Ionization Processes In Collisions Research Articles

Influence of electric fields on background photo- and collisional ionization processes in Rydberg-atom collision studies

Studies of Rydberg atom collision processes, especially those with small reaction rate constants, are frequently complicated by spurious background signals associated with photoionization by thermal blackbody radiation and with ionization resulting from collisions with background gas. Here we demonstrate that the magnitude and relative importance of the signals that result from these processes are strongly influenced by the presence of an applied electric field. Possible causes of this dependence are discussed.

Suggesting a di-omega dibaryon search in heavy ion collision experiments

The structure of a new dibaryon $(\ensuremath{\Omega}\ensuremath{\Omega}{)}_{{0}^{+}}$ is studied theoretically in the framework of the chiral SU(3) quark model by solving a resonating group method equation. The binding energy of this dibaryon is predicted to be around 100 MeV, the mean-square root of the distance between two \ensuremath{\Omega}'s is 0.84 fm, and the preliminary estimated mean lifetime is about two times that of the free $\ensuremath{\Omega}$'s. All these interesting properties, as well as the two negative charge units it carries, could make it easily identifiable experimentally in the heavy ion collision process. The production probability of this new dibaryon in a 158 GeV $\mathrm{Pb}+\mathrm{Pb}$ collision is estimated using the thermal model. The rate is of the order of ${10}^{\ensuremath{-}6}$ to ${10}^{\ensuremath{-}5}$ per event. It is expected that, with the increase of the temperature, the production rate will also be increased.

X-ray photoionized plasma diagnostics with helium-like ions. Application to warm absorber-emitter in active galactic nuclei

We present He-like line ratios (resonance, intercombination and forbidden lines) for totally and partially photoionized media. For solar plasmas, these line ratios are already widely used for density and temperature diagnostics of coronal (collisional) plasmas. In the case of totally and partially photoionized plasmas, He-like line ratios allow for the determination of the ionization processes involved in the plasma (photoionization with or without an additional collisional ionization process), as well as the density and the electronic temperature. With the new generation of X-ray satellites, Chandra/AXAF, XMM and Astro-E, it will be feasible to obtain both high spectral resolution and high sensitivity observations. Thus in the coming years, the ratios of these three components will be measurable for a large number of non-solar objects. In particular, these ratios could be applied to the Warm Absorber-Emitter, commonly present in Active Galactic Nuclei (AGN). A better understanding of the Warm Absorber connection to other regions (Broad Line Region, Narrow Line Region) in AGN (Seyferts type-1 and type-2, low- and high-redshift quasars...) will be an important key to obtaining strong constraints on unified schemes. We have calculated He-like line ratios, for Z=6, 7, 8, 10, 12 and 14, taking into account the upper level radiative cascades which we have computed for radiative and dielectronic recombinations and collisional excitation. The atomic data are tabulated over a wide range of temperatures in order to be used for interpreting a large variety of astrophysical plasmas.

Elastic electron–ion collisions in a Lorentzian plasma

Plasma screening effects on elastic electron–ion collision processes in a Lorentzian (kappa)-distribution plasma are investigated using the eikonal method. The electron–ion interaction potential is obtained by introducing a plasma dielectric function εκ. The semiclassical straight-line trajectory method is applied to the path of a projectile electron in order to investigate the variation of the eikonal phase as a function of impact parameter and spectral index κ. In the first-order eikonal approximation, the dynamic plasma screening effect on the eikonal phase is found to be a static screening effect, and depends only on the Debye length and spectral index. The position of the maximum of the differential elastic eikonal cross-section recedes from the ion core with increasing spectral index. The plasma screening effect on the eikonal cross-section increases with decreasing spectral index.

Three‐dimensional plasma simulation of Io's interaction with the Io plasma torus: Asymmetric plasma flow

