Ionization Dynamics Research Articles

Relativistic Electrons from Vacuum Laser Acceleration Using Tightly Focused Radially Polarized Beams.

We generate a tabletop pulsed relativistic electron beam at 100Hz repetition rate from vacuum laser acceleration by tightly focusing a radially polarized beam into a low-density gas. We demonstrate that strong longitudinal electric fields at the focus can accelerate electrons up to 1.43MeV by using only 98GW of peak laser power. The electron energy is measured as a function of laser intensity and gas species, revealing a strong dependence on the atomic ionization dynamics. These experimental results are supported by numerical simulations of particle dynamics in a tightly focused configuration that take ionization into consideration. For the range of intensities considered, it is demonstrated that atoms with higher atomic numbers like krypton can favorably inject electrons at the peak of the laser field, resulting in higher energies and an efficient acceleration mechanism that reaches a significant fraction (≈14%) of the theoretical energy gain limit.

Hollow ion atomic structure and X-ray emission in dense hot plasmas

Hollow ion X-ray emission is of great interest in high-energy-density research, since negligible opacity allows studies from the interior of very dense objects. In this paper, ionization potential depressions of the isoelectronic sequences for single and double K-shell vacancies are obtained from a pure ab initio multiconfiguration Hartree–Fock simulation including exact exchange terms and finite temperature dense plasma effects. It is demonstrated that the simultaneous representation of these ab initio data in the form of a map of hollow ion X-ray transition energies enables identification of important steps in the matter evolution and ionization dynamics. Mapping along the isoelectronic sequence as a function of the pumping energy of a X-ray free electron laser also enables visualization of the impact of ionization potential depression on the pathways of hollow ion formation.

Electron momentum correlation along laser magnetic field direction as a probe of nondipole effect in strong-field nonsequential double ionization

The electric dipole approximation is commonly adopted in the theoretical investigation of light-atom/molecule interaction, wherein the magnetic component of the driving electromagnetic field is neglected. Our study highlights the significant role of the magnetic field effect in the recollision dynamics of nonsequential double ionization (NSDI) driven by a mid-infrared laser. Due to the magnetic component of the laser field, in the multiple-returning events, the tunneling electron with a large initial momentum along the laser magnetic field direction at some specific tunneling time is inefficient for NSDI. The corresponding footprint is revealed in the correlated electron momentum distribution along the magnetic field direction. Moreover, we show that this effect becomes more obvious with increasing laser wavelength, leading to a notable reduction in the NSDI yield. Our findings provide an alternative perspective for studying the recollision dynamics involving the magnetic field effect.

Nonsequential double ionization of Ar in two-color inhomogeneous laser fields

With the classical ensemble method, we investigate the correlated electron dynamics in nonsequential double ionization (NSDI) of Ar in parallel two-color inhomogeneous laser fields with different inhomogeneity parameter ϵ. Numerical results show that at low laser intensities, the probability of NSDI is higher for two-color laser fields with a higher inhomogeneity parameter ϵ. At high laser intensities, however, the probability of NSDI decreases as ϵ increases. It is also shown that under low laser intensities, the two-color inhomogeneous laser fields drives the NSDI predominantly dominated by the RII channel, and the ratio of the RII channel increases as ϵ increases. More interestingly, an arc-like structure is found in the correlated electron momentum distributions for the two-color inhomogeneous laser fields. The underlying dynamics are revealed by tracing back to classical trajectories.

Strong Field Ionization Dynamics Resolved by Two-color Elliptical Phase-of-phase Spectroscopy

Strong Field Ionization Dynamics Resolved by Two-color Elliptical Phase-of-phase Spectroscopy

