Showing papers on "Ionization published in 2002"
TL;DR: An electron generated by x-ray photoionization can be deflected by a strong laser field and this phase dependence can be used to measure the duration and chirp of single sub100-attosecond x-rays.
Abstract: An electron generated by x-ray photoionization can be deflected by a strong laser field Its energy and angular distribution depends on the phase of the laser field at the time of ionization This phase dependence can be used to measure the duration and chirp of single sub100-attosecond x-ray pulses
823 citations
TL;DR: In this paper, the authors extended the tunneling ionization model of Ammosov-Delone-Krainov (ADK) for atoms to diatomic molecules by considering the symmetry property and the asymptotic behavior of the molecular electronic wave function.
Abstract: We have extended the tunneling ionization model of Ammosov-Delone-Krainov (ADK) for atoms to diatomic molecules by considering the symmetry property and the asymptotic behavior of the molecular electronic wave function. The structure parameters of several molecules needed for calculating the ionization rates using this molecular ADK model have been obtained. The theory is applied to calculate the ratios of ionization signals for diatomic molecules with their companion atoms that have nearly identical binding energies. The origin of ionization suppression for some molecules has been identified. The predicted ratios for pairs with suppression $({\mathrm{D}}_{2}:\mathrm{Ar},$ ${\mathrm{O}}_{2}:\mathrm{Xe})$ and pairs without suppression $({\mathrm{N}}_{2}:\mathrm{Ar},$ CO:Kr) are in good agreement with the measurements. However, the theory predicts suppression for ${\mathrm{F}}_{2}:\mathrm{Ar},$ which is in disagreement with the experiment. The ionization signals of NO, ${\mathrm{S}}_{2},$ and of SO have also been derived from the experimental data, and the results are also shown to be in agreement with the prediction of the present molecular ADK theory.
622 citations
TL;DR: In this paper, the authors presented selected values of thermodynamic quantities for the aqueous ionization reactions of 64 buffers, many of which are used in biological research. But the selection of the values of the thermodynamics quantities for each buffer is discussed.
Abstract: This review contains selected values of thermodynamic quantities for the aqueous ionization reactions of 64 buffers, many of which are used in biological research. Since the aim is to be able to predict values of the ionization constant at temperatures not too far from ambient, the thermodynamic quantities which are tabulated are the pK, standard molar Gibbs energy ΔrG∘, standard molar enthalpy ΔrH°, and standard molar heat capacity change ΔrCp∘ for each of the ionization reactions at the temperature T=298.15 K and the pressure p=0.1 MPa. The standard state is the hypothetical ideal solution of unit molality. The chemical name(s) and CAS registry number, structure, empirical formula, and molecular weight are given for each buffer considered herein. The selection of the values of the thermodynamic quantities for each buffer is discussed.
574 citations
TL;DR: This work proposes that the electrons themselves can be exploited for ultrafast measurements, and uses a ‘molecular clock’, based on a vibrational wave packet in H2+, to show that distinct bunches of electrons appear during electron–ion collisions with high current densities, and durations of about 1 femtosecond (10-15 s).
Abstract: Experience shows that the ability to make measurements in any new time regime opens new areas of science Currently, experimental probes for the attosecond time regime (10-18–10-15 s) are being established The leading approach is the generation of attosecond optical pulses by ionizing atoms with intense laser pulses This nonlinear process leads to the production of high harmonics during collisions between electrons and the ionized atoms The underlying mechanism implies control of energetic electrons with attosecond precision We propose that the electrons themselves can be exploited for ultrafast measurements We use a ‘molecular clock’, based on a vibrational wave packet in H2+ to show that distinct bunches of electrons appear during electron–ion collisions with high current densities, and durations of about 1 femtosecond (10-15 s) Furthermore, we use the molecular clock to study the dynamics of non-sequential double ionization
481 citations
TL;DR: A background-free observation of cold antihydrogen atoms is made using field ionization followed by antiproton storage, a detection method that provides the first experimental information aboutAntihydrogen atomic states, suggesting that the antiHydrogen is formed via three-body recombination.