A three‐dimensional, stationary, two‐fluid plasma model for electrons and one ion species was developed to understand the local interaction of Io's atmosphere with the Io plasma torus and the formation of Io's ionosphere. Our model calculates, self‐consistently, the plasma density, the velocity and the temperatures of the ions and electrons, and the electric field for a given neutral atmosphere and imposed Io plasma torus conditions but assumes for the magnetic field the constant homogeneous Jovian field. With only photoionization in a pure SO2 atmosphere it is impossible to correctly model the plasma measurements by the Galileo spacecraft. With collisional ionization and photoionization the observations can be successfully modeled when the neutral atmospheric column density is Ncol = 6 × 1020 m−2 and the atmospheric scale height is H = 100 km. The energy reservoir of the Io plasma torus provides via electron heat conduction the necessary thermal energy for the maintenance of the collisional ionization process and thus the formation of Io's ionosphere. Anisotropic conductivity is shown numerically as well as analytically to be essential to understand the convection patterns and current systems across Io. The electric field is very greatly reduced, because the ionospheric conductances far exceed the Alfvén conductance ΣA, and also strongly twisted owing to the Hall effect. We find that the electric field is twisted by an analytic angle tan Θtwist = Σ2/(Σ1 + 2ΣA) from the anti‐Jupiter direction toward the direction of corotation for constant values of the Pedersen and Hall conductances Σ1 and Σ2 within a circle encompassing Io's ionosphere. Because the electron velocity is approximately equal to the E × B drift velocity, the electron flow trajectories are twisted by the same angle toward Jupiter, with E and B the electric and magnetic fields, respectively. Since Σ1 ∼ Σ2, the electron flow is strongly asymmetric during convection across Io, and the magnitude of this effect is directly due to the Hall conductivity. In contrast, the ions are diverted slightly away from Jupiter when passing Io. Large electric currents flow in Io's ionosphere owing to these substantially different flow patterns for electrons and ions, and our calculations predict that a total electric current of 5 million A was carried in each Alfvén wing during the Galileo flyby. We also find a total Joule heating rate dissipated in Io's ionosphere of P = 4.2 × 1011 W.

Simultaneous Calculation of Reflection, Physical Sputtering and Secondary Electron Emission from a Metal Surface due to Impact of Low-Energy Ions

A computer simulation code which treats elastic and inelastic collision processes of low-energy ions in solids is presented. In the code, the direct excitation of electrons by a penetrating ion and recoiling atoms is simulated using the Monte Carlo technique, in addition to the simulation of elastic collisions of the moving particles with solid atoms. Electron cascades of the excited electrons and collision cascades of the recoil atoms are also taken into account, and as a result, the code allows us to simulate ion-solid interactions such as ion reflection, physical sputtering and secondary electron emission from the solids. This code is applied to calculations of the energy and angular distributions of emitted particles and the total particle yields of aluminum by impact of ions with the atomic numbers Z1 of 1 to 17 and energies Ei of 10 eV to 10 keV at normal incidence. The calculated sputtering yield and ion reflection coefficient are in reasonable agreement with empirical formulae which have been recently presented. The calculated electron yield shows the clear dependence on Z1 and Ei, but the Ei-dependence is different from that of the electronic stopping power at such low impact energies. The energy and angular distributions of emitted particles indicate the similarities of the secondary electron emission and the physical sputtering, as observed in recent experiments.

Hydrodynamic Simulations of Hα Emission in Algol‐Type Binaries

Two-dimensional hydrodynamic simulations of mass transfer in short-period Algol-type binaries were performed using the numerical code Virginia Hydrodynamics 1. This code uses the piecewise parabolic method with a Lagrangian remap. Our version of the code also accounts for radiative cooling and collisional ionization and excitation processes. The purpose of performing the simulations was to study the Hα emission from circumstellar gas in the Algols. Using observational evidence from the literature to constrain the gas stream properties, hydrodynamic maps of the Hα emissivity in the two systems β Per (P = 2.87 days) and TT Hya (P = 6.95 days) were made in both Cartesian and velocity coordinates from the simulation data. The velocity maps were then compared to Doppler tomograms constructed from observed Hα line emission in these systems. Since the tomograms cannot be directly transformed to maps of emission in spatial coordinates, the simulated Cartesian maps enabled us to interpret the dynamical processes that produce the features observed in the Doppler tomograms. We find that the simulations produce asymmetric accretion structures with many features similar to those found in the Doppler tomograms of Algol systems.

