Ionization Processes In Collisions Research Articles

Atom-Rydberg Atom Collisions in Hydrogen Plasmas: Cross Sections and Rate Coefficients

Non-LTE modelling requires accurate atomic data e.g. collisional excitation and ionization cross-sections and rate coefficients. In order to improve the modeling of the solar photosphere, as well as to model atmospheres of other similar and cooler stars where the main constituent is also hydrogen, it is necessary to take into account the influence of all the relevant collisional processes on the excited-atom populations in weakly ionized hydrogen plasmas. In this context we present the data needed for the inclusion of the specific atomic collisional processes in the investigation of the optical and kinetic properties of weakly ionized stellar atmospheres layers. The ionization processes in collisions of excited hydrogen atoms with atoms in ground states were considered for the principal quantum numbers 2 ≤ n ≤ 20 and temperatures 4000 K ≤ T ≤ 20000 K.

Hydrodynamical interaction of stellar and planetary winds: effects of charge exchange and radiation pressure on the observed Ly α absorption

ABSTRACT Lyman α observations of the transiting exoplanet HD 209458b enable the study of exoplanet exospheres exposed to stellar extreme ultraviolet (EUV) fluxes, as well as the interacting stellar wind properties. In this study we present 3D hydrodynamical models for the stellar–planetary wind interaction including radiation pressure and charge exchange, together with photoionization, recombination, and collisional ionization processes. Our models explore the contribution of the radiation pressure and charge exchange to the Ly α absorption profile in a hydrodynamical framework, and for a single set of stellar wind parameters appropriate for HD 209458. We find that most of the absorption is produced by the material from the planet, with a secondary contribution of neutralized stellar ions by charge exchange. At the same time, the hydrodynamic shock heats up the planetary material, resulting in a broad thermal profile. Meanwhile, the radiation pressure yields a small velocity shift of the absorbing material. While neither charge exchange nor radiation pressure provides enough neutrals at the velocity needed to explain the observations at −100 km s−1 individually, we find that the two effects combined with the broad thermal profile are able to explain the observations.

Special Issue on Atomic and Ionic Collisions with Formation of Quasimolecules

Many areas of science today, like atomic and molecular physics, nuclear physics, astrophysics, laboratory plasma research etc., depend on data for ionic, atomic, and molecular collision processes. The purpose of the Special Issue “Atomic and Ionic Collisions with Formation of Quasimolecules” in Atoms is to engage a broad community of researchers to consolidate knowledge, make new discoveries, and to continue the exchange of ideas.

Numerical analysis of effects of ion-neutral collision processes on RF ICP discharge

The discharge process of a radiofrequency (RF) inductively coupled plasma (ICP) has been modeled by an ElectroMagnetic Particle-in-Cell Monte Carlo Collision method (EM PIC-MCC). Although the simulation had been performed by our previous model to investigate the discharge mode transition of the RF ICP from a kinetic point of view, the model neglected the collision processes of ions (H+ and H2+) with neutral particles. In this study, the RF ICP discharge process has been investigated by the latest version of the model which takes the ion-neutral collision processes into account. The basic characteristics of the discharge mode transition provided by the previous model have been verified by the comparison between the previous and present results. As for the H-mode discharge regime, on the other hand, the ion-neutral collisions play an important role in evaluating the growth of the plasma. Also, the effect of the ion-neutral collisions on the kinetic feature of the plasma has been investigated, which has highlighted the importance of kinetic perspective for modeling the RF ICP discharge.

Rydberg Atoms: From Determinism to Chaos

R.F. Stebbings and F.B. Dunning were the editors of Rydberg States of Atoms and Molecules. Issued in 1983, this book actually reflected the emergence of a new field in atomic and molecular physics, which subsequently resulted in a number of applications in academic sciences and technology. In this paper, we analyze the results obtained in recent years for ionization processes in collisions of heavy particles with the participation of Rydberg atoms in a wide energy range. These results have been actively used in modern applications of atomic physics. In addition, we consider the influence of nonlinear resonant interaction of Rydberg complexes with electromagnetic waves on the rate of propagation of satellite signals in the atmosphere.

