Showing papers on "Ionization published in 1985"

X-ray secondary heating and ionization in quasar emission-line clouds

Abstract: Accurate Monte Carlo computations of the X-ray secondary electron heating, ionization, and excitation of H and He gas in interstellar space and in quasar emission-line clouds, are presented. The fraction of energy deposited in each form is sensitive to the background ionization fraction, x = n(H+)/n(Htot), and can affect the temperature, ionization state, and line emissivities at large depths in X-ray photoionized clouds. Analytic fits are provided for these energy fractions over the range 0.0001-1 for primary electron energies up to many keV. In both broad-line and narrow-line clouds, emission lines sensitive to the energy budget and electron density may be strongly affected.

Theory of negative corona in oxygen.

Abstract: Theoretical predictions are given of the development of the current and the distributions of charge and electric field in negative corona, or Trichel current pulses [G W Trichel, Phys Rev 54, 1078 (1938)], in oxygen at a pressure of 667 kPa (50 Torr) For a 10-mm-diam negative sphere located 20 mm from a positive plane, the calculated current pulse has a rise time of 11 ns, a pulse width of 50 ns, and a peak amplitude of 13 mA These results agree satisfactorily with experimental values The predicted velocity of the cathode-directed light pulse also agrees well with observations The theory is based on the accurate numerical solution of Poisson's equation in conjunction with the continuity equations for electrons, positive ions, and negative ions The effects of ionization, attachment, recombination, electron diffusion, and photoemission and ion secondary-electron emission from the cathode are all included The initial steep rise of the current pulse is largely due to rapid ionization and electron motion in the high Laplacian field near the cathode As the discharge develops, a dense plasma forms near the cathode, leading to strong space-charge distortion of the field A prominent cathode fall region is formed immediately adjacent to the cathode, an almost zero field is formed within the plasma and the field is enhanced over the region to the anode The current pulse is quenched because the low electric field in the plasma immobilizes the majority of the electrons which then undergo three-body attachment; furthermore, the cathode fall region becomes reduced to such a short distance that insignificant current is produced from this region Because of the low mobility of the negative ions, the current remains low and the structure of the space-charge fields changes only slowly with time between pulses

A two-sublattice model for molten solutions with different tendency for ionization

Abstract: Different models accounting for the introduction of an excess of cations or anions in an ionic melt are considered. A new model which describes the variation in the thermodynamic properties in a melt as it gradually changes its composition from purely metallic state to complete ionization is developed. The model is based upon Temkin’s expression for the configurational entropy of mixing. Neutral species and hypothetical vacancies with an induced charge are introduced on the anion sublattice. The induced charge is equal to the average valency of the species occupying the cation sublattice. When the tendency of ionization is weak, the model approaches the substitutional model. For binary systems the new model is formally identical to the associate solution model if the associates are defined in such a way that they contain one atom of the electronegative element. For higher-order systems the new model works with a lower number of composition variables and parameters than the associate solution model.

An updated evaluation of recombination and ionization rates

Abstract: Nouvelle evaluation des taux d'ionisation et de recombinaison par collision electronique des elements abondants en astrophysique

On convergence of computation of chemically reacting flows

Abstract: The computational problems associated with high-temperature flows undergoing finite-rate ionization reactions is investigated. The conservation equations governing chemical species and vibrational and electron energies are solved simultaneously with those for overall mass, momentum, and energy for a one-dimensional subsonic flow, through a constant-area duct, originating behind a normal shock wave, using an implicit time-marching technique. Boundary conditions are imposed in the form of characteristic wave variables accounting for the effects of chemical reactions on the speed of sound. Converging solutions are obtained for cases in which chemical reactions are weak, but difficulty is encountered in other cases. The cause of the difficulty is investigated and shown to be the sharp pressure disturbances produced by such reactions.

Correspondence between electron binding energy and chemisorption reactivity of iron clusters.

Abstract: New experiments in a fast-flow reactor have uncovered a strong correlation between the reactivity of free iron clusters and cluster ionization thresholds: Clusters with low thresholds efficiently add molecular hydrogen, and the relative rates of this reaction closely follow variations in cluster-electron binding energy. This correspondence can be understood in terms of a requirement for metal-to-hydrogen charge transfer in the activation of the ${\mathrm{H}}_{2}$ bond.

