THE PHYSICS OF IONIZED GASES: 22nd Summer School and International Symposium on the Physics of Ionized Gases; Invited Lectures, Topical Invited Lectures and Progress Reports
740(2004); http://dx.doi.org/10.1063/1.1843491View Description Hide Description
This paper reviews some of the non‐equilibrium processes governing the dynamics of molecular formation and destruction in the interstellar medium. In the first part, it is shown how the existence of interstellar grains is mainly responsible for the formation of H2 and how a small fractional ionization of H and H2 by cosmic rays explains satisfactorily how most of the simple common molecular species are subsequently formed by a series of exoergic ion‐molecule reactions. The second part of the review is devoted to the problem of the fragmentation of molecules by the interstellar photons whose energy is less than the ionization potential of atomic H. For most of the common interstellar molecules, photodissociation occurs via an indirect mechanism, due to non‐adiabatic coupling between a bound valence state with a dissociative channel. The resonant character of the process leads to a very frequency selective photodissociation cross section, which can induce isotopic fractionation. A brief description of the theoretical methods used to treat photodissociation induced by non adiabatic coupling is discussed. The results of our calculated photodissociation cross sections for the CH+ molecule are compared with recent experiments using an ion storage ring. The excellent agreement between theory and experiment confirms that such non‐adiabatic processes are well understood.
740(2004); http://dx.doi.org/10.1063/1.1843492View Description Hide Description
Electron‐impact excitation plays a major role in emission from aurora and a less significant but nonetheless crucial role in the dayglow and nightglow. For some molecules, such as N2, O2 and NO, electron‐impact excitation can be followed by radiative cascade through many different sets of energy levels, producing emission with a large number of lines. We review the application of our statistical equilibrium program to predict this rich spectrum of radiation, and we compare results we have obtained against available independent measurements. In addition, we also review the calculation of energy transfer rates from electrons to N2, O2 and NO in the thermosphere. Energy transfer from electrons to neutral gases and ions is one of the dominant electron cooling processes in the ionosphere, and the role of vibrationally excited N2 and O2 in this is particularly significant. The importance of the energy dependence and magnitude of the electron‐impact vibrational cross sections in the calculation of these rates is assessed.
740(2004); http://dx.doi.org/10.1063/1.1843493View Description Hide Description
This paper deals with the presentation and discussion of recent research on the transport of electrons and ions in gases at low energies. Particular emphasis is placed on electron swarm experiments related with the negative differential conductivity of electrons in some gas mixtures, and with secondary ionisation processes due to the impact of metastables with neutrals (Penning ionisation). Ion transport is firstly adressed through some recent measurements on atomic and molecular systems for which both theory and experiment have reached a high degree of agreement, and also on those in which the ranges of the density‐normalized electric field intensity E/N have been increased susbtantially. Also, the recent advances on the application of transport theories dealing with inelastic collisions are presented, as well as some recent measurements of negative ions and charged clusters in gaseous mixtures, leading to the successful test of Blanc’s law at low fields, to the experimental mobiliites.
740(2004); http://dx.doi.org/10.1063/1.1843494View Description Hide Description
Recent progress in theoretical and experimental investigations of photoabsorption by atoms and ions is presented. Specifically, examples of near‐chaotic behavior in photoionization of positive ions, low‐energy manifestations of nondipole effects, high‐energy breakdown of the single particle picture and new phenomenology uncovered in the inner‐shell photoabsorption by negative ions are discussed.
740(2004); http://dx.doi.org/10.1063/1.1843495View Description Hide Description
The influence of the target density on the electron‐capture (EC) processes in collisions of fast ions with atoms and molecules is considered. The partial EC cross sections σ n on the principal quantum number n of the scattered projectile, as well as the total σtot values are calculated for highly charged ions interacting with gaseous and solid targets in the energy range of E = 100 keV/amu to 10 MeV/amu. It is shown that with the target density increasing, the population of the excited states of the scattered projectiles, formed via the EC channel, is suppressed due to projectile ionization by the target particles and, as a result, the effective EC cross sections drastically decrease.