Abstract: A background-free observation of cold antihydrogen atoms is made using field ionization followed by antiproton storage, a detection method that provides the first experimental information about antihydrogen atomic states. More antihydrogen atoms can be field ionized in an hour than all the antimatter atoms that have been previously reported, and the production rate per incident high energy antiproton is higher than ever observed. The high rate and the high Rydberg states suggest that the antihydrogen is formed via three-body recombination.
446 citations
TL;DR: In this article, the column densities of HCO+, DCO+, N2H+, and N2D+ were estimated as a function of distance from the dust peak.
Abstract: The maps presented in Paper I are here used to infer the variation of the column densities of HCO+, DCO+, N2H+, and N2D+ as a function of distance from the dust peak. These results are interpreted with the aid of a crude chemical model that predicts the abundances of these species as a function of radius in a spherically symmetric model with radial density distribution inferred from the observations of dust emission at millimeter wavelengths and dust absorption in the infrared. Our main observational finding is that the N(N2D+)/N(N2H+) column density ratio is of order 0.2 toward the L1544 dust peak as compared to N(DCO+)/N(HCO+) = 0.04. We conclude that this result, as well as the general finding that N2H+ and N2D+ correlate well with the dust, is caused by CO being depleted to a much higher degree than molecular nitrogen in the high-density core of L1544. Depletion also favors deuterium enhancement, and thus N2D+, which traces the dense and highly CO-depleted core nucleus, is much more enhanced than DCO+. Our models do not uniquely define the chemistry in the high-density depleted nucleus of L1544, but they do suggest that the ionization degree is a few times 10-9 and that the ambipolar diffusion timescale is locally similar to the free-fall time. It seems likely that the lower limit, which one obtains to ionization degree by summing all observable molecular ions, is not a great underestimate of the true ionization degree. We predict that atomic oxygen is abundant in the dense core and, if so, H3O+ may be the main ion in the central highly depleted region of the core.
429 citations
TL;DR: The reorganization energy upon positive ionization of pentacenes is determined both experimentally and theoretically to be remarkably low, one key element that allows one to rationalize the extremely high hole mobilities recently measured in pentacene single crystals.
Abstract: The reorganization energy in pentacene is reported on the basis of a joint experimental and theoretical study of pentacene ionization using high-resolution gas-phase photoelectron spectroscopy, semiempirical intermediate neglect of differential overlap calculations, and first-principles correlated quantum-mechanical calculations at MP2 and density functional theory levels. The reorganization energy upon positive ionization of pentacene is determined both experimentally and theoretically to be remarkably low. This is one key element that allows one to rationalize the extremely high hole mobilities recently measured in pentacene single crystals.
418 citations
TL;DR: Initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters are presented, finding that, whereasXe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced.
Abstract: Intense radiation from lasers has opened up many new areas of research in physics and chemistry, and has revolutionized optical technology. So far, most work in the field of nonlinear processes has been restricted to infrared, visible and ultraviolet light, although progress in the development of X-ray lasers has been made recently. With the advent of a free-electron laser in the soft-X-ray regime below 100 nm wavelength, a new light source is now available for experiments with intense, short-wavelength radiation that could be used to obtain deeper insights into the structure of matter. Other free-electron sources with even shorter wavelengths are planned for the future. Here we present initial results from a study of the interaction of soft X-ray radiation, generated by a free-electron laser, with Xe atoms and clusters. We find that, whereas Xe atoms become only singly ionized by the absorption of single photons, absorption in clusters is strongly enhanced. On average, each atom in large clusters absorbs up to 400 eV, corresponding to 30 photons. We suggest that the clusters are heated up and electrons are emitted after acquiring sufficient energy. The clusters finally disintegrate completely by Coulomb explosion.
380 citations
TL;DR: The hole-vibrational coupling is found to be significantly smaller than the electron-vibational coupling in the case of these oligomers; however, both quantities are predicted to converge to the same value when increasing the chain length.