Kα emission in equilibrium and non-equilibrium plasmas

A detailed spectral model has been under development for the computer simulation of 2 p → ls Kα x-ray emissions from highly-charged Fe ions in plasmas. Particular interest has been directed at the Ka emissions produced by the ions from Fe XVIII to Fe XXIV, which occur in the spectral range from 1.84 to 1.94 Å. Account has been taken of the fundamental radiative emission processes associated with radiationless electron capture or dielectronic recombination, inner-shell-electron collisional excitation, and inner-shell-electron collisional ionization. In high-temperature plasmas, small or moderate departures from steady-state corona-model charge-state distributions may occur due to the effects of ion transport processes. However, the assumption of equilibrium (Maxwellian) electron energy distributions is usually accepted to be valid. Particular emphasis has now been directed at the identification of spectral features that can serve as diagnostics of extreme non-equilibrium or transient-ionization conditions, which can occur in both low-density and high-density plasmas as well as in electron-ion beam interactions. In the development of the general theoretical description of the emission spectra, it has been necessary to allow for an arbitrary (non-Maxwellian) electron energy distribution. In order to provide a microscopic investigation of the fundamental Ka line-formation processes that can play a dominant role under extreme non-equilibrium conditions, the observed x-ray emission spectra from Fe ions in the Electron Beam Ion Trap (EBIT) facility at the Lawrence Livermore National Laboratory have been simulated. For the precise interpretation of the high-resolution x-ray observations, which may involve the analysis of blended spectral features composed of many lines, it has been necessary to take into account the multitude of fine-structure components of the Ka radiative transitions in the ions from Fe XVIII to Fe XXIV. A complex situation can be encountered for densities that are higher than the validity range of the simple corona-model approximation. With increasing density, collisionally-induced transitions among the low-lying fine-structure states can play an important role. We have developed a hierarchy of simplified statistical-population models for the distribution of the initial ions among the different low-lying fine-structure states. The inadequacies of this simple approximation can be fundamentally remedied only by the application of a detailed (and possibly time-dependent) collisional-radiative-model description of the fine-structure excitation, de-excitation, and ionization processes. It has been found that inner-shell-electron collisional excitation and ionization processes involving the complex intermediate ions from Fe XVIII to Fe XXI produce spectral features, in the wavelength range from 1.89 to 1.94 Å, which are particularly sensitive to density variations and transient-ionization conditions. For a precise analysis of the x-ray observations produced by anisotropic (directed) electron collisional excitations, it will be necessary in a future extension of this investigation to take into account the angular distribution and polarization of the emitted radiation.

Atomic collisions at ultrarelativistic energies: 6.4 TeV S and 33.2 TeV Pb

We have used beams of 6.4 TeV S ions and 33.2 TeV Pb ions obtained from the CERN-SPS facility to study collision processes of heavy ions at ultrarelativistic energies. Measurements have been made of electron-positron pair production, and capture of an electron produced from the negative continuum onto bound states of the moving ion. Here we report on some aspects of these results. In a new formulation for the theory of electronic stopping power of ions at ultrarelativistic energies/Lindhard and Sørensen (LS) find that nuclear size effects should act to reduce the momentum transfer to electrons and hence the stopping power. Using the 33.2-Pb ions, we measured dE dx in C, Si, Cu, Sn and Pb targets. The LS theory was confirmed.

X-Ray Emission from Dust in Hot Plasmas

Energetic (few keV) thermal electrons easily penetrate interstellar dust grains immersed in hot plasmas. These electrons create K-shell vacancies in atoms locked in dust grains through the K-shell collisional ionization process. Heavy elements such as Fe and Ca produce fluorescent Kα emission in the subsequent decay of the inner shell vacancy. We predict that this emission from Fe atoms in dust grains should be present in X-ray spectra emitted by dusty plasma, in addition to Kα emission from highly ionized Fe ions. This affects the strength and shape of the Fe Kα complex. We also predict a low ratio of L-shell/Kα emission for dust grains, because of low fluorescent yields for L-shell transitions. These effects should be most pronounced in young supernova remnants (SNRs). We discuss observational evidence for the fluorescent Fe Kα emission from dust in X-ray spectra of Tycho's, Kepler's, and Cassiopeia A SNRs, the three youngest remnants in our Galaxy.