Transverse coherence of a natural metastable‐atom nozzle beam : Scattering and van der Waals‐Zeeman transitions740(2004); http://dx.doi.org/10.1063/1.1843496View Description Hide Description
By use of the resonant metastability‐exchange process, a metastable‐atom beam possessing all genuine qualities of a “ natural ” ground‐state atom nozzle beam is prepared. Owing to the angular narrowness (0.35 mrad) and smallness of the effective source diameter (15 μm) of this beam, the scattering of metastable atoms by a silicon‐nitride nano‐slit grating is investigated in detail, in a partially coherent regime. The elastic scattering exhibits high‐order diffraction peaks combined with a standard van der Waals deflection effect. When a static magnetic field is present, surface‐induced exo‐energetic transitions among Zeeman sub‐levels are observed.
740(2004); http://dx.doi.org/10.1063/1.1843497View Description Hide Description
This report presents an investigation on the modeling of stationary DC and microwave nitrogen discharges and their afterglows, operating at pressures around one Torr and ionization degrees between 10−7–10−4. The model is based on the self‐consistent solutions to the electron Boltzmann equation coupled to the rate balance equations for the most important neutral and charged species, the wave electrodynamics characteristics and the gas thermal balance equation. The results are obtained as a function of the usual discharge operating parameters, namely gas pressure, discharge current or electron density, and tube radius. It is shown that the vibrationally excited molecules play a central role in the whole problem, ensuring a strong link between different kinetics and directly contributing to the mechanisms of dissociation and gas heating. Furthermore, vibrationally excited molecules in high vibration levels are in the origin of the peaks observed in the flowing afterglow for the concentrations of several species, such as , N2(B 3Π g ), and electrons, which occur downstream from the discharge after a dark zone as a consequence of the V‐V up‐pumping mechanism.
Application of Momentum Transfer Theory for Ion and Electron Transport in Pure Gases and in Gas Mixtures740(2004); http://dx.doi.org/10.1063/1.1843498View Description Hide Description
In this paper we have presented two applications of Momentum Transfer Theory (MTT), which were both aimed at obtaining reliable data for modeling of non‐equilibrium plasma. Transport properties of ion swarms in presence of Resonant Charge Transfer (RCT) collisions are studied using Momentum Transfer Theory (MTT). Using the developed MTT we tested a previously available anisotropic set of cross‐sections for Ar++Ar collisions bay making the comparisons with the available data for the transverse diffusion coefficient. We also developed an anisotropic set of Ne++Ne integral cross‐sections based on the available data for mobility, longitudinal and transverse diffusion. Anisotropic sets of cross‐sections are needed for Monte Carlo simulations of ion transport and plasma models. Application of Blanc’s Law for drift velocities of electrons and ions in gas mixtures at arbitrary reduced electric field strenghts E/n0 was studied theoretically and by numerical examples. Corrections for Blanc’s Law that include effects of inelastic collisions were derived. In addition we have derived the common mean energy procedure that was proposed by Chiflikian in a general case both for ions and electrons. Both corrected common E/n0 and common mean energy procedures provide excellent results even for electrons at moderate E/n0 where application of Blanc’s Law was regarded as impossible. In mixtures of two gases that have negative differential conductivity (NDC) even when neither of the two pure gases show NDC the Blanc’s Law procedure was able to give excellent predictions.
740(2004); http://dx.doi.org/10.1063/1.1843499View Description Hide Description
Ion beam deposition is a process far from thermodynamic equilibrium and is in particular suited to grow metastable thin films with diamond‐like properties, such as tetrahedral amorphous carbon (ta‐C) and cubic boron nitride (c‐BN). In this contribution the atomistic description of the deposition and growth processes are reviewed and compared to experimental results, obtained from mass selected ion beam deposition. The focus will be set to the nucleation and growth processes of boron nitride as a model system for ion based thin film formation. Furthermore, recent examples for nanostructure formation in ion deposited compound thin films will be presented. Ion beam deposited metal‐carbon nano‐composite thin films exhibit a variety of different morphologies such as rather homogeneous nanocluster distributions embedded in an a‐C matrix, but also the self‐organized formation of nanoscale multilayer structures.
740(2004); http://dx.doi.org/10.1063/1.1843500View Description Hide Description
Molecular Dynamics computer simulation is used to demonstrate the behaviour of surfaces upon impact by energetic molecules. At low energies and glancing angles the fullerene molecules can be made to scatter from the surface intact. The coupling of the deposited energy into the surface vibrational modes, particularly for layered materials like HOPG graphite, can lead to what appears to be anomalous behaviour. This is explored and compared with experimental results.
Not all fullerene molecules are spherical. The C76 fullerene is elliptical in shape. Computer simulations are used to investigate the effects of shape on the scattering of molecules from a graphite surface.