Populations in the metastable and the resonance levels of argon and stepwise ionization effects in a low‐pressure argon positive column

Abstract: The contribution of the ionization from the two metastable and the two resonance levels of argon to the total ionization rate in a low‐pressure argon positive column is investigated theoretically and experimentally. A simple yet self‐consistent discharge model is developed by coupling the balance between the electron total production and loss rates to the steady‐state rate balance equations for the excited states. The predictions of this model, i.e., the maintenance field for the positive column and the populations in the individual excited states as a function of the gas pressure and the discharge current are compared with the experiment (0.05

Ionization frequencies for solar cycle 21: Revised

Abstract: Detailed spectra of the extreme ultraviolet solar flux at the earth were provided by instruments on the Atmosphere Explorer satellites. These data have been used for aeronomical purposes in a large number of studies. An important parameter for such studies is the rate of production of various ions through the photoionization process. This parameter, known as an ionization frequency, is the integral over wavelength of the product of the solar flux and the cross section for the ionization of the particular constituent. Thus, the determination of the ionization rate is dependent on a good knowledge of the solar EUV intensities for the solar period in question. Over the past few years the EUV solar spectra that have been developed for use by aeronomers as reference spectra for such photochemical and ionospheric studies have been improved. The results of a redetermination of the most important ionization frequencies using the revised solar fluxes are reported. The impact is found to be more significant at solar maximum, amounting to a reduction of 12-21 percent in the ionization frequencies of the major terrestrial thermospheric constituents for solar-minimum conditions and 21-33 percent for solar-maximum conditions. The corrections are apart from the ongoing debate concerning the absolute intensity of EUV solar-flux measurements for the solar-cycle 21 maximum period.

Electron Production in Proton Collisions: Total Cross Sections

Abstract: Existing data on the ionization of neutral atoms and molecules by proton impact are reviewed, and electron production cross-section data are collected. The three major experimental methods are discussed and possible sources of error identified. Some theoretical cross sections are discussed, and well-established methods of relating them to measured cross sections are reviewed. A mathematical equation is fitted to the weighted experimental data for each target, and these fits are adjusted to be consistent with appropriate theoretical calculations and with electron impact and photoionization data. Recommended values of total cross sections for proton-impact ionization are given.

Semiconductor cluster beams: One and two color ionization studies of Six and Gex

Abstract: Supersonic beams of clusters of Si and Ge atoms have been produced by laser vaporization followed by supersonic expansion in a helium carrier. The cluster beams were characterized by F2(7.9 eV) and ArF(6.4 eV) excimer laser ionization accompanied by time-of-flight mass analysis. In addition, the feasibility of a resonant two-photon ionization (R2PI) spectroscopic study was explored by two-color experiments involving initial excitation with the second (2.36 eV) and third (3.54 eV) harmonics of the Nd:YAG followed by excimer laser ionization. All two-photon ionization processes were found to produce extensive fragmentation of the larger clusters. The observed fragmentation pattern for the silicon and germanium clusters were remarkably similar to each other, but drastically different from that seen for metal clusters in the same apparatus. Unlike metal clusters, which tend to lose one atom at a time, these semiconductor clusters appear to fragment by a fission process, the daughter ions falling almost exclusively in the size range from 6 to 11 atoms. Time delay studies in the two-color experiments established that clusters of both Si and Ge have excited electronic states with lifetimes of approximately 100 ns. This again is dramatically different from the behavior found with metal clusters, and indicates the feasibility of R2PI spectroscopy on these cold semiconductor particles. The existence of such long-lived excited states indicates that there is probably an energy gap between the band of electronic states being excited and the ground electronic state.

