Ionization Processes In Collisions Research Articles

The Uranian corona as a charge exchange cascade of plasma sheet protons

Models of magnetospheric convection and interparticle interactions are used in a study of the collisional interactions between atmospheric neutral hydrogen and magnetospheric charged particles observed by Voyager to be convecting through the Uranian magnetosphere. The most important reactions are e−‐H collisional ionization and p+‐H charge exchange with the atmosphere. The e−‐H collisional ionization process, continually reenergized by compressional heating of the electrons as they drift toward Uranus, produces a cascade of new plasma. This process has been suggested by many authors as the source of the warm (10 eV at L = 5) plasma and is found by this work to continue in a cascade to even cooler and more abundant plasma. This newly created plasma consists almost entirely of electrons and protons because He and H2 are nearly absent from the very upper layers of the atmosphere. The densities computed for the warm plasma are within an order of magnitude of the values observed by Voyager. The cold plasma should occur only near or sunward from Uranus, where it is too cold to have been detected by Voyager on its inbound trajectory. Low‐altitude p+‐H charge exchange between the atmosphere and the new plasma produces a significant flux of hot H. This hot H may comprise the Uranian H corona, and its production serves as a high‐altitude thermospheric heat source, both of which are implied by Voyager UVS observations (although this heat source is apparently inadequate to produce the high thermospheric temperature that was observed). The generation of significant new plasma near Uranus leads to the following speculation: If this plasma crosses the dayside magnetopause and mixes with magnetopause boundary layers such as the plasma mantle, there to be swept back along the magnetotail, reincorporated into the magnetotail by the same processes postulated for solar wind plasma entry, and reenergized in the magnetotail current sheet, it would constitute an important source for the hot plasma (1 keV at L = 5) observed by Voyager.

Ion energy distributions on surfaces exposed to plasmas: An experimental and theoretical investigation

The plasma sheath is a space region between a plasma and the surface of a solid next to the plasma. The plasma sheath influences the plasma assisted deposition processes of thin films. It is also a determining factor for particle extraction as a plasma diagnostic tool. Ions were extracted from a plasma, produced by a dc hollow cathode discharge. The extracted ions were mass analyzed and simultaneously their energy distributions were measured. During each experiment, the discharge parameters were kept constant, whereas the sheath voltage and therefore the thickness of the sheath in front of the extraction orifice was varied. In this way the influence of the sheath on the composition of the ion flux and on the energy distributions of the various ion species could be observed. The gases used were hydrogen, hydrogen-argon, and hydrogen-silan mixtures in the pressure range of 0.08–1 mbar. The ion current densities were about 1 mA/cm2 and the sheath voltages were varied from floating potential to 160 V. A numerical model was developed to calculate the potential distribution in the sheath and the energy distributions of the various ion species. These calculations were done by an iterative method to obtain a self consistent solution. By comparing the results of the calculations with the measured ion energy distributions and ion intensities, conclusions about collision processes of ions and neutrals could be made. The investigations show that due to collision processes, the composition of the ion flux passing the sheath is considerably changed and the energy distributions of the various ion species can be explained by a few simple collision processes.

Pulsed discharge source of supersonically cooled transient species

A source combining dc discharge with pulsed supersonic expansion is shown to be a versatile tool for spectroscopic studies of a variety of transient species. High concentrations of cold small radicals have been generated using suitable precursors, and detected by laser induced fluorescence. Discharging of halogenated methanes provides an efficient source of halogenated carbenes. Molecular radical cations are similarly efficiently produced, probably as a result of the Penning ionization process in collisions with metastable rare-gas atoms. Reactive sputtering of electrodes is shown to result in the production of metal halides and similar simple metal compounds. An application of the discharge source as a useful tool for matrix isolation studies is proposed and discussed.

Electron transfer collision studies on stored ions produced by synchrotron radiation

Studies of electron transfer collisions of low energy stored ions with target atoms have been carried out as a function of the well depth of the Penning ion trap. The ions were produced by vacancy cascades following inner-shell photoionization by synchrotron radiation. The ion storage data, following an analysis based on the details of the ion production, storage, and collision processes in the trap, provides information for optimizing the accuracy of electron transfer rate coefficient measurements when several charge state are present. Low trap well depths exceeding a well-defined minimum value are the most useful.

