

physics.atom-ph↗

arXiv2018

Quantum Mechanical Potentials and Inactive Electron Effects in Resonant Charge Exchange Collisions

Scattering angle differential cross sections for the He+ + He single electron capture process are studied using plane wave Born approximation models for projectile energies between 30 keV and 1.89 MeV. Within this simplistic framework, we study the effects of the frozen core approximation by performing a full 5-Body calculation that explicitly includes all particles in the collision and comparing it with a single bound state model that neglects the bound electron in the projectile and a double bound state model that neglects the inactive electron in the target atom. Results are compared with experiment and we show that inclusion of the inactive electron in the perturbation potential is more important than inclusion in the wave functions. We also introduce a semi-quantum mechanical perturbation potential that treats the atomic electrons as a quantum mechanical electron cloud rather than point particles. The semi-quantum mechanical perturbation removes the deep, unphysical minimum that exists in cross sections calculated with Born-type models, but also has the effect of greatly reducing the magnitude of the small scattering angle cross sections.

physics.atom-ph↗

arXiv2018

Ionization of Hydrogen by Electron Vortex Beam

Optical vortex beams have an extensive history in terms of both theory and experiment, but only recently have electron vortex beams been proposed and realized. The possible applications of these matter vortex waves are numerous, but a fundamental understanding of their interactions with atoms and molecules has not yet been developed. In this work, fully differential cross sections for fast (e,2e) collisions using electron vortex projectiles with small amounts of quantized orbital angular momentum are presented. A comparison is made with the fully differential cross sections using plane wave projectiles and a detailed study of angular momentum transfer is included. Results indicate that ionization by electron vortex beam projectiles is much less likely than for plane wave projectiles, and the angular momentum of the incident electron is transferred directly to the ionized electron.

physics.atom-ph↗

arXiv2013

Improved Numerical Method for Calculation of 4-Body Transition Amplitudes

In order to study 4-body atomic collisions such as excitation-ionization, transfer with target excitation, and double electron capture, the calculation of a nine-dimensional numerical integral is often required. This calculation can become computationally expensive, especially when calculating fully differential cross sections (FDCS), where the positions and momenta of all the particles are known. We have developed a new technique for calculating FDCS using fewer computing hours, but more memory. This new technique allows for much more efficient calculations and the use of many fewer resources.

physics.comp-ph↗