Abstract: The hole-vibrational coupling is reported for anthracene, tetracene, and pentacene on the basis of a joint experimental and theoretical study of ionization spectra using high-resolution gas-phase photoelectron spectroscopy and first-principles correlated quantum-mechanical calculations. The hole-vibrational coupling is found to be significantly smaller than the electron-vibrational coupling in the case of these oligomers; however, both quantities are predicted to converge to the same value when increasing the chain length.
361 citations
Pennsylvania State University1, Tel Aviv University2, University of Maryland, Baltimore County3, Goddard Space Flight Center4, Georgia State University5, The Catholic University of America6, University of Florida7, Johns Hopkins University8, Space Telescope Science Institute9, Ohio State University10, Universities Space Research Association11, Ohio University12
TL;DR: In this paper, the authors present results from a 900 ks exposure of NGC 3783 with the High-Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory.
Abstract: We present results from a 900 ks exposure of NGC 3783 with the High-Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory. The resulting X-ray spectrum, which covers the 0.5-10 keV energy range, has the best combination of signal-to-noise ratio and resolution ever obtained for an AGN. This spectrum reveals absorption lines from H-like and He-like ions of N, O, Ne, Mg, Al, Si, and S. There are also possible absorption lines from H-like and He-like Ar and Ca as well as H-like C. We also identify inner-shell absorption from lower ionization ions such as Si VII-Si XII and S XII-S XIV. The iron absorption spectrum is very rich; L-shell lines of Fe XVII-Fe XXIV are detected, as well as probable resonance lines from Fe XXV. A strong complex of M-shell lines from iron ions is also detected in the spectrum. The absorption lines are blueshifted relative to the systemic velocity by a mean velocity of -590 ± 150 km s-1. We resolve many of the absorption lines, and their mean FWHM is 820 ± 280 km s-1. We do not find correlations between the velocity shifts or the FWHMs with the ionization potentials of the ions. Most absorption lines show asymmetry, having more extended blue wings than red wings. In O VII we have resolved this asymmetry to be from an additional absorption system at approximately -1300 km s-1. The two X-ray absorption systems are consistent in velocity shift and FWHM with the ones identified in the UV lines of C IV, N V, and H I. Equivalent width measurements for all absorption and emission lines are given and column densities are calculated for several ions. We resolve the narrow Fe Kα line at 6398.2 ± 3.3 eV to have an FWHM of 1720 ± 360 km s-1, which suggests that this narrow line may be emitted from the outer part of the broad-line region or the inner part of the torus. We also detect a "Compton shoulder" redward of the narrow Fe Kα line, which indicates that it arises in cold, Compton-thick gas.
360 citations
TL;DR: In this article, the evolution of large atomic clusters exposed to a super-intense ultrashort laser pulse is considered, and the optimal conditions are analyzed for efficient absorption of laser radiation by large clusters.
TL;DR: In this article, the authors presented the first high-resolution, soft X-ray spectrum of the prototypical Seyfert 2 galaxy, NGC 1068, obtained with the XMM-Newton Reflection Grating Spectrometer (RGS).
Abstract: We present the first high-resolution, soft X-ray spectrum of the prototypical Seyfert 2 galaxy, NGC 1068. This spectrum was obtained with the XMM-Newton Reflection Grating Spectrometer (RGS). Emission lines from H-like and He-like low-Z ions (from C to Si) and Fe L-shell ions dominate the spectrum. Strong, nar- row radiative recombination continua (RRCs) for several ions are also present, implying that most of the observed soft X-ray emission arises in low-temperature plasma (kTea few eV). This plasma is photoion- ized by the inferred nuclear continuum (obscured along our line of sight), as expected in the unified model of active galactic nuclei (AGNs). We find excess emission (compared to pure recombination) in all resonance lines (1s!np) up to the photoelectric edge, demonstrating the importance of photoexcitation as well. We introduce a simple model of a cone of plasma irradiated by the nuclear continuum; the line emission we observe along our line of sight perpendicular to the cone is produced through recombination/radiative cas- cade following photoionization and radiative decay following photoexcitation. A remarkably good fit is obtained to the H-like and He-like ionic line series, with inferred radial ionic column densities consistent with recent observations of warm absorbers in Seyfert 1 galaxies. Previous Chandra imaging revealed a large (extending out to � 500 pc) ionization cone containing most of the X-ray flux, implying that the warm absorber in NGC 1068 is a large-scale outflow. To explain the ionic column densities, a broad, flat distribu- tion in the logarithm of the ionization parameter (� ¼ LX=ner 2 ) is necessary, spanning log � ¼ 0-3. This sug- gests either radially stratified ionization zones, the existence of a broad density distribution (spanning a few orders of magnitude) at each radius, or some combination of both. Subject headings: galaxies: individual (NGC 1068) — galaxies: Seyfert — line: formation — X-rays: galaxies
TL;DR: In this article, the authors investigate the ionization of hydrogen in a dynamic solar atmosphere and show that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state.