Characteristics of the Single-Gap Pseudospark Discharge Under Nanosecond Pulsed Voltages

In this paper, a single-gap pseudospark discharge chamber was built and tested with several electrode gap distances and pressures to explore the characteristics of the pseudospark under nanosecond pulsed voltages. Experimental results show that with the development of the discharge, anode plasma firstly forms near the anode, and then moves towards the cathode. The ionization initial voltage decreases with the increase of the electrode gap distance and pressure in our pseudospark discharge configuration. By analysing the emission spectra, it is concluded that the major emission spectra are composed of the second positive bands, and the electron energy of the plasma is in the level of several tens of electron volts. The mechanism of how the pressure and gap distance influence the characteristics has been discussed with the simulation of the collision ionization process using the Monte Carlo Collision method. Based on the simulation and experimental results, it is inferred that the breakdown voltages is decided by the ionization process of the electron with the neutral gas in the main gap, and a possible reason is put forward to explain the reason that U is a function of p2d.

Allowance for the tunnel effect in the entrance channel of fusion–fission reactions

A two-stage model is developed in order to describe fusion–fission reactions. The process in the course of which colliding ions approach each other is simulated at the first stage, the deformations and relative orientations of the ions being taken into account. The first stage of the calculation is completed as soon as colliding nuclei touch each other. A continuous nuclear system (monosystem) is formed at this instant. The emerging distributions of the angular momenta of this system and of its potential and internal energies at the point of touching are used as input data that are necessary for triggering the second stage of the calculation. The evolution of collective coordinates that describe the shape of the monosystem is calculated at the second stage. The description of this evolution is terminated either at the instant of its fission or upon the release of a major part of its excess energy via particle and photon emission. In the latter case, the probability for the fission of the monosystem or a further decrease in its excitation energy becomes extremely small. The ion-collision process and the evolution of the monosystem formed after primary nuclei come into contact are simulated on the basis of stochastic Langevin equations. The quantities appearing in them (which include the potential energy and inertial and friction parameters) are determined with allowance for the shell structure of nuclei. The tunneling of colliding nuclei through the Coulomb barrier is taken into account, and the effect of this phenomenon on model predictions is studied.

Storage-ring experiments on dielectronic recombination at the interface of atomic and nuclear physics

A brief review about topical developments in the exploitation of the resonant electron–ion collision process of dielectronic recombination (DR) as a sensitive spectroscopic tool is given. The focus will be on DR storage-ring experiments of few-electron highly charged ions. Among others, the questions addressed in these studies cover diverse topics from the areas of strong-field quantum electrodynamics, of lifetime studies using DR resonances, and of nuclear physics. Examples from the storage rings CRYRING in Stockholm, TSR in Heidelberg, and ESR in Darmstadt are given. In addition, an overview is provided about the ongoing developments and future perspectives of DR collision spectroscopy at the upcoming Facility for Antiproton and Ion Research in Darmstadt, Germany.

Spectroscopy of berylliumlike xenon ions using dielectronic recombination

Be-like ions have been investigated employing the resonant electron–ion collision process of dielectronic recombination (DR) as a spectroscopic tool. The experiments were performed at the experimental storage ring in Darmstadt, Germany, using its electron cooler as a target for free electrons. DR Rydberg resonance series for the associated intra-L-shell transitions and were observed with high resolution. In addition to these excitations from the ground state we determined resonances associated with excitations of ions initially in the metastable state. The corresponding excitation energies were determined to be and and . These excitation energies are compared with previous measurements and with recent state-of-the-art atomic structure calculations.

Molecular dynamic simulation of low-energy FIB irradiation induced damage in diamond

In this article, a large scale multi-particle molecular dynamics (MD) simulation model was developed to study the dynamic structural changes in single crystal diamond under 5keV Ga+ irradiation in conjunction with a transmission electron microscopy (TEM) experiment. The results show that the thickness of ion-induced damaged layer (∼9.0nm) obtained from experiments and simulations has good accordance, which demonstrates the high accuracy achieved by the developed MD model. Using this model, the evolution of atomic defects, the spatial distributions of implanted Ga particles and the thermal spike at the very core collision area were analysed. The local thermal recrystallizations observed during each single ion collision process and the increase of the density of the non-diamond phase (mostly sp2 bonded) at irradiation area are fund to be the underling mechanisms responsible for ion fluence dependent amorphization of diamond observed in previous experiments.

Ionization of lithium vapor by nanosecond resonant laser pulses tuned to 2S → 2P transition

A theoretical study is reported for the resonant excitation and ionization of dense lithium vapor induced by nanosecond laser pulses, tuned to the resonance transition 2S–2P. The lithium vapor with density (1014–1016 cm−3) is assumed to be excited and ionized by a laser beam with laser power of (105–106 W cm−2) according to the experimental conditions of Skenderovic et al. (Phys Rev A 62:052707, 2000). The time evolution of electron energy distribution function and the electron density, the population density of the excited states as well as the atomic ion, are solved numerically. The numerical calculations of the electron energy distribution function show that nonequilibrium plasmas are produced in lithium vapor by laser irradiation of the nS–nP resonance line. The electrons in these plasmas are heated by superelastic collisions with atoms in the nP state giving a distribution of electrons in energy that is characterized by a series of spikes at energy separated by the nS–nP transition energy. In addition, the competition between photoionization processes and collisional ionization processes for producing the Li+ as well as energy pooling collisions process of the excited lithium atoms plays essential roles in populating the highly excited states. Moreover, the results are found to be consistent with the experimental observations.

LASER PHOTOIONIZATION SENSOR OF THE SEPARATING ISOTOPES AND NUCLEAR REACTIONS PRODUCTS: THEORETICAL FOUNDATIONS OF NEW SCHEME

It is proposed and studied a new optimal theoretical scheme for the laser photoionization sensor of the separating isotopes, nuclear isomers and nuclear reactions products, which is based on the selective laser excitation of the isotope atoms into excited Rydberg states and further stochastic collisional ionization mechanism. The operator perturbation theory and FockerPlank stochastic equation method are used for description of stochastic collisional ionization process. The optimal laser action model and density matrices formalism is used for numeric calculation of the optimal scheme of the Na, U, Cs and Cf isotopes (ions, nuclei) sensing.

The effects of electron transfer on the energy loss of slow He 2+, C 2+, and C 4+ ions penetrating a graphene fragment

Electronic energy loss in the collision processes of slow ions with a graphene fragment is investigated by combining ab initio time-dependent density functional theory calculations for electrons with molecular dynamics simulations for ions in real time and real space. We study the electronic energy loss of slow He2+, C2+, and C4+ ions penetrating the graphene fragment as a function of the ion velocity, and establish the velocity-proportional energy loss for low-charged ions down to 0.1 a.u. One mechanism clarified in the simulations for electron transfer is polarization capture, which is effective for bare ions at low velocities. The other one is resonance capture, by which the incident ion can capture electrons from the graphene fragment to its electron affinity levels, which have the same, or nearly the same, energy as those of the electron donor levels. The results demonstrate that the nonlinear behavior of energy loss of C4+ is attributed to the large number of electrons captured by this multi-charged ion during the collision.

Probing nuclear properties by resonant atomic collisions between electrons and ions

The utilization of the resonant atomic electron–ion collision process of dielectronic recombination (DR) as a tool to probe nuclear properties via isotope shifts and hyperfine effects is discussed. Based on DR, this resonance reaction spectroscopy at electron coolers of heavy-ion storage rings denotes a versatile approach to access nuclear parameters such as charge radius, spin, magnetic moment or lifetimes of long-lived excited nuclear states (isomers). The high sensitivity of DR allows for experiments with artificially synthesized rare isotopes and isomers. Recent experimental progress in the preparation of such exotic species at the ESR storage ring in Darmstadt is presented. The DR technique is exemplified for the case of 234Pa88+ (Z = 91).

Cross Sections of the Processes Induced by Electron Collisions with H2+, HeH+, and Their Isotopes

The accuracy of cross sections given by theoretical calculations is evaluated on the collision processes of molecular ions and electrons. The processes focused on are dissociative recombination, dissociative excitation, and rotational and vibrational transitions of the molecular ions of H2+, HeH+, and their isotopes, which are relevant to divertor plasmas. Adopting the multichannel quantum defect theory, we calculated the state-selective cross sections for various states and energies. The validity of those calculations is investigated by comparing with experimental data under some limited conditions, and the calculations are verified from physical viewpoints.

Molecular dynamics simulation of Ga+ ion collision process

Molecular dynamics (MD) simulations of 30keV accelerated Ga ions has been carried out, colliding with a Si surface. Using this procedure the amorphous structural region of the Si was found to expand with the progression of the interface region, that lie between the amorphous structure and the crystalline structure, as fluence increased in the depth direction. The height of the structure is increased and has a peak value around 1.6×1015ion/cm2, the height becomes the negative value beyond the peak, that is, the phenomenon changes from a deformation to a remove. The tendency of distribution for height and depth is a good agreement with an experimental data.

The differential cross sections and mechanisms of ionization and in the collisions of He2++H(1s) with strong magnetic fields

The ionization processes in collisions of H (1s) atoms with He2+ are studied by the classical trajectory Monte Carlo method. The first-order differential cross sections for ionization are calculated for different field strengths of both transverse and parallel magnetic field, and they increase obviously with the fields applied. Instead of monotonic decline, a peak appears for the differential cross section, with the fields applied. According to the analysis of trajectories, the ionization mechanisms become quite different. In this paper, the influences the strength and direction of the applied field including the incident energy on the ionization mechanisms are also explained.

Multiple Ionization of Ne in coincidence with partially stripped B2+ ions in the intermediate energy regime

A projectile ion-recoil ion and electron-recoil ion coincidence technique has been used to study the multiple ionization process in collisions of 0.75-4.0MeV B2+ with Ne atoms. Absolute total and partial pure ionization cross sections are reported for the first time at these impact energies with the present setup. The relative contribution of each recoil charge state has been investigated. The experimental data are compared to CDW-EIS calculation and with He2+ projectiles. The data throw further light on the effect of projectile screening in the ionization of multi-electronic targets and provide a support for the validity of the CDW-EIS model in this intermediate-to low velocity regime.

Limitations of coherent diffractive imaging of single objects due to their damage by intense x-ray radiation

During the coherent diffraction imaging (CDI) of a single object with an intense x-ray free-electron laser (FEL) pulse, the structure of the object changes due to the progressing radiation damage. Electrons are released from atoms and ions during photo-, Auger- and collisional ionization processes. More and more ions appear in the sample. The repulsive force between ions makes them move apart. Form factors of the created ions are reduced when compared with the atomic form factors. Additional scattering of energetic photons from the free electrons confined within the beam focus deteriorates the obtained diffractive signal. Here, we consider pulses short enough to neglect ionic movement and investigate how (i) the decrease of atomic form factors due to the progressing ionization of the sample and (ii) the scattering from the free electrons influence the signal obtained during the CDI. We quantify the loss of structural information about the object due to these effects with hydrodynamic simulations. Our study has implications for the experiments planned on high-resolution three-dimensional imaging of single reproducible particles with x-ray FELs.

Collisional Ionization in Lithium Vapor Excited by Nanosecond Laser Pulses

We report a theoretical study of the collisional ionization processes which occur under 2s → 2p excitation of lithium vapor excited with a nanosecond pulsed laser. The time evolution of the electron energy distribution function and the atomic ion density (Li) as well as the molecular ion density (Li+2 ) at di erent lithium vapor densities was investigated. The results show a nonlinear behavior of the energy spectra of the electrons created during the interaction of the laser with the lithium vapor. The nonlinear behavior results from superelastic collisions between the free electrons produced by collisional ionization and photoionization with Li(2p). The results also show that a competition exists between the collisional ionization and photoionization processes in producing the atomic ion and the molecular ions.