摘要:

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 H2and how a small fractional ionization of H and H2by 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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843491

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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, O2and 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, O2and 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 N2and O2in 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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843492

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843493

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843494

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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 &sgr;non the principal quantum numbernof the scattered projectile, as well as the total &sgr;totvalues are calculated for highly charged ions interacting with gaseous and solid targets in the energy range ofE= 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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843495

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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 &mgr;m) 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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843496

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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 asN2(A 3&Sgr;g+), N2(B3&Pgr;g),N2+(B 2&Sgr;u+)and electrons, which occur downstream from the discharge after a dark zone as a consequence of the V‐V up‐pumping mechanism. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843497

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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 strenghtsE/n0was 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 commonE/n0and common mean energy procedures provide excellent results even for electrons at moderateE/n0where 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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843498

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843499

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

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 C76fullerene 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. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1843500

出版商:AIP    

年代:1904

数据来源: AIP