Abstract: We investigate the ionization of hydrogen in a dynamic solar atmosphere. The simulations include a detailed non-LTE treatment of hydrogen, calcium, and helium but lack other important elements. Furthermore, the omission of magnetic fields and the one-dimensional approach make the modeling unrealistic in the upper chromosphere and higher. We discuss these limitations and show that the main results remain valid for any reasonable chromospheric conditions. As in the static case, we find that the ionization of hydrogen in the chromosphere is dominated by collisional excitation in the Lyα transition followed by photoionization by Balmer continuum photons—the Lyman continuum does not play any significant role. In the transition region, collisional ionization from the ground state becomes the primary process. We show that the timescale for ionization/recombination can be estimated from the eigenvalues of a modified rate matrix where the optically thick Lyman transitions that are in detailed balance have been excluded. We find that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state. Throughout the chromosphere the timescale is long (103-105 s), except in shocks where the increased temperature and density shorten the timescale for ionization/recombination, especially in the upper chromosphere. Because the relaxation timescale is much longer than dynamic timescales, hydrogen ionization does not have time to reach its equilibrium value and its fluctuations are much smaller than the variation of its statistical equilibrium value appropriate for the instantaneous conditions. Because the ionization and recombination rates increase with increasing temperature and density, ionization in shocks is more rapid than recombination behind them. Therefore, the ionization state tends to represent the higher temperature of the shocks, and the mean electron density is up to a factor of 6 higher than the electron density calculated in statistical equilibrium from the mean atmosphere. The simulations show that a static picture and a dynamic picture of the chromosphere are fundamentally different and that time variations are crucial for our understanding of the chromosphere itself and the spectral features formed there.
TL;DR: The testing of different solvents showed that addition of buffers such as ammonium acetate, ammonium hydroxide, or acetic acid may suppress ionization in APPI, and the reactions are discussed in detail in light of thermodynamic data.
Abstract: The ionization mechanism in dopant-assisted atmospheric pressure photoionization and the effect of solvent on the ionization efficiency was studied using 7 naphthalenes and 13 different solvent systems. The ionization efficiency was 1−2 orders of magnitude higher with dopant than without, indicating that the photoionization of the dopant initiates the ionization process. In positive ion mode, the analytes were ionized either by charge exchange or by proton transfer. Charge exchange was favored for low proton affinity solvents (water, hexane, chloroform), whereas the addition of methanol or acetonitrile to the solvent initiated proton transfer. In negative ion mode, the compounds with high electron affinity were ionized by electron capture or by charge exchange and the compounds with high gas-phase acidity were ionized by proton transfer. In addition, some oxidation reactions were observed. All the reactions leading to ionization of analytes in negative ion mode are initiated by thermal electrons formed in...
TL;DR: In this article, the ionization fraction of the protostellar a disc is calculated by taking into account vertical temperature structure and the possible presence of trace metal atoms, both thermal and X-ray ionization are considered.