Electron Interactions with CHF3

In this paper we assess and synthesize the available information on the cross sections and the rate coefficients for collisional interactions of trifluoromethane (CHF3) with electrons in an effort to build a database on electronic and ionic collision processes that will aid the understanding of the behavior of CHF3 in its use in manufacturing semiconductor devices and other applications. The limited data on the total and partial electron impact ionization cross sections, total and partial cross sections for electron impact dissociation of CHF3 into neutral species, electron-impact induced line and continuous light emission from CHF3, negative ion states of CHF3, and the energetics of ionization, dissociation, and attachment are summarized and discussed. Besides some recent unpublished measurements of the total electron scattering cross section below 20 eV, to our knowledge no measurements are available of the cross sections of any of the electron scattering processes (elastic, momentum, vibrational, inelastic, etc.) or the electron transport, attachment, and ionization coefficients. While the available information is meager, the synthesis of the existing knowledge and the background information provided in the paper can be helpful for modeling plasma reactors. Clearly, more measurements and calculations are needed of the cross sections for virtually all fundamental electron impact processes for this plasma processing gas. Measurements of the transport, attachment, and ionization coefficients over wide ranges of the density reduced electric field are also needed.

Coupled-channel study of collision processes involving highly-charged ions

Collision processes of multiply-charged ions with atomic hydrogen are studied by the triple-center coupled-channel (cc) method in addition to the conventional two-center cc method. Each basis function for the scattering equations is further expanded as a linear combination of the Gaussian-type orbitals (GTO). The third center is located at the saddle point of the potential between the two nuclei and the basis functions on it are chosen to be the eigenstates of united atoms. The triple-center cc method improves the overestimated ionization cross sections of the two-center cc calculations at intermediate energies. As the ionic charge increases, the difference between the triple-and the two-center cc cross sections becomes smaller. The overestimation of the two-center cc ionization cross sections is due to the overcompleteness of pseudocontinuum states attached to both the centers and the effect of the overcompleteness becomes smaller as the asymmetry of the system becomes larger.

CROSS SECTIONS FOR COLLISION PROCESSES OF Li ATOMS INTERACTING WITH ELECTRONS, PROTONS, MULTIPLY CHARGED IONS, AND HYDROGEN MOLECULES

The available experimental and theoretical cross section data for collision processes of Li atoms in the ground state and excited (up ton= 3) states with electrons, protons, and multiply charged ions have been collected and critically assessed. The electron-impact processes include excitation from the ground state, transitions between excited states withn= 2, 3, and ionization from all states considered. The heavy-particle-impact processes include excitation, ionization (for the proton case), and electron removal (the sum of ionization and electron capture). Well-established cross section scaling relationships have been used to generate the cross sections for processes for which no information was found in the literature. This approach was particularly extensively used for the collision processes involving excited Li states, and for collisions with protons and multiply charged ions. In addition, available data for collision processes of Li atoms and ions with neutral hydrogen molecules are shown. The “recommended” cross sections for the considered processes, generated either by critical assessment of available data or by a scaling procedure, have been fitted to simple analytic expressions which ensure correct asymptotic behavior of the cross sections. These recommended cross sections are presented by giving their analytic-fit expressions and tabulating the values of the parameters entering these analytic fits. For those reactions from the ground state for which there is a significant amount of cross section information, we also present the recommended cross sections in graphical form together with the cross section data used for their generation. The criteria for assessing the accuracy of the experimental data, theoretical calculations, and procedures used in determining the recommended cross sections are discussed.

Application of new spectroscopic technique for the study of interaction of multicharged ions with gaseous targets in the extreme ultraviolet

A new optical technique for the extreme ultraviolet (EUV) spectral region based on glass capillary converters has been tested for studies of highly charged ion collision processes, plasma diagnostics and synchrotron radiation applications. Specifically a complex glass capillary converter system has been utilized in conjunction with a high resolution 2.2m grazing incidence monochromator to record EUV (10-100 nm) projectile and target spectra following multicharged ion collisions with He, leading to an enhancement of detected EUV photon flux between 10 and 30.

Eikonal differential scattering cross sections for elastic electron–ion collisions in strongly coupled plasmas

The eikonal approximation is applied to investigate the elastic electron–ion collision processes in strongly coupled plasmas. Plasma-screening effects are investigated for the eikonal differential elastic scattering cross sections. The electron–ion interaction potential in strongly coupled plasmas has been approximated by the ion-sphere model potential. The classical straight-line trajectory approximation is applied to the motion of the projectile electron in order to investigate the variation of the eikonal differential elastic scattering cross section as a function of the impact parameter and ion-sphere radius. A modified eikonal approximation called the Wallace correction is also considered. The eikonal differential elastic scattering cross sections substantially decrease with an increase of the energy of the projectile electron and increase as the plasma-screening effect decreases through the ion-sphere radius.