Molecular species have been used in ion implantation for doping shallow layers in silicon. There are two contradictory things that can happen when a cluster or molecule is implanted. The molecule will damage the crystal structure with each impact and in so doing could prevent the channelling of the implanted ions, thereby reducing the over‐all range of the implantation. It is also possible that the atoms in the “front” of the cluster/molecule will interact with the surface first, pushing aside the surface atoms so that the atoms of the cluster/molecule following behind might not interact with them so strongly and hence be able to penetrate the solid more deeply. This will result in a deeper implantation range profile. Simulations are compared between single atom and molecular species to investigate which of these mechanisms, if any is operating at low implantation energies.
Two clearly observed vibrational modes are excited in a graphite surface by molecular impacts. It is shown that these vibrational modes can assist in the desorption of loosely bound adsorbates from the surface. At higher impact energies it is shown that the surface disruption caused by the impact can both aid and inhibit the desorption process depending upon the position and energy of the initial fullerene impact in relation to the position of the adsorbate. Some simple conclusions about the “desorbing power” of a fullerene impact as a function of energy are drawn.
740(2004); http://dx.doi.org/10.1063/1.1843501View Description Hide Description
Plasma‐based ion implantation and deposition (PBII&D) technology has been developed rapidly in the past decade. This technique is especially promising for modifying three‐dimensional components. In PBII&D, plasma is generated in the entire processing chamber and then surrounds the components. When a train of negative voltage pulses are applied to the parts, ions are drawn to all the surfaces exposed to the plasma. At a high energy, ions are implanted to the surfaces, but at a low energy and with a proper precursor gases, ions are deposited to form a film. This technology has found applications in many areas including semiconductors, automotive, aerospace, energy and biomedical. This article reviews PBII&D fundamentals, describes features of various PBII&D systems and plasma sources, and discusses implantation and deposition techniques. The paper will also present application examples of this technology.
740(2004); http://dx.doi.org/10.1063/1.1843502View Description Hide Description
Surface treatment by means of pulsed laser beams in reactive atmospheres is an attractive technique to enhance the surface features, such as corrosion and wear resistance or the hardness. Many carbides and nitrides play an important role for technological applications, requiring the mentioned property improvements. Here we present a new promising fast, flexible and clean technique for a direct laser synthesis of carbide and nitride surface films by short pulsed laser irradiation in reactive atmospheres (e.g. methane, nitrogen). The corresponding material is treated by short intense laser pulses involving plasma formation just above the irradiated surface. Gas‐Plasma‐Surface reactions lead to a fast incorporation of the gas species into the material and subsequently the desired coating formation if the treatment parameters are chosen properly. A number of laser types have been used for that (Excimer Laser, Nd:YAG, Ti:sapphire, Free Electron Laser) and a number of different nitride and carbide films have been successfully produced. The mechanisms and some examples will be presented for Fe treated in nitrogen and Si irradiated in methane.
740(2004); http://dx.doi.org/10.1063/1.1843503View Description Hide Description
JANNUS (Joint Accelerators for Nano‐Science and Nuclear Simulation) is a project designed to study the modification of materials using multiple ion beams and in‐situ TEM observation. It will be a unique facility in Europe for the study of irradiation effects, the simulation of material damage due to irradiation and in particular of combined effects. The project is also intended to bring together experimental and modelling teams for a mutual fertilisation of their activities. It will also contribute to the teaching of particle‐matter interactions and their applications. JANNUS will be composed of three accelerators with a common experimental chamber and of two accelerators coupled to a 200 kV TEM.
740(2004); http://dx.doi.org/10.1063/1.1843504View Description Hide Description
Swift heavy ions with energies of about 5 MeV/u cause a high energy deposition into solid targets. As a consequence first the electronic system receives energy and afterwards the energy is transferred to the atomic system by electron phonon coupling or Coulomb repulsion. After a short introduction of what is denoted ion track, different approaches for investigating the ion track formation are presented. Typical experimental data corresponding to these approaches are discussed with respect to the thermal‐spike and Coulomb‐explosion models.