Abstract: We calculate the ionization fraction of protostellar a discs, taking into account vertical temperature structure and the possible presence of trace metal atoms. Both thermal and X-ray ionization are considered. Previous investigations of layered discs used radial power-law models with isothermal vertical structure. But a models are used to model accretion, and the present work is a step towards a self-consistent treatment. The extent of the magnetically uncoupled ('dead') zone depends sensitively on a, on the assumed accretion rate and on the critical magnetic Reynolds number, below which magnetohydrodynamic (MHD) turbulence cannot be self-sustained. Its extent is extremely model-dependent. It is also shown that a tiny fraction of the cosmic abundance of metal atoms can dramatically affect the ionization balance. Gravitational instabilities are an unpromising source of transport, except in the early stages of disc formation.
TL;DR: In this paper, a fluid model of the homogeneous barrier discharge for nitrogen at atmospheric pressure is constructed and the primary excitation and ionization processes specific for this discharge are pointed out.
Abstract: A fluid model of the homogeneous barrier discharge is constructed for nitrogen at atmospheric pressure. The primary excitation and ionization processes specific for this discharge are pointed out. The calculations show that, in a wide range of external conditions, the homogeneous barrier discharge in nitrogen has a form of Townsend discharge which is easy to study. The influence of different mechanisms of electron emission from dielectric barriers and surface recombination over the electrical characteristics of a barrier discharge is studied. Introduction of a finite lifetime at the surface for adsorbed electrons allows us to obtain the results qualitatively corresponding to the experimental data.
TL;DR: In this article, the electron-impact ionization cross sections of helium, neon, argon, krypton, and xenon are reported for energies ranging from the first ionization threshold to 1000 eV.
Abstract: Accurate values of the electron-impact ionization cross sections for the rare gases are needed in a variety of contexts. However, despite numerous investigations over many decades, uncertainty as to the correct values has persisted. The pioneering total-cross-section measurements of Rapp and Englander-Golden are generally regarded as the most reliable but no comprehensive study has independently verified their correctness. In this paper, measurements of electron-impact ionization cross sections of helium, neon, argon, krypton, and xenon are reported for energies ranging from the first ionization threshold to 1000 eV. These data confirm the essential correctness of Rapp and Englander-Golden's total measurements and at the same time provide a complete set of consistent absolute partial cross sections.
Book•
08 Oct 2002
TL;DR: The theory of photoionized regions was introduced in this article and the theory of collisional ionization equilibrium was discussed in the paper "The Diffuse Universe: Answers to Selected Exercises".
Abstract: 1. What Is the Diffuse Universe?.- 2. Line Emission Processes.- 3. Collisional Excitation.- 4. Line Transfer Effects.- 5. Collisional Ionization Equilibrium.- 6. Continuum and Recombination Line Processes.- 7. Cooling Plasmas.- 8. Interstellar Shocks.- 9. The Theory of Photoionized Regions.- 10. Parameters of Photoionized Regions.- 11. Photoionizing Shocks.- 12. Interstellar Dust.- 13. Introduction to Astrochemistry.- 14. Thermal Phases of Diffuse Matter.- A. Physical Constants.- B. Useful Atomic Data.- B.1 Hydrogenic Spectra.- B.1.1 The Hydrogen Spectrum.- B.1.2 The Helium II Spectrum.- B.2 Abundances.- B.3 Collisional Excitation Data.- B.4 Ionization Potentials.- C. Continuum Data.- C.1 Free-Free Emission.- C.2 Total Free-Free Emission.- C.3 Two-Photon Emission.- D. Rotational Wave function Symmetry.- E. Answers to Selected Exercises.
TL;DR: The propagation of short, intense laser pulses in the atmosphere is investigated theoretically and numerically and an equilibrium for a single optical-plasma filament is obtained, which involves a balancing between diffraction, nonlinear self-focusing, and plasma defocusing.