The hybrid anti-symmetrized coupled channels method (haCC) for the tRecX code

We present a new implementation of the hybrid antisymmetrized Coupled Channels (haCC) method in the framework of the tRecX (Scrinzi, 2022 [6]). The method represents atomic and molecular multi-electron functions by combining CI functions, Gaussian molecular orbitals, and a numerical single-electron basis. It is suitable for describing high harmonic generation and the strong-field dynamics of ionization. Fully differential photoemission spectra are computed by the tSurff method. The theoretical background of haCC is outlined and key improvements compared to its original formulation are highlighted. We discuss control of over-completeness resulting from the joint use of the numerical basis and Gaussian molecular orbitals by pseudo-inverses based on the Woodbury formula. Further new features of this tRecX release are the iSurff method, new input features, and the AMOS gateway interface. The mapping of haCC into the tRecX framework for solving the time-dependent Schrödinger equation is shown. Use, performance, and accuracy of haCC are discussed on the examples of high-harmonic generation and strong-field photo-emission by short laser pulses impinging on the Helium atom and on the linear molecules N2 and CO. Program summaryProgram title: tRecX — time-dependent Recursive indeXing (tRecX=tSurff+irECS)CPC Library link to program files:https://doi.org/10.17632/m9g2jc82sw.1Developer's repository link:https://gitlab.physik.uni-muenchen.de/AG-Scrinzi/tRecXLicensing provisions: GNU General Public License 2Programming language: C++External libraries: Eigen, arpack, lapack, blas, boost, FFTW (optional)Journal Reference of previous version: A. Scrinzi, Comp. Phys. Comm., 270:108146, 2022.Does the new version supersede the previous version: YesReasons for the new version: Major new functionality: haCC — hybrid antisymmetrized coupled channels methodSummary of revisions: Main additions are haCC and iSurff. Code usage and compilation were improved.Nature of problem: tRecX is a general solver for time-dependent Schrödinger-like problems, with applications mostly in strong field and attosecond physics. There are no technical restrictions on the spatial dimension of the problem with up to 6 spatial dimensions realized in the strong-field double ionization of Helium. Gaussian-based quantum chemical multi-electron atomic and molecular structure can be combined with the numerical basis. A selection of coordinate systems is available and any Hamiltonian involving up to second derivatives and arbitrary up to three dimensional potentials can be defined on input by simple scripts.Solution method: The method of lines is used with spatial discretization by a flexible combination of one dimensional basis sets, DVR representations, discrete vectors, expansions into higher-dimensional eigenfunctions of user-defined operators, and Gaussian based molecular orbitals. Multi-electron Gaussian-based CI (configuration interaction) functions for neutrals and ions are combined with the numerical basis. Photo-emission spectra are calculated using the time-dependent surface flux method (tSurff) in combination with infinite range exterior complex scaling (irECS) for absorption. The code is object oriented and makes extensive use of tree-structures and recursive algorithms. Parallelization is by MPI. Code design and performance allow use in production as well as for graduate level training.

Criticality of Spray Solvent Choice on the Performance of Next Generation, Spray-Based Ambient Mass Spectrometric Ionization Sources: A Case Study Based on Synthetic Cannabinoid Forensic Evidence

Mass spectrometry (MS) is a highly selective and sensitive analytical tool with a myriad of applications, but such techniques are typically used in laboratory settings due to the handling and preparations that are necessary. The merging of two streams of robust research, portable MS systems and next-generation ambient ionization methods, now provides the ability to perform high-performance chemical screening in an on-site and on-demand manner, with natural applications in disciplines such as forensic science, where samples of interest are typically found in field environments (i.e., traffic stops, crime scenes, etc.). Correspondingly, investigations regarding the suitability and robustness of these methodologies when they are utilized for authentic forensic evidence processing are prudent. This work reports critical insights into the role that choice of spray solvent system plays regarding analytical performance of two spray-based ambient ionization sources, paper spray ionization (PSI) and filter cone spray ionization (FCSI), when employed for evidence types containing emerging synthetic cannabinoids. The systematic characterization studies reported herein show that the applied spray solvent can dramatically affect both spectral intensity and signal duration, and in some circumstances, yield deleterious false negative responses. Overall, acetonitrile-based systems are shown to strike a balance between analyte solubility concerns and spray ionization dynamics of the novel ion sources employed on portable MS systems.