Optical Spectroscopic Studies on Penning Ionization and Charge-Transfer Reactions at Thermal Energy by Using Flowing Afterglow

The flowing afterglow apparatus has been coupled with a low pressure chamber by a small orifice for optical spectroscopic studies on Penning ioniza­ tion and charge-transfer reactions under single collision conditions. Nascent rovibrational distributions are determined from a spectral simulation. As selected examples, detailed results are reported about non-Franck-Condon type He(2 3 S)/02 Penning ionization and near-resonant He+ /H20, D20 charge-transfer reactions. A brief summary is also given of our optical spectroscopic studies by using the conventional flowing afterglow apparatus. The flowing afterglow (FA) method was developed in 1963 by Ferguson and his co-workers of NOAA Laboratories in Boulder, Colorado, for the study of ion-molecule reactions in the earth's atmosphere.o The main emphasis of their studies was placed upon measurements of formation rate constants and branching ratios of ionic products by using a mass spectrometer. More than a few thousands kinetic data at (near) thermal energy were compiled in the reported tables. 2 ) In our laboratory, the FA coupled with a DV and visible emission detection system has been used for studying the following ionization processes in collisions of metastable atoms (M*) and ions (M+) with a molecule (AB): M*+AB~ABH+M+e-, (Penning ionization) (1) ~ A H + B + M + e-, (Dissociative Penning ionization) (2)

Ionization processes in ground-state alkali-noble gas collisions. I Na-Ne

The authors report the results of an experimental study of ionization processes in collisions between fast sodium atoms and thermal neon atoms in a crossed-beam experiment (250, 289 and 392 eV laboratory collision energies). Energy loss spectra of scattered ions at selected scattering angles as well as differential cross sections for some specific ionization channels have been measured. Observed structures in the energy loss spectra are ascribed to scattered Na+ ions and recoiled Ne+ ions. The results are interpreted qualitatively using a molecular orbital (MO) correlation diagram. Most of the Ne+ structures are ascribed to ion pair exit channels of the type Na-(*)+Ne+(*) and to autoionization of doubly excited Ne** states. These exit channels arise from a single or double electron promotion along the 4f sigma MO, correlating with 2pNe in the separated atom limit. Na+ production is believed to arise from an L-vacancy-sharing mechanism following a double 4f sigma promotion.

Ionization processes in ground-state alkali-noble gas collisions. II. K-Ar

For pt.I see ibid., vol.24, no.2, p.439-58 (1991). The authors present the results of an experimental investigation of ionization processes in collisions between fast potassium atoms and thermal argon atoms. The experiments were performed in a crossed-beam apparatus at laboratory collision energies of 100-400 eV. Energy loss spectra of scattered ions at selected scattering angles as well as differential cross sections for some specific ionization channels have been measured. Observed structures in the energy loss spectra are ascribed to K+ ions and recoiled Ar+ ions. In some cases the analysis is complicated due to overlap of projectile and target structures in the spectra. The Ar+ structures are attributed to ion-pair formation of the type K-(3p6nln'l')+Ar+(3p5) and to autoionization of doubly excited Ar**(3p4nln'l') states. Molecular autoionization or Penning ionization in the exit channel might also play a role. Formation of excited ion-pair states like K-(3p6nln'l')+Ar+*(3p4nl) appeared to be unimportant, in contrast with the results of a previous study of similar processes in Na-Ne.