Abstract: The propagation of short, intense laser pulses in the atmosphere is investigated theoretically and numerically. A set of three-dimensional (3D), nonlinear propagation equations is derived, which includes the effects of dispersion, nonlinear self-focusing, stimulated molecular Raman scattering, multiphoton and tunneling ionization, energy depletion due to ionization, relativistic focusing, and ponderomotively excited plasma wakefields. The instantaneous frequency spread along a laser pulse in air, which develops due to various nonlinear effects, is analyzed and discussed. Coupled equations for the power, spot size, and electron density are derived for an intense ionizing laser pulse. From these equations we obtain an equilibrium for a single optical-plasma filament, which involves a balancing between diffraction, nonlinear self-focusing, and plasma defocusing. The equilibrium is shown to require a specific distribution of power along the filament. It is found that in the presence of ionization a self-guided optical filament is not realizable. A method for generating a remote spark in the atmosphere is proposed, which utilizes the dispersive and nonlinear properties of air to cause a low-intensity chirped laser pulse to compress both longitudinally and transversely. For optimally chosen parameters, we find that the transverse and longitudinal focal lengths can be made to coincide, resulting in rapid intensity increase, ionization, and white light generation in a localized region far from the source. Coupled equations for the laser spot size and pulse duration are derived, which can describe the focusing and compression process in the low-intensity regime. More general examples involving beam focusing, compression, ionization, and white light generation near the focal region are studied by numerically solving the full set of 3D, nonlinear propagation equations.
TL;DR: In this paper, an inner-valence ionized cluster, which releases its excess energy by emitting an electron, is characterized by an efficient energy transfer between monomers in the cluster.
TL;DR: In this paper, the properties of dense plasmas characteristic of the atmospheres of neutron stars and of the interior of massive white dwarfs are examined, and the problem of pressure ionization of hydrogen in a strong magnetic field and the crystallization of the quantum one-component-plasma at finite temperature is discussed.
Abstract: We briefly examine the properties of dense plasmas characteristic of the atmospheres of neutron stars and of the interior of massive white dwarfs. These astrophysical bodies are natural laboratories to study respectively the problem of pressure ionization of hydrogen in a strong magnetic field and the crystallization of the quantum one-component-plasma at finite temperature.
TL;DR: In this article, dust and the radiation pressure acting upon it provide the controlling factor in moderating the density, excitation, and surface brightness of photoionized narrow line regions (NLRs) structures.
Abstract: The remarkable similarity between emission spectra of narrow-line regions (NLRs) in Seyfert galaxies has long presented a mystery. In photoionization models, this similarity implies that the ionization parameter is nearly always the same, about U ~ 0.01. Here we present dusty, radiation pressure-dominated photoionization models that can provide natural physical insight into this problem. In these models, dust and the radiation pressure acting upon it provide the controlling factor in moderating the density, excitation, and surface brightness of photoionized NLR structures. Additionally, photoelectric heating by the dust is important in determining the temperature structure of the models. These models can also explain the coexistence of the low-, intermediate-, and coronal ionization zones within a single self-consistent physical structure. The radiation pressure acting on dust may also be capable of driving the fast (~3000 km s-1) outflows such as are seen in the HST observations of NGC 1068.
TL;DR: In this paper, a series of simultaneous ionization and scintillation measurements were performed for a variety of particles in liquid argon and liquid xenon, and the results verified the existence of the same flat top response level for relativistic heavy particles from Ne to La.
Abstract: For the determination of the absolute scintillation yields –the number of scintillation photons per unit absorbed energy– for a variety of particles in liquid argon, a series of simultaneous ionization and scintillation measurements were performed. The results verified that scintillation yields for relativistic heavy particles from Ne to La are constant despite their extensive range of linear energy transfer. Such a constant level, called "flat top response" level, manifests the maximum absolute scintillation yield in liquid argon. The maximum absolute scintillation yield is defined by the average energy to produce a single photon, Wph(max) = 19.5±1.0 eV. In liquid xenon, the existence of the same flat top response level was also found by conducting scintillation measurements on relativistic heavy particles. The Wph(max) in liquid xenon was evaluated to be 13.8±0.9 eV using the Wph for 1 MeV electrons, obtained experimentally. The ratio between the two maximum scintillation yields at the flat top response level obtained in liquid argon and xenon is in good agreement with the estimation by way of the energy resolutions of scintillation due to alpha particles in both liquids.