Steering the energy sharing of electrons in nonsequential double ionization with orthogonally polarized two-color field

Abstract Using the semiclassical ensemble model, the dependence of relative amplitude for the recollision dynamics in nonsequential double ionization (NSDI) of neon atom driven by the orthogonally polarized two-color field (OTC) laser field is theoretically studied. And the dynamics in two typical collision pathways, recollision-impact-ionization (RII) and recollision-excitation with subsequent ionization (RESI), is systematically explored. Our results reveal that the V-shaped structure in the correlated momentum distribution is mainly caused by the RII mechanism when the relative amplitude of the OTC laser field is zero, and the first ionized electrons will quickly skim through the nucleus and share few energy with the second electron. As the relative amplitude increases, the V-shaped structure gradually disappears and electrons are concentrated on the diagonal in the electron correlation spectrum, indicating that the energy sharing after electrons collision is symmetric for OTC laser fields with large relative amplitudes. Our studies show that changing the relative amplitude of the OTC laser field can efficiently control the electron–electron collisions and energy exchange efficiency in the NSDI process.

Deciphering the conformational preference and ionization dynamics of tetrahydrofuran: Insights from advanced spectroscopic techniques.

Tetrahydrofuran (THF) has garnered significant attention due to its pivotal role in biological and chemical processes. The diverse array of conformations exhibited by THF profoundly impacts its reactivity and interactions with other molecules. Understanding these conformational preferences is crucial for comprehending its molecular behavior. In this study, we utilize infrared (IR) resonant vacuum ultraviolet photoionization/mass-analyzed threshold ionization (VUV-PI/MATI) mass spectroscopies to capture distinctive vibrational spectra of individual conformers, namely, "twisted" and "bent," within THF. Our conformer-specific vibrational spectra provide valuable insights into the relative populations of these two conformers. The analysis reveals that the twisted (C2) conformer is more stable than the bent (CS) conformer by 17 ± 15cm-1. By precisely tuning the VUV photon energy to coincide with vibrational excitation via IR absorption, we selectively ionize specific conformers, yielding two-photon IR + VUV-PI/MATI spectra corresponding to the twisted and bent conformers. This investigation conclusively affirms that both the twisted and bent conformers coexist in the neutral state, while only the twisted conformer exists in the cationic state. These findings not only bridge gaps in existing knowledge but also provide profound insights into the behavior of this pivotal molecule in the realms of biology and medicine.

Impact of Coulomb interactions on ionization rates of noble gases in multi-cycle laser fields

In this study, we delve into the ionization dynamics of noble gases exposed to an 800 nm sine-squared laser pulse, placing emphasis on the influence of the Coulomb interaction on the ionization rate. Calculations are conducted under two conditions: one with and the other without considering the Coulomb potential. The results reveal the significant role of the Coulomb interaction in the dynamics of ionization. Our findings foster a deeper understanding of ionization processes within intense laser fields and aid in enhancing our existing theoretical frameworks.

Simulation of laser-induced ionization in wide bandgap solid dielectrics with a particle-in-cell code.

Modeling the laser-plasma interaction within solids is crucial in controlling ultrafast laser processing of dielectrics, where the pulse propagation and plasma formation dynamics are highly intricate. This is especially important when dealing with nano-scale plasmas where specific phenomena of plasma physics, such as resonance absorption, can significantly impact the energy deposition process. In this article, we report on adapting of a Particle-In-Cell code, EPOCH, to model the laser-plasma interaction within solids. This is performed by implementing a background permittivity and by developing and validating adapted field ionization and impact ionization modules. They are based on the Keldysh ionization theory and enable the modeling of ionization processes within solids. The implementation of these modules was validated through comparisons with a hydrodynamic code and existing literature. We investigate the necessary number of super-particles per cell to model realistic ionization dynamics. Finally, we apply the code to explore the dynamics of plasma formation in the regime of of quantized structuring of transparent films. Our study elucidates how a stack of nano-plasma layers can be formed by the interference of a pulse with its reflection on the exit surface of a high refractive index material.

Electro-Nuclear Dynamics of Single and Double Ionization of H2 in Ultrafast Intense Laser Pulses.