Collective kinetic energy in the heavy ion collision process

Collective kinetic energy in the heavy ion collision process

Observation of collisional ionization of chromium atoms in high Rydberg states and the N4 scaling relation

We have used mass selected multiphoton dissociation/excitation/collisional ionization (MPD/MPE/CI) spectroscopy to study benzene chromium tricarbonyl (BCT) in an effusive molecular beam under collisionless and collisional conditions. Under collisionless conditions, our results are entirely consistent with, but not identical to, our earlier nonmass selected published experiments under bulk gas conditions. The spectroscopic results confirm that we can produce chromium atoms in a variety of well defined Rydberg states (7S3[Ns3d5], N=30–43). Following the state selective preparation of the chromium Rydberg atoms, by MPD of BCT followed by MPE of the ground state chromium atoms, they can be allowed to collide with ground electronic state benzene chromium tricarbonyl molecules producing ground state chromium ions and other products. We observe the scaling of the cross section for this collisional ionization process as a function of the Rydberg electron principal quantum number. The cross sections are a strong function of the Rydberg electron principal quantum number N, even when N is very large. Scaling as N4, the cross sections suggest that dipolar interactions with molecular colliders can be quite efficient at inducing collisional ionization of Rydberg atoms.

Electron removal from molecular hydrogen by fully stripped ions at intermediate energies.

A classical phase-space model of the hydrogen molecule is presented and applied to the study of the electron-capture and -ionization processes in collisions of fully stripped ions with ${\mathrm{H}}_{2}$ at intermediate impact energies and charge states from 1 to 10. The model is based on the independent-electron and the impact-parameter approximations. The electron--impinging-ion and electron--target-nuclei interactions are exactly taken into account. The interaction between the electrons is approximated by model potentials. The calculated total cross sections for production of free electrons and capture of one electron are in good agreement with different experimental data. The ratio between the capture cross sections from molecular and atomic hydrogen targets is also analyzed and compared with available empirical scalings. It is found that this ratio varies from a value less than 1 at low impact energy to 4 at higher energies. The reasons for these differences are discussed. A comparison is made between the capture cross sections for different orientations of the hydrogen molecule.

Dynamical features of medium energy heavy ion collision processes

A semiclassical dynamical simulation for medium and highenergy heavy ion collisions is presented and used to discuss in detail the features of evolution for 12C(25GeV; b = 0.0fm)+63 Cu reaction. From the calculated results it is shown that the model proposed can give a reasonable picture of the compression, expansion and fragmentation processes of the system in heavy ion reaction.

Photo‐ and Electro‐Production of a New Light Boson

AbstractIn GSI experiments, a light pseudoscalar boson of mass Φ 1.6 MeV may have been observed. In order to examine this possibility from a different aspect we propose to study the photo‐and electro‐production process of Φ‐particles. We calculate the cross section of neutral and charged Φ‐particles. We examine the possible role of these production mechanisms in the heavy ion collision processes.

Stored multi-charged ion research: current status and plans

Low energy multi-charged ions have been produced in Penning ion traps by electron-impact ionization, and as recoil ions from fast, highly-charged heavy ion impact on target gases. Studies of charge transfer from gases and hydrogen atoms to these ions have been completed and are being extended. New multi-charged ion production methods using photoionization by synchrotron radiation, and ionization of atomic beams, are being considered. Certain potential precision spectroscopy measurements on ions stored in radio-frequency traps are also being developed. These new measurements and techniques are expected to be difficult to implement, but may lead to significant advances in the understanding of multi-charged ion collision processes at near-thermal energies, and to spectroscopic precisions not previously possible with high charge states.

Collisional ionization under strong laser irradiation

The main laser-induced collisional ionization processes are examined according to the shape and the relative position of the input and output channel potential curves. The different cases are illustrated by a few systems under experimental investigation, with emphasis on beam experiments.