TL;DR: In this paper, the authors collected and reviewed cross section data for electron collisions with carbon monoxide, including total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational and electronic states, ionization, and dissociation.
Abstract: Cross section data are collected and reviewed for electron collisions with carbon monoxide. Collision processes included are total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational and electronic states, ionization, and dissociation. For each process, recommended values of the cross sections are presented, when possible. The literature has been surveyed through to the end of 2013.
TL;DR: Using Monte Carlo and ab initio simulations to interpret the spectra, this work is able to identify slightly stretched HCl molecules, strongly stretched molecules on the verge of ionization, contact ions comprising H3O+ and Cl-, and an ionic surface phase rich in Zundel ions, H5O2+.
Abstract: Ionization and dissociation reactions play a fundamental role in aqueous chemistry. A basic and well-understood example is the reaction between hydrogen chloride (HCl) and water to form chloride ions (Cl(-)) and hydrated protons (H(3)O(+) or H(5)O(2)(+)). This acid ionization process also occurs in small water clusters and on ice surfaces, and recent attention has focused on the mechanism of this reaction in confined-water media and the extent of solvation needed for it to proceed. In fact, the transformation of HCl adsorbed on ice surfaces from a predominantly molecular form to ionic species during heating from 50 to 140 K has been observed. But the molecular details of this process remain poorly understood. Here we report infrared transmission spectroscopic signatures of distinct stages in the solvation and ionization of HCl adsorbed on ice nanoparticles kept at progressively higher temperatures. By using Monte Carlo and ab initio simulations to interpret the spectra, we are able to identify slightly stretched HCl molecules, strongly stretched molecules on the verge of ionization, contact ion pairs comprising H(3)O(+) and Cl(-), and an ionic surface phase rich in Zundel ions, H(5)O(2)(+).
TL;DR: In this article, the authors used the binary encounter Bethe (BEB) model to calculate the Born cross-sections for direct ionization and then used the sum of the BEB cross sections weighted according to the statistical weights of the final ion states.
Abstract: Total ionization cross sections of neutral carbon, nitrogen, and oxygen atoms by electron impact are presented. In our theory we have included the possibilities that (a) some target atoms used in an experiment were in metastable states close to the ground state, (b) excitation-autoionization of ${2s2p}^{m}$ exited states may be substantial, and (c) ions produced in experiments may be in excited, low-lying metastable states. The binary-encounter Bethe (BEB) model of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] is used to calculate the cross sections for direct ionization. Plane-wave Born cross sections scaled by the method developed by Kim [Phys. Rev. 64, 032713 (2001)] are used to determine the contributions from excitation-autoionization. A sum of the BEB cross sections for direct ionization weighted according to the statistical weights of the final ion states is used to modify the direct ionization cross sections. The combination of the BEB model and the scaled Born cross sections is in excellent agreement with available experimental data. The present method can easily be extended to heavier open-shell atoms.
TL;DR: In this article, the authors formulate the physical basis for the streamline-by-streamline integration of the ionization and heat equations of the steady X-wind, including X-ray heating and ionization, mechanical heating and a revised calculation of ambipolar diffusion heating.
Abstract: In order to compare the X-wind with observations, one needs to be able to calculate its thermal and ionization properties. We formulate the physical basis for the streamline-by-streamline integration of the ionization and heat equations of the steady X-wind. In addition to the well-known processes associated with the interaction of stellar and accretion funnel hot spot radiation with the wind, we include X-ray heating and ionization, mechanical heating, and a revised calculation of ambipolar diffusion heating. The mechanical heating arises from fluctuations produced by star-disk interactions of the time-dependent X-wind that are carried by the wind to large distances where they are dissipated in shocks, MHD waves, and turbulent cascades. We model the time-averaged heating by the scale-free volumetric heating rate, Γmech = αρv3s-1, where ρ and v are the local mass density and wind speed, respectively, s is the distance from the origin, and α is a phenomenological constant. When we consider a partially revealed but active young stellar object, we find that choosing α ~ 10-3 in our numerical calculations produces temperatures and electron fractions that are high enough for the X-wind jet to radiate in the optical forbidden lines at the level and on the spatial scales that are observed. We also discuss a variety of applications of our thermal-chemical calculations that can lead to further observational checks of X-wind theory.