Modified Fokker-Planck approach to steady-state distributions in plasmas.

A diffusion-equation approach to the collisional radiative recombination and ionization processes in plasmas is improved in order to be applicable to low density plasmas. Dense excited levels are assumed to be quasicontinuous and thus treated by Fokker-Planck equations in the nearest neighbor (nn) approximation, and a few levels lying below the bottleneck are retained explicitly as a discrete set. These coupled equations are solved iteratively, which in turn allows incorporation of the corrections to the nn approximation. The method has been tested for the simple hydrogen plasma in a steady state. The numerical results agree with the exact solutions. Extensions of this approach to nonequilibrium plasmas and complex plasmas of heavy ions are discussed.

Tandem mass spectrometry of peptides: Mechanistic aspects and structural information based on neutral losses. II—Tri‐ and larger peptides

AbstractThe neutral products arising during the collisionally activated dissociation of protonated oligopeptides (MH+) are post‐ionized by collision and detected in neutral fragment‐reionization (+NfR+) mass spectra. For the isomeric tripeptides Ala‐Gly‐Gly, Gly‐Ala‐Gly and Gly‐Gly‐Ala, the amino acid and dipeptide losses from the C‐terminus and the diketopiperazine losses from the N‐terminus allow for differentiation. These neutral fragments are identified in the corresponding +NfR+ spectra by comparison to reference collision‐induced dissociative ionization (CIDI) mass spectra of individual amino acids, dipeptides and diketopiperazines. Peptides with distinct C‐termini but otherwise identical sequences are found to yield +NfR+ products that are characteristic of the respective C‐terminal amino acid. This is demonstrated for several peptide pairs, including leucine‐ and methionine‐enkephalin. In general, +NfR+ spectra are dominated by the heavier neutral losses; further, +NfR+ and CIDI cause extensive dissociation, indicating that the collisional ionization process imparts high average internal energies.

Excitation and ionization of H(2s) by proton impact

The two-centre atomic orbital close-coupling expansion method is used to study the excitation and ionization processes in collisions of protons with metastable hydrogen H(2s). The excitation cross sections to n=3 and 4 and the averaged dipole moments are compared with the eikonal impulse distorted wave approximation calculations by Rodriguez and Miraglia and the one-centre expansion calculation by Ford et al. General agreement is found at high impact energies, but the difference in dipole moments between the present calculation and the EIA calculation at low energies is very large. We also examine the propensity rule for the magnetic substate distributions in the excitation cross sections in the 'natural' frame of reference.

Dissipative processes in an expanding massive gluon gas

The temperature dependence of the kinetic coefficients is obtained in the nonperturbative region with the help of Green-Kubo-type formulae in the model of massive gluon gas motivated by numerical results from simulations of lattice QCD. The entropy production rate is estimated using scaling hydrodynamics. It is shown that the increase in the viscosity coefficients leads to entropy generation in heavy ion collision processes which could be big, especially for temperatures close to the critical one.

Kinetic-energy distributions of ions sampled from argon plasmas in a parallel-plate, radio-frequency reference cell

Kinetic-energy distributions are presented for ions sampled from 13.56-MHz discharges in argon in a capacitively-coupled, parallel-plate, Gaseous Electronics Conference (GEC) radio-frequency reference cell. The cell was modified to allow sampling of ions through an orifice in the grounded electrode. Kinetic-energy distributions are presented for Ar+, Ar++, Ar+2, ArH+, and several trace ions for plasma pressures ranging from 1.3 Pa, where ion-atom collisions in the plasma sheath are not important, to 33.3 Pa, where collisions are important. Applied peak-to-peak radio-frequency (rf) voltages of 50, 100, and 200 V were used, and the current and voltage waveforms at the powered electrode were measured. Dependences of the ion fluxes, mean energies, and kinetic-energy distributions on gas pressure and applied rf voltage are interpreted in terms of possible ion-collision processes. The results agree with previously measured kinetic-energy distributions of ions sampled from the side of the plasma through a grounded probe for similar discharge conditions, verifying that ion kinetics are characteristic of the plasma sheath independent of where it is formed [J. K. Olthoff, R. J. Van Brunt, and S. B. Radovanov, J. Appl. Phys. 72, 4566 (1992)].