We present an efficient method for modeling the single and double ionization dynamics of the H2 molecule in ultrashort, intense laser fields. This method is based on a semianalytical approach to calculate the time-dependent single and double molecular ionization rates and on a numerical approach to describe the vibrational motion that takes place in the intermediate molecular ion H2+. This model allows for the prediction of the single and double ionization probabilities of the H2 molecule to be made over a wide range of frequencies and laser intensities with limited computational time while providing a realistic estimate of the energy of the products of the dissociative ionization and of the Coulomb explosion of the H2 molecule. The effect of vibrational dynamics on ionization yields and proton kinetic energy release spectra is demonstrated and, in the case of the latter, is discussed in terms of basic strong-field molecular fragmentation mechanisms.

Nonsequential double ionization of Ar atoms in counter-rotating two-color elliptically polarized laser fields

Electron correlation behaviors and recollision dynamics in nonsequential double ionization (NSDI) of Ar atoms in counter-rotating two-color elliptically polarized (TCEP) laser fields are investigated by using the classical ensemble model. The combined electric field in counter-rotating TCEP laser pulses traces out a trefoil pattern, i.e. the waveform in a period shows three leaves in different directions, and each leaf is called a lobe. Unlike counter-rotating two-color circularly polarized laser field, the combined electric field has no spatial symmetry. The amplitudes of the three field lobes and the angles between them are different. Thus, the returning electron mainly returns to the parent ion from one direction, and the electron momentum distributions show strong asymmetry. Numerical results show that the NSDI yield gradually decreases as the ellipticity increases, and the correlated behavior of the correlated electron momentum along the long axis of the laser polarization plane gradually evolves from correlation behavior mainly located in the first quadrant and the third quadrant to anti-correlation behavior mainly located in the second quadrant and fourth quadrant. In order to further understand the correlation behaviors of electron pairs, different characteristic times in the NSDI processes are discussed, respectively. It is found that single ionization events and recollision events gradually decrease, but single ionization time and recollision time change slightly. This may be the main reason for the decrease in NSDI yield. And as the ellipticity increases, the traveling time and the recollision energy gradually decrease, while the delay time increases. Therefore, we can conclude that ellipticity may be responsible for the NSDI process. In addition, further analysis finds that the shape of the trajectory becomes more and more triangular as the ellipticity increases due to the counter-rotating TCEP laser fields of the specific dynamical symmetries of the total net electric field. And it is found that whether it is a “short trajectory” or a “long trajectory”, more populations move to the second quadrant and the fourth quadrant as the ellipticity increases. The results show that increasing the ellipticity will gradually change the two electrons from emitting in the same direction to emitting in the opposite direction. This well demonstrates that both ellipticity and travelling time are responsible for the formation of the electron momentum distribution at the recollision time, meaning that both of them affect the emitted directions of both electrons.

Research Advances in Ionization Dynamics of Molecules for Bi-Wavelength Circularly Polarized Laser Field

Research Advances in Ionization Dynamics of Molecules for Bi-Wavelength Circularly Polarized Laser Field

Extreme Ultraviolet Radiation Sources from Dense Plasmas

The concept of dense and hot plasmas can be used to build up powerful and brilliant radiation sources in the soft X-ray and extreme ultraviolet spectral range. Such sources are used for nanoscale imaging and structuring applications, such as EUV lithography in the semiconductor industry. An understanding of light-generating atomic processes and radiation transport within the plasma is mandatory for optimization. The basic principles and technical concepts using either a pulsed laser or a gas discharge for plasma generation are presented, and critical aspects in the ionization dynamics are outlined within the framework of a simplified atomic physics model.

The electrical asymmetry effect in electronegative CF4 capacitive RF plasmas operated in the striation mode

The Electrical Asymmetry Effect (EAE) provides control of the mean ion energy at the electrodes of multi-frequency capacitively coupled radio frequency plasmas (CCP) by tuning the DC self-bias via adjusting the relative phase(s) between the consecutive driving harmonics. Depending on the electron power absorption mode, this phase control affects the ion flux in different ways. While it provides separate control of the mean ion energy and flux in the α-mode, limitations were found in the γ- and Drift-Ambipolar modes. In this work, based on experiments as well as kinetic simulations, the EAE is investigated in the striation-mode, which is present in electronegative CCPs driven by low frequencies. The discharge is operated in CF4 and is driven by two consecutive harmonics (4/8 MHz). The simulation results are validated against measurements of the DC self-bias and the spatio-temporally resolved dynamics of energetic electrons. To include heavy particle induced secondary electron emission realistically, a new computationally assisted diagnostic is developed to determine the corresponding secondary electron emission coefficient from a comparison of the DC self-bias obtained experimentally and from the simulations. Based on the validated simulation results, the EAE is found to provide separate control of the mean ion energy and flux in the striation mode, while the axial charged particle density profiles and the number of striations change as a function of the relative phase. This is understood based on an analysis of the ionization dynamics.