Carbon‐containing compounds on fusion‐related surfaces: Thermal and ion‐induced formation and erosion740(2004); http://dx.doi.org/10.1063/1.1843505View Description Hide Description
The deposition of carbon on metals is the unavoidable consequence of the application of different wall materials in present and future fusion experiments like ITER. Presently used and prospected materials besides carbon (CFC materials in high heat load areas) are tungsten and beryllium. The simultaneous application of different materials leads to the formation of surface compounds due to the erosion, transport and re‐deposition of material during plasma operations. The formation and erosion processes are governed by widely varying surface temperatures and kinetic energies as well as the spectrum of impinging particles from the plasma. The knowledge of the dependence on these parameters is crucial for the understanding and prediction of the compound formation on wall materials. The formation of surface layers is of great importance, since they not only determine erosion rates, but also influence the ability of the first wall for hydrogen isotope inventory accumulation and release. Surface compound formation, diffusion and erosion phenomena are studied under well‐controlled ultra‐high vacuum conditions using in‐situ X‐ray photoelectron spectroscopy (XPS) and ion beam analysis techniques available at a 3 MV tandem accelerator. XPS provides chemical information and allows distinguishing elemental and carbidic phases with high surface sensitivity. Accelerator‐based spectroscopies provide quantitative compositional analysis and sensitivity for deuterium in the surface layers. Using these techniques, the formation of carbidic layers on metals is studied from room temperature up to 1700 K. The formation of an interfacial carbide of several monolayers thickness is not only observed for metals with exothermic carbide formation enthalpies, but also in the cases of Ni and Fe which form endothermic carbides. Additional carbon deposited at 300 K remains elemental. Depending on the substrate, carbon diffusion into the bulk starts at elevated temperatures together with additional carbide formation. Depending on the bond nature in the carbide (metallic in the transition metal carbides, ionic e.g. in Be2C), the surface carbide layer is dissolved upon further increased temperatures or remains stable. Carbide formation can also be initiated by ion bombardment, both of chemically inert noble gas ions or C+ or CO+ ions. In the latter case, a deposition‐erosion equilibrium develops which leads to a ternary surface layer of constant thickness. A chemical erosion channel is also discussed for the enhanced erosion of thin carbon films on metals by deuterium ions.
The Interpretation of the Broad Maximum in the Energy Spectra of Ar+ Ions Scattered from the Clean Metal Surfaces740(2004); http://dx.doi.org/10.1063/1.1843506View Description Hide Description
The broad maximum in the energy spectra of Ar+ ions scattered from the clean metal surfaces to the scattering angles smaller than 90° is studied. Besides the Ar+ ions, other ion species (He+, N+ and Ne+) are also used as projectiles. The targets in the experiments were polycrystalline silver and copper as well as the monocrystalline nickel (110). According to the analysis of the obtained energy spectra, the maximum is interpreted as the consequence of the symmetric binary collisions between the Ar+ projectiles and the former implanted Ar atoms placed under the outermost atomic layer. This type of experiments gives a unique opportunity to study the process of low energy Ar+ ions implantation in real time, and it could be extended to other projectile‐target systems of the greater technological interest.
Magnetron Plasma Sputtered Nanocomposite Thin Films: Structural Surface Studies by In Vacuo Photoelectron Spectroscopy740(2004); http://dx.doi.org/10.1063/1.1843507View Description Hide Description
The experimental system that enables thin film deposition by chemical vapor deposition combined with magnetron sputtering and sample surface characterization by photoelectron spectroscopy (PES), without breaking the vacuum between the deposition and the characterization stage, is described. The particular goal of this work was study of the surface arrangement of embedded metallic nanoclusters of 1B group (Au, Ag, and Cu) in amorphous hydrogenated carbon (a‐C:H). From the range of applied material characterization tools, we present here the results of several PES‐based experiments used to reveal cluster properties at the surface: as‐deposited sample PES measurements, off‐normal take‐off angle XPS, and in situ in‐depth XPS profiling by Ar+ ion etching. Clear distinction in all PES results of the samples deposited on the grounded substrates from those deposited on −150 V dc biased ones is obtained, revealing that keeping the substrate grounded during deposition results in topmost metallic clusters covered with a very thin layer of a‐C:H, while applying negative bias voltage to the substrate results in partially bald clusters on the surface.
740(2004); http://dx.doi.org/10.1063/1.1843508View Description Hide Description
A part of incident energy is absorbed within the irradiated sample when a solid is exposed to the influence of laser radiation, to more general electromagnetic radiation within the wide range of wavelengths (from microwaves, to infrared radiation to X‐rays), or to the energy of particle beams (electronic, protonic, or ionic). The absorption process signifies a highly selective excitation of the electronic state of atoms or molecules, followed by thermal and non‐thermal de‐excitation processes. Non‐radiation de‐excitation‐relaxation processes induce direct sample heating. In addition, a great number of non‐thermal processes (e.g., photoluminescence, photochemistry, photovoltage) may also induce heat generation as a secondary process. This method of producing heat is called the photothermal effect.