The cometary atmosphere and its interaction with the solar wind

The present state of both our knowledge and our understanding of cometary atmospheres and their interactions with the solar wind is critically reviewed. Following the introduction (section 1) the spectrophotometry of the cometary atmosphere and the tail is reviewed in section 2. The existing, rather fragmentary, knowledge about atmospheric abundances and the production rates of various molecular species is given in section 3. The deductions that can be made from these values about the chemical composition of the nucleus and their uncertainties are discussed. Existing models of the cometary atmosphere including the ionosphere are critically evaluated in section 4. The continuing dichotomy between models that concentrate on the chemistry while oversimplifying the dynamics and those that concentrate on the dynamics and thermodynamics while minimizing the chemistry is discussed. The need to combine these two approaches in the future is emphasized. The ionization processes in the cometary atmosphere are discussed in section 5. It is emphasized that besides photoionization and charge exchange ionization with the solar wind protons, other (collisional) ionization processes may play a role in the rapid ionization of cometary neutrals in certain situations. The overall nature of the solar wind‐comet interaction is discussed in section 6 with special emphasis on the strong coupling between the solar wind and cometary plasmas, which is responsible for the observed accelerations in the cometary tails. The complex and time‐varying fine structures observed in the plasma tails of comets are discussed next in section 7. The uses of these to infer the strength and morphology of the magnetic field and the associated currents in the plasma tail on one hand, and the varying conditions in the solar wind on the other, are emphasized. The detailed models of the comet‐solar wind interaction, from the earlier fluid dynamic models to the latest MHD models are critically evaluated in section 8, and their limitations are underscored. It is also pointed out that the nature of the solar wind interaction with the cometary atmosphere is highly variable and that in certain circumstances the nature of both the outer and the inner shocks could change. Also, the need for a detailed study of the solar wind interaction with the fledgling cometary atmosphere at larger heliocentric distances is stressed. The review is concluded with section 9, where the emphasis is on the outstanding problems of cometary atmospheres and their interactions with the solar wind.

Decay of fast ion structure in the ionosphere

Both proton auroral and downward flowing ion events have significant fast ion components. For these phenomena we demonstrate that charge exchange and collisional ionization processes can exert a strong effect on the imaging of ionized density structure to the lower ionosphere. This has value in relating ionized density spectral measurements at altitudes of the F region maximum and below to plasma behavior at higher altitudes, as well as providing initial conditions for locally driven plasma instabilities. It is shown that charge exchange of fast ions into fast neutrals results in the rapid attenuation of structure with neutral mean free paths much larger than the structure dimensions and the generation of structure of larger size. For structure with dimensions much larger than the neutral mean free path, fast neutral loss is diffusive rather than convective and attenuation of structure is less rapid.

Effect of collisional transitions on the population distributions of excited atoms in a plasma

The effect of collisional transitions on the population distribution of excited atoms has been studied by considering a plasma in which the electron density varies with time. It has been found that for highly excited states, for which the ionization rate is much larger than that of electron-density variations, the collisional transitions are not important compared with the collisional ionization and recombination processes. However, for intermediate and lower excited states, they play an important role in making the population distribution approach Saha equilibrium with the electrons.

Energy dependence of differential cross sections in endoergic ion-molecule collision processes of negative ions

For laboratory kinetic energies between 0.2 and 100 eV the energy dependence of differential cross sections in endoergic ion-molecule collision processes of the negative halide ions F −, Cl −, Br − and I − on O 2 has been measured. With F − no reactions have been observed, whereas with the other ions charge exchange as well as ion-molecule reactions were found. Ion-molecule reactions yield the product ions O −, ClO −, BrO − and IO −. For the last three ions the experimental thresholds obtained were used to evaluate their electron affinities. Considering the influence of Doppler broadening of the effective reaction energy one obtains EA(IO) > 2.1 ± 0.3 eV, EA(BrO) > 1.5 ± 0.2 eV, EA(ClO) > 1.6 ± 0.2 eV. The O − product ion could be observed with the primary ions I − and Br −. The associated energy dependent cross sections show structure which can be related to excited states of the reaction products.

Ionic collision processes in simple gas mixtures containing hydrogen: K R Ryan,J Chem Phys, 61 (4), 1974, 1559–1570.

Ionic collision processes in simple gas mixtures containing hydrogen: K R Ryan,J Chem Phys, 61 (4), 1974, 1559–1570.

Ionic collision processes in mixtures of hydrogen and rare gases: K R Ryan and I G Graham, J Chem Phys, 59 (8), 1973, 4260–4271

Ionic collision processes in mixtures of hydrogen and rare gases: K R Ryan and I G Graham, J Chem Phys, 59 (8), 1973, 4260–4271