Identifying photoelectron releasing order in strong-field dissociative ionization of H2.

Driven by intense laser fields, the outgoing photoelectrons in molecules possess a quiver motion, resulting in the rise of the effective ionization potential. The coupling of the field-dressed ionization potential with abundant molecular dynamics complicates the laser-molecule interactions. Here, we demonstrate an approach to resolve photoelectron releasing order in the dissociative and non-dissociative channels of multiphoton ionization driven by an orthogonally polarized two-color femtosecond laser pulse. The photoelectron kinetic energy releases and the regular nodes in the photoelectron angular distributions due to the participation of different continuum partial waves allow us to deduce the field-dressed ionization potential of various channels. It returns the ponderomotive energy experienced by the outgoing electron and reveals the corresponding photoionization instants within the laser pulse. Our results provide a route to explore the complex strong-field ionization dynamics of molecules using two-dimensional photoelectron momentum spectroscopy.

Pulse Cycle Dependent Nondipole Effects in Above-Threshold Ionization

In this study, we employ strong field approximation (SFA) to investigate the influence of the number of pulse cycles on above-threshold ionization within the framework of nondipole theory. The SFA enables the analysis of the ionization process under the dominance of the electric field, compared to other factors such as the binding potential of an atom. Nondipole effects, including higher-order multipole fields, can significantly impact ionization dynamics. However, the interaction between nondipole effects and pulse cycles remains unclear. Therefore, we investigate the pulse cycle dependence of ionization and examine peak shifts in Kr and Ar atoms. Our findings have implications for comprehensively understanding the effects of electromagnetic fields on electron behavior. The insights gained from this study provide valuable guidance for future research in strong field ionization.

Strong-field nonsequential double photoionization using virtual-detector theory with path summation

We present an ab initio study of the nonsequential strong-field ionization dynamics of a model two-electron atom with helium character. Single- and double-ionization events are characterized and displayed using detector signals extracted at different points in the two-electron two-dimensional space. The double photoelectron momentum distribution is calculated via coherent path-summation over virtual-particle trajectories. A comparison is made between the momentum distributions obtained with the virtual-detector method, the Schr\"odinger wave function, and the time-dependent surface flux method developed by Tao and Scrinzi [New J. Phys. 14, 013021 (2012)]. Insights into different ionization and electron recollision pathways are gained from detailed virtual-particle tracking and energy-time readouts. This study demonstrates the extension of virtual-detector theory to strong-field multielectron quantum dynamics and highlights the importance of the evolving quantum phase in quasiclassical electron propagation.

Machine-learning calibration of intense x-ray free-electron-laser pulses using Bayesian optimization

X-ray free-electron lasers (XFELs) have brought new ways to probe and manipulate atomic and molecular dynamics with unprecedented spatial and temporal resolutions. A quantitative comparison of experimental results with their simulated theoretical counterpart, however, generally requires a precise characterization of the spatial and temporal x-ray pulse profile, providing a nonuniform photon distribution. The determination of the pulse profile constitutes a major, yet inevitable, challenge. Here, we propose a calibration scheme for intense XFEL pulses utilizing a set of experimental charge-state distributions of light noble gas atoms at a series of pulse energies in combination with first-principles simulations of the underlying atomic x-ray multiphoton ionization dynamics. The calibration builds on Bayesian optimization, which is a powerful, machine-learning-based tool particularly well suited for computationally expensive numerical optimization. We demonstrate the presented scheme to calibrate the pulse duration as well as the spatial fluence distribution profile of XFEL pulses. Our proposed method can serve as a comprehensive tool for characterizing ultraintense and ultrafast x-ray pulses.