Recombinative desorption dynamics: Molecular hydrogen from Cu(110) and Cu(111)

Abstract: The rotational and vibrational distributions of H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces following atomic permeation are studied using multiphoton ionization combined with time‐of‐flight mass spectrometry. Rotational distributions are found to be non‐Boltzmann and to possess mean rotational energies which are 80%–90% of the surface temperature, Ts. These distributions are identical to within the experimental accuracy for H2 and D2 and also for desorption from the (110) and (111) faces. Moreover, the ortho and para nuclear spin modifications of both isotopes are statistically populated. In contrast, the vibrational population ratio, Pv‘=1/Pv‘=0, is found to be as much as 100 times greater than the ratio corresponding to a Boltzmann vibrational population at Ts. Specifically, the Pv‘=1/Pv‘=0 ratio for H2 (D2) is 0.052±0.014 (0.24±0.20) desorbing from Cu(110), and 0.084±0.030 (0.35±0.20) desorbing from Cu(111). For comparison the Boltzmann‐at‐Ts ratios would be 0.0009 for H...

Pressure Ionization, Resonances, and the Continuity of Bound and Free States

Abstract: Publisher Summary This chapter discusses several techniques and addresses several questions concerning the continuous evolution of bound eigenstates into free states under the influence of increasing pressure. As bound states move into the continuum, they evolve into low-energy scattering resonances; these shape resonances occur only at high densities where there is strong screening of the atomic potential. Electrons which occupy resonance states are neither strictly localized nor totally free. For short times the resonance electrons are trapped by the centrifugal barrier; over longer times they escape to large distance. Pressure ionization, resonances, and the continuity principle are linked because the scattering resonances interpolate smoothly between localized bound states and freely moving continuum states. Many plasma properties, including equations of state, electrical conductivity, bremsstrahlung emission, and x-ray opacities are strongly affected by resonances. Even the meaning of the plasma ionization state is clarified by understanding the physical character of resonance states. The continuity theorem implies that there is no discontinuous boundary between low- and high-density behaviors.

Atomic energy levels of the iron-period elements, potassium through nickel

Abstract: Experimentally derived energy levels of the elements from potassium to nickel in all stages of ionization are critically compiled. The data for each level include its position in /cm (relative to the ground state), configuration, term designation, J-value, and, where available, the g-value and two leading percentages of the eigenvector composition in the most appropriate coupling scheme. For the He I and H I isoelectronic sequences, calculated level positions are given because they are considered more accurate than the measurements presently available. Ionization energies for each ion are derived either from Rydberg series, extrapolation, or calculation. Complete references are given for the compiled data.

Quantum yield of electron impact ionization in silicon

Abstract: p‐channel Si‐gate metal‐oxide‐semiconductor transistors of very thin oxides are used for the study of quantum yield of electron impact ionization in silicon. Electrons are injected into silicon from the polysilicon gate by tunneling to give an approximate δ‐function energy distribution. This energy distribution is preserved when electrons pass through the oxide by direct tunneling. Using the carrier‐separation properties of the induced junction, we are able to experimentally measure the number of generated electron‐hole pairs as a function of the incident electron energy, up to 5 eV. Our results are found to be in excellent agreement with recent theoretical calculations of quantum yield. Beyond 5 eV, the interpretations on the experimental data are difficult due to the broadening of the incident electron energy distribution. This broadening effect is caused by strong scattering in the oxide when electrons tunnel by the Fowler–Nordheim (F–N) process. It is observed that the average energy of those electron...

Mechanisms for multiple ionization of atoms by strong pulsed lasers.

Abstract: It is shown that substantial ionization during the rise of the pulse of a strong laser is inevitable. Thus only multiply charged ions are likely to be exposed to fields above ${10}^{14}$W/${\mathrm{cm}}^{2}$. The behavior of the $N$-photon generalized cross section for $N\ensuremath{\gg}1$ and its connection to tunneling are discussed.

s-wave threshold in electron attachment - Observations and cross sections in CCl4 and SF6 at ultralow electron energies

Abstract: The threshold photoionization method was used to study low-energy electron attachment phenomena in and cross sections of CCl4 and SF6 compounds, which have applications in the design of gaseous dielectrics and diffuse discharge opening switches Measurements were made at electron energies from below threshold to 140 meV at resolutions of 6 and 8 meV A narrow resolution-limited structure was observed in electron attachment to CCl4 and SF6 at electron energies below 10 meV, which is attributed to the divergence of the attachment cross section in the limit epsilon, l approaches zero The results are compared with experimental collisional-ionization results, electron-swarm unfolded cross sections, and earlier threshold photoionization data

Ionospