Theory Of Ionization Research Articles

Theory of strong-field sequential double ionization of polyatomic molecules

Theory of strong-field sequential double ionization of polyatomic molecules

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.

Modeling of the femtosecond pulsed laser-induced damage of multi-layer dielectric gratings

The laser damage resistance of multi-layer dielectric gratings (MDGs) and metal multi-layer dielectric gratings (MMDGs) for femtosecond lasers directly determines the performance and life span of the high-energy laser system. It remains a significant challenge to predict the laser-induced damage threshold (LIDT) of MDGs and MMDGs for femtosecond lasers. Based on the nonlinear ionization and electric field intensity model, an effective method is proposed to calculate the LIDT of MDGs and MMDGs by introducing the dual temperature model. We take into account the evolutions of thermophysical parameters including the absorption coefficient, heat capacity, and thermal conductivity of the dielectric material as a function of the free-electron density and temperature. The free electron density of the conduction band combined with the electric field strength inside the grating can be calculated by the laser-induced ionization theory, more importantly, the transient distribution of the lattice temperature field generated by femtosecond laser irradiated grating excitation is obtained by the two-temperature model (TTM). This work is beneficial for guiding the design and fabrication of MDGs and MMDGs with high-LIDTs for ultra-short ultra-intense laser systems.

Quantum-corrected semiclassical theory for strong-field ionization

Quantum-corrected semiclassical theory for strong-field ionization

Influence of nanoparticles’ polarization on the streamer branching and propagation in nanofluids

The mutual shielding effect between branches is thought to be one of the reasons for reduced propagation of streamer in nanofluids. However, the mechanism of how these nanoparticles induce streamer branching in oil is not clear. In this paper, a numerical model was established based on charge carrier continuity equations and field-dependent molecular ionization theory to study the propagation and branching of streamer. The effects of isolated particle with different size, permittivity, and location on streamer branching in oil are discussed in detail. It is found that isolated particle with larger size, or greater permittivity, or closer position from streamer would cause strong enough distortion of electric field around the particle. This changes the ionization distribution during the propagation of streamer to induce a side-branch. To simulate the state of “global uniformity and local clumping” of nanoparticles in nanofluids, a multi-nanoparticle system is established. The local clumped nanoparticles play a synergistic effect on electric field distortion. The reduced electric field, the increased propagating path, and the mutual shielding effect of streamer branches are responsible for the improved breakdown voltage of nanofluids.

Imaging surfaces at the space–time limit: New perspectives of time-resolved scanning tunneling microscopy for ultrafast surface science

Many fundamental processes in nature occur on ultrashort time scales within picoseconds to attoseconds, and on intrinsic length scales from nanometers to picometers. The structure of crystalline solids is dictated by long range order and the periodic arrangement of atoms, but the elementary excitations that define its interaction with the environment may vary locally at the atomic scale. Multiple domains and phases can coexist on length scales down to a few nanometer, and impurities and defects can influence the collective many-body response of solids at the single-atom level. Ultrafast pump–probe techniques provide valuable information about fundamental many-body interactions in solids and at surfaces, but spatially average over macroscopic spot sizes such that the influence of local order or disorder at angstrom scales is not directly accessible. Therefore, real-space observation of ultrafast dynamics with atomic spatial resolution is highly desirable, and motivates the development of time-resolved ultrafast scanning tunneling microscopy (USTM) since the early 1990’s. Tremendous progress has been made in this field in the past decade, and a number of breakthrough achievements have significantly advanced our possibilities to add ultrafast time resolution to the angstrom spatial resolution of STM. This article reviews new technical approaches and developments in the field of USTM. A particular focus will be the classification of light-matter interaction in tunnel junctions, based on the criteria for adiabatic tunneling from Keldysh's theory of strong-field ionization and a tunneling time as defined by Büttiker and Landauer, and on Tucker's definition of quantum detection in a tunnel junction mixer. Moreover, various mechanisms to generate an ultrafast tunneling current in USTM are discussed and are to some extent related to those from other techniques such as optical spectroscopy or photoemission spectroscopy. The resulting new possibilities for imaging the ultrafast dynamics of electronic and vibrational excitations at surfaces with USTM will be highlighted. Finally, the article outlines possible future directions of USTM for studying ultrafast processes and light-induced phenomena at surfaces and in quantum materials.

Theory of ionization in a crystal with a linear and a circular laser polarization: Influence of the laser wavelength

Theory of ionization in a crystal with a linear and a circular laser polarization: Influence of the laser wavelength

Adiabatic theory of generation and rescattering of vortex electrons in strong-field ionization by elliptically polarized pulses

The adiabatic theory of strong-field ionization is extended to the specific configuration in which ionization occurs from a vortex orbital in an axially symmetric target by an elliptically polarized pulse with small ellipticity and the major axis of the polarization ellipse directed along the vortex axis of the initial state. Generation of vortex electrons in the continuum, their propagation in the laser field, and subsequent rescattering on the parent ion are described. Adiabatic asymptotics of the rescattering parts of the solution to the time-dependent Schr\"odinger equation and the ionization amplitude are obtained. On the basis of these results, the factorization formula giving the photoelectron momentum distribution (PEMD) in the vicinity of a backward rescattering caustic is derived. Our interest in the present configuration stems from the fact that, because of the nonzero ellipticity of the field, liberated vortex electrons arrive for rescattering with a nonzero impact parameter. The corresponding scattering amplitude generalizes the recently introduced vortex scattering amplitude characterizing head-on vortex-target rescattering in the linear polarization case [O. I. Tolstikhin and T. Morishita, Phys. Rev. A 99, 063415 (2019)]. Using the factorization formula, one can extract the absolute value of this generalized scattering amplitude from the observable PEMD, which opens a new window for target structure imaging in strong-field physics. The theory is illustrated by calculations for two atomic targets, ${\mathrm{He}}^{+}(2p,m=1)$ and $\mathrm{Xe}(5p,m=1)$. We show that in the case of the initial $\ensuremath{\pi}$ orbital and nonzero ellipticity not only the absolute values of two head-on scattering amplitudes with $m=0$ and 1, but also their phase difference as functions of the scattering angle can be extracted from the PEMD.

EI-MS fragmentation routes of CWC-related organoarsenic compounds: The electron ionization and DFT studies.

For unambiguous identification of Chemical Weapons Convention-related chemicals in environmental and biological samples, access to mass spectra and expertise in spectral interpretation is required. As of today, there is no mass spectrum available for dialkyl alkylarsonodithioites in the literature. Therefore, it is essential to obtain spectral information about these compounds. The CWC-related organoarsenic compounds were synthesized and analyzed by GC-MS, and their retention index (RI) was calculated by Van Den Dool's method. The relationship between RI and DFT-calculated polarizability (⍺) and molecular weight was also studied. Electron ionization (EI) mode was used to investigate the general fragmentation patterns of the synthesized compounds. A DFT analysis of some major fragment ions was performed in order to confirm the fragmentation pathways. A linear relationship between RI and ⍺ is observed. The mass spectra of compounds 5-7 are affected by alkyl groups on sulfur and arsenic atom. The molecular ion of compounds 5-7 can be observed in their EI-MS with relatively good abundance. EI-MS studies revealed some interesting fragmentation pathways, such as the formation of arsenic analogue of benzilic and tropylium in EI-MS of chemicals 6. By analyzing the mass spectra and RI values, it is possible to detect CWC-related chemicals in aqueous, blood, and urine samples during an on-site or off-site inspection.

The influence of solar-modulated regional circulations and galactic cosmic rays on global cloud distribution

The influence of solar forcing and Galactic Cosmic Rays (GCR) ionization on the global distribution of clouds is investigated using 42 years ERA-5 data (1979–2020). In the mid-latitudes over Eurasia, GCR and cloudiness are negatively correlated, which argues against the ionization theory of enhanced cloud droplet nucleation due to increased GCR during minima in the solar cycle. In the tropics, the solar cycle and cloudiness are positively correlated in regional Walker circulations below 2 km altitude. The phase relationship between amplification of regional tropical circulations and the solar cycle is consistent with total solar forcing, rather than modulation of GCR. However, in the intertropical convergence zone, changes in the cloud distribution are consistent with a positive coupling with GCR in the free atmosphere (2–6 km). This study opens some future challenges and research directions, and clarifies how atmospheric circulation at the regional scale can help in understanding solar-induced climate variability.

Mechanical stability parameters of chalcogenides and pnictides based optoelectronic materials

A study of experimental data reveals that the bulk modulus of chalcogenides and pnictides based chalcopyrites (AIIBIVC2 V and AI BIIIC2 VI) can be explained by a simple scaling rule that rely only on the crystal ionicity, ionic charge product, and the melting temperature. PVV theory of crystal ionicity, temperature dependence of elasticity and product of ionic charge theory are taken into account for the study. Based on this result, a simple microhardnessbulk modulus relation is applied to evaluate the microhardness of the complex compounds; which correspond well with the experimental data and other published results. The proposed findings support in the modeling of emerging semiconductor materials and even understanding of their mechanical properties for optoelectronics, photovoltaic, electromagnetic (EM) screening, and spintronic applications. PACS: 62.20.-x; 62.20.Qp

Spectroscopic properties of Cr,Yb:YAG nanocrystals under intense NIR radiation

Laser induced white emission (LIWE) was thoroughly studied in recent decades. However, despite the progress in understanding of this phenomenon, the mechanism behind LIWE remains unclear. The present paper focuses on the influence of Yb content on the LIWE properties of Cr,Yb:YAG nanocrystals. Microstructure and optical properties of the samples were characterized and the influence of the concentration of Yb3+ ions on the spectroscopic properties of Cr,Yb:YAG and energy transfer processes between Cr3+ and Yb3+ ions was revealed. Multiphoton ionization theory was used to explain the findings of the paper.

Fast electron contribution in electronic stopping for p-He and p-Ar collisions

Our study has shown that in collisions p-He and p-Ar with energies above 50 keV fast electrons carry away as kinetic energy up to 70 % of energy released during particle stopping. Ignoring the fast electron contribution makes impossible bringing independently measured cross sections of elementary processes of ionization, excitation, stripping and charge exchange into compliance with measured cross sections of electronic stopping. Fast electron emission in collisions of energetic ions with solids may induce extra radiation effects (create additional vacancies and defects in solid).Mean kinetic energies of ejected electrons We obtained from the analysis of cross section ratio agree well with directly measured energy spectra of the emitted electrons. The dynamic ionization theory implies the We proportionality to initial velocity v. In the considered cases, such a dependence takes place at the velocities below 2 a.u. At higher velocities, the observed dependence is successfully described in the framework of the Born approximation.

Determination of Leaf pH without Grinding the Sample: Is It Closer to the Reality?

This study recommends a non-grinding measurement method of leaf pH which can reduce the destructive interference to the measured fresh-leaf pH values. To verify the accuracy of this method, we measured leaf pH with the non-grinding and grinding method and further assessed the dilution effect on leaf pH in the grinding process. Compared with the non-grinding method, the grinding method significantly increased the measured pH value; leaf pH increased with decreasing leaf–water ratio in the procedure of the grinding method, but gradually stabilized. The grinding effects of severe physical damage and thereafter oxidization of leaf samples, and the dilution effects according to the Debye–Hückel limiting law and acid-base ionization theory, may both contribute to the increased leaf pH measured with the grinding method. Thus, leaf pH measured with the non-grinding method was expected to be much closer to those of leaf sap in vivo and be more suitable to indicating the dynamic variation or instant response of leaf pH to the environmental changes. Finally, considering that non-significant difference had been proved in the measured leaf pH between dried, frozen, refrigerated, and fresh ground samples, a conversion equation was provided to facilitate mutual conversion of the results with non-grinding fresh samples (y) against those with grinding dried samples (as representative) (x): y = 1.097x − 0.722.

Cohesion energy simulation of inorganic layered alkaline-earth fluorohalides

Within the framework of an interionic potential model, electrostatic and repulsive energies, lattice self-potentials for distinct ions, and the Madelung constant were calculated for several technologically important layered alkaline-earth fluorohalide scintillators belonging to the matlockite family and crystallizing with the PbFCl-type structure. The Coulombic term was computed using the conventional Ewald method, where the formulas were adjusted to optimize the computer calculation, and the overlap repulsive term was computed by means of an empirical approach based on the compressible ionic theory. The dispersive contribution was quantified using well-known formulations, and the polarization contribution was determined purely by geometric considerations based on both the size and anisotropic coordination of the highly polarizable halogen atom. In general, the obtained results were found to be in close agreement with the available data, except for the contribution of short-range interactions in the lattice. A quantitative explanation has been proposed to elucidate the differences detected. It was pointed out that the structural stability of these lamellar structures can be understood in terms of the anisotropic coordination of halogen anions, especially the high coordination of metal cations combined with their sizes. Our calculations led to an accurate evaluation of the cohesive energy, which, to the best of our knowledge, has never been measured before. Finally, the corresponding results will be useful for a better understanding of the chemical bonds and structural behavior of PbFCl-type compounds at high pressures.

Inner-shell ionisation of Xe[formula omitted]–Au and Pb collision systems: MO picture

The generation of collision-induced vacancies and their transfer within the electronic inner shells are the source of several unprecedented phenomena, which have been revealed in recent experimental studies conducted using state-of-the-art next-generation accelerators and detectors. Despite the multitude of studies conducted over the years, our understanding of the atomic vacancy transfer mechanisms remains exiguous. The detection and analysis of collision-induced X-rays is a widely-used powerful technique for analysing atomic-scale phenomena. However, both experimental and theoretical investigations on ion–atom collision-induced X-rays have remained mostly restricted to high-energy light-ion impact on heavy atoms. To date, only a few studies have been reported on the investigation of very-low-energy heavy-ion impact on heavy atoms using X-rays, especially M-shell X-rays of the heavy atom. This study was conducted to evaluate the inner-shell ionisation of Au (135 and 508 μg/cm2) and Pb (107, 157 and 390 μg/cm2) targets due to low-energy (2–5 MeV) 54Xeq+-ion impact. The collision-induced X-ray spectra of both the collision partners were examined, and the corresponding intensity ratios and cross-sections were compared with those calculated using two well-known ionisation theories, viz. PWBA and ECPSSR. The measured cross-sections were underestimated by these theories. Moreover, the cross-sections measured in this study were found to be several orders of magnitude higher than those obtained in other reported studies on low-energy lighter-ion-impact on Au and Pb. The significant discrepancies observed between the experimental and theoretical values have been explained qualitatively within the framework of quasi-molecular orbital formation using level correlations. The insights obtained from the results of this study can be applied to analyse the vacancy transfer mechanisms in very-heavy systems (combined atomic number > 100).

Numerical simulation of femtosecond laser ablation of quartz glass and silicon nitride.

The ablation threshold and depth models were established based on the electron density evolution equation of dielectric and ionization theory. The volume model was then deduced, and the models were validated. For quartz glass and silicon nitride, the dependences of the threshold with pulse duration, ablation depth, and volume with fluence were analyzed. Also, the relations of scallop height and scanning velocity were predicted. The results show that the thresholds of quartz glass at durations of 12, 35, and 220 fs were 3.3, 2.9, and 2.3J/cm2, respectively, while the thresholds of silicon nitride at the same durations were 2.0, 1.8, and 1.4J/cm2, respectively. The ablation depth of quartz glass decreased with the increase of duration when the fluence deviated from the threshold. The depth of silicon nitride was opposite to it, and the ablation volume of silicon nitride was larger than that of quartz glass in the same condition. When the scanning velocity and fluence were determined, the scallop height decreased with the increase of duration. However, when the velocity and duration were determined, the scallop height increased with the increase of fluence. The shorter the duration was, the greater the variation of the scallop height.

Study of the length and influencing factors of air plasma ignition time

Abstract When a high-energy laser acts on a film surface, plasma flashes of both the air and film can be generated simultaneously. However, when the conventional plasma flash method is used to identify thin film damage, there is a misjudgment problem caused by the inability to distinguish the air and film plasma flashes. In order to solve the problem of misjudgment, the ignition times of air and thin film plasma flashes can be obtained, respectively. If the ignition times of air and thin film plasma flashes are not equal, they can be distinguished from the time difference. In this paper, a nanosecond Nd:YAG pulse laser is used to break down the air at room temperature and pressure, and the theoretical and experimental values of the ignition time of air plasma flash are obtained. The curves of the ignition time of air plasma flash with the laser wavelength, incident energy, focusing spot, and pulse width are simulated. The reasons for the changes are analyzed from the perspectives of multiphoton absorption, cascade ionization theory, and electromagnetic theory of laser breakdown gas. The results show that when the laser pulse width is 10 ns, the energy is 160 mJ, and the spot radius is 0.015 cm. The theoretical and experimental values of the ignition time of air plasma flash are 2.146 and 2 ns, respectively, which are in good agreement. Larger values of laser focus spot size and pulse width relate to a longer ignition time of the air plasma flash, whereas larger values of laser wavelength and incident energy are related to a shorter ignition time. The research reflects the characteristics and electronic transition of air plasma, as well as the micromorphological evolution of the interaction between laser and air, presents the process of air plasma flash generation and growth, and reveals the ignition mechanism of air plasma. It not only provides a basis for improving the traditional plasma flash identification method to identify film damage but also has a certain scientific significance for studying the generation mechanism of laser-supported combustion waves and detonation waves.

Relativistic Two-Photon Matrix Elements for Attosecond Delays

The theory of one-photon ionization and two-photon above-threshold ionization is formulated for applications to heavy atoms in attosecond science by using Dirac–Fock formalism. A direct comparison of Wigner–Smith–Eisenbud delays for photoionization is made with delays from the Reconstruction of Attosecond Beating By Interference of Two-photon Transitions (RABBIT) method. Photoionization by an attosecond pulse train, consisting of monochromatic fields in the extreme ultraviolet range, is computed with many-body effects at the level of the relativistic random phase approximation (RRPA). Subsequent absorption and emission processes of infrared laser photons in RABBIT are evaluated by using static ionic potentials as well as asymptotic properties of relativistic Coulomb functions. As expected, light elements, such as argon, show negligible relativistic effects, whereas heavier elements, such a krypton and xenon, exhibit delays that depend on the fine-structure of the ionic target. The relativistic effects are notably close to ionization thresholds and Cooper minima with differences in fine-structure delays predicted to be as large as tens of attoseconds. The separability of relativistic RABBIT delays into a Wigner–Smith–Eisenbud delay and a universal continuum–continuum delay is studied with reasonable separability found for photoelectrons emitted along the laser polarization axis in agreement with prior non-relativistic results.

L-shell ionisation and X-ray production cross-sections for some low Z-elements by proton impact

L-shell ionisation and x-ray production cross-sections of various targets are useful in many Proton Induced X-ray Emission (PIXE) applications. Generally, x-ray production cross sections are measured directly but ionisation cross sections are only evaluated using theoretical models. The use of ionisation models has been instrumental in the ion-atom description of the subshells which are then converted to x-ray production cross sections, however, certain limitations exist for currently available models. For instance, at extremely low energy and high energy of the incident particle, some modifications are made to these models to correctly obtain these values. While existing ionisation theory coupled with associated computed codes or data tables have been used successfully in determining cross-sections of medium Z and high Z-elements, targets of low atomic number have not been fully studied, especially when bombarded with proton energy less than 1 MeV. In this study, a revised model using computer codes and algorithm are used in obtaining cross-sections of some light elements (Neon, Magnesium and Argon) whose experimental values are difficult to measure. To verify the certainty of these values, a comparison was made with some experimental values.