TL;DR: In this article, the authors analyzed the high-resolution X-ray spectrum of the Seyfert 1 galaxy NGC 5548, for the full 0:1{10 keV band, using improved calibration results of the Chandra-LETGS instrument.
Abstract: We analyze the high-resolution X-ray spectrum of the Seyfert 1 galaxy NGC 5548, for the full 0:1{10 keV band, using improved calibration results of the Chandra-LETGS instrument. The warm absorber consists of at least three ionization components, with a low, medium and high ionization parameter. The X-ray absorbing material, from an outflowing wind, covers the full range of velocity components found from UV absorption lines. The presence of redshifted emission components for the strongest blue-shifted resonance absorption lines indicate that the absorber is located at a distance larger than the edge of the accretion disk. We derive an upper limit to the edge of the accretion disk of 1 light year. Absorption lines from ions of at least ten chemical elements have been detected, and in general there are no strong deviations from solar abundances. The narrow emission lines from the O vii and Ne ix forbidden and intercombination lines probably originate from much larger distances to the black hole. We nd evidence for weak relativistically broadened oxygen and nitrogen emission lines from the inner parts of the accretion disk, but at a much smaller flux level than those observed in some other active galactic nuclei. In addition, there is a broad, non-relativistic C vi Ly emission line that is consistent with emission lines from the inner part of the optical/UV broad line region.
TL;DR: In this paper, a detailed modeling of spatio-temporal dynamics of pulsed discharges reveal that voltage displacement into the cathode sheath plays a critical role in the formation of a large group of high-energy electrons in high-voltage nanosecond pulse and dramatic increase in ionization rate.
Abstract: Minimization of the power required to sustain weakly ionized plasmas can be achieved if the energy of ionizing electrons is high, from tens to thousands of electronvolts. These electrons spend about a half of their energy on ionization cascades, in contrast to low-energy (1-3 eV) electrons in conventional discharges that dissipate most of their energy in nonionizing inelastic collisions. High-energy electrons can be injected into the gas as beams. Alternatively, they can be created in situ by applying a very strong electric field for a short time, with a repetition rate matching the rate of recombination. Analytical calculations show that the power budget in the high-voltage, repetitive pulse mode can be significantly lower than in the dc regime, but still much higher than in the case of electron beam ionization. For each pulse length, there exists an optimum electric field that minimizes the power budget. A detailed modeling of spatio-temporal dynamics of pulsed discharges reveal that voltage displacement into the cathode sheath plays a critical role. Fully coupled modeling of nonlocal kinetics of high- and low-energy electrons, ionization processes, charge particle transport, and electrodynamics was performed for a high-voltage pulse developing with a substantial pre-existing plasma. The kinetic modeling shows formation of a large group of high-energy electrons in high-voltage nanosecond pulse and dramatic increase in ionization rate. New effects are indicated, such as field reversal, "two-cathode" effect, and interpulse ionization.
TL;DR: In this paper, a nonadiabatic multi-electron (NME) dynamics model was proposed for strong field ionization in polyatomic molecules, which greatly affects molecular ionization, its fragmentation and its energetic properties.
Abstract: We report the observation and characterization of a new nonresonant strong field ionization mechanism in polyatomic molecules: Nonadiabatic multi-electron (NME) dynamics. The strong field response of a given molecule depends on important properties such as molecular geometry and bonding, the path length of delocalized electrons and/or ionization potential as well as on basic laser pulse parameters such as wavelength and intensity. Popular quasi-static tunnelling models of strong field molecular ionization, based upon the adiabatic response of a single active electron, are demonstrated to be inadequate when electron delocalization is important. The NME ionization mechanism greatly affects molecular ionization, its fragmentation and its energetics. In addition, multi-electron effects are shown to be present even in the adiabatic long wavelength limit.