The photothermal effect and subsequent propagation of thermal waves on the surface and in the volume of the solid absorbing the exciting beam may produce the following: variations in the temperature on the surfaces of the sample; deformation and displacement of surfaces; secondary infrared radiation (photothermal radiation); the formation of the gradient of the refractivity index; changes in coefficients of reflection and absorbtion; the generation of sound (photoacoustic generation), etc. These phenomena may be used in the investigation and measurement of various material properties since the profile and magnitude of the generated signal depend upon the nature of material absorbing radiation. A series of non‐destructive spectroscopic, microscopic and defectoscopic detecting techniques, called photothermal methods, is developed on the basis of the above‐mentioned phenomena.
This paper outlines the interaction between the intensity modulated laser beam and solids, and presents a mathematical model of generated thermal sources. Generalized models for a photothermal response of optically excited materials have been obtained, including thermal memory influence on the propagation of thermal perturbation. Focus is on optically opaque media. The derived models are compared to existing models neglecting the thermal memory effect. In this way it has been possible to determine the range of value of existing models and to indicate the additional employment of photothermal methods in determining through experimentation the thermal memory properties of solids. These properties have not as yet been experimentally determined in any medium, nor has the methodology for the experimental measurement of thermal memory parameters been suggested in the literature. Their recognition is highly significant not only for further fundamental research, but for many modern applications as well.
740(2004); http://dx.doi.org/10.1063/1.1843509View Description Hide Description
If swift heavy ions (5 MeV/u) hit solid targets energy is transferred to the electrons of the solid. Caused by the high electronic energy deposition along the flight path of the ion a cylindrical shaped chemical or structural defect cluster is formed. This kind of defect cluster together with its electronic and atomic precursors is called ion track. Ion‐track formation is mainly described by two different mechanisms: At the beginning of the ion‐track formation, within a timescale of 10−17 s, the energy is transferred into the electronic system and the electrons move away from the center of the track where a positively charged inner cylinder remains. If the charge‐neutralization time of these positive ions along the track is sufficiently long so that they can repel each other by Coulomb forces, the Coulomb‐explosion model can describe the track formation. Otherwise (short charge‐neutralization time) the energy is transferred into the atomic system by electron‐phonon coupling (within a picosecond timescale), what is included in the thermal‐spike model. During the energy transfer from the electronic to the atomic system atoms are set in motion and charged as well as neutral particles are sputtered. By detecting these particles energy resolved information on the track forming mechanism can be achieved.
Within the talk different experimental approaches to the ion‐track formation realized in the ion‐beam laboratory (ISL) of the Hahn‐Meitner‐Institut (HMI) are presented.
The electron dynamics takes place within a femtosecond timescale. Auger‐electrons leaving the target surface are suitable for investigating the short‐time dynamics because typical Auger‐decay times are in the order of a few femtoseconds. The width of the Auger‐profiles mainly reflects a convolution of the energy distribution of the electrons involved in the Auger process. An energy transfer from the projectile to the electronic system of the target causes a change of the energy distribution of the valence electrons and the shape of Auger‐profiles respectively.
For analyzing the atom dynamics (picosecond timescale) sputtered ions as well as neutral particles have to be detected. The desorbed ions are detected by a conventional energy resolved mass‐spectrometer. For the detection of the sputtered neutrals a new experimental setup is installed. The neutrals are ionized by non‐resonant multiphoton ionization with an amplified pulsed Ti:Sapphire laser and analyzed by a time‐of‐flight mass spectrometer.
New and Expanded Spectroscopic Databases of the National Institute of Standards and Technology (NIST)740(2004); http://dx.doi.org/10.1063/1.1843510View Description Hide Description
Three atomic spectroscopic databases of the U.S. National Institute of Standards and Technology are reviewed. First, our principal spectroscopic database, the Atomic Spectra Database (ASD), is discussed, which will soon be put on the internet in a new, entirely redesigned and expanded version 3.0. This new edition will include several additional critical compilations that were recently completed at NIST. Furthermore, two new smaller specialized databases will be reviewed, — a handbook of basic atomic spectroscopic data and a tabulation of data for soft x‐ray lines, tailored to the needs of x‐ray space observatories. Finally, an overview of our plans for new critical data compilations during the next two‐to‐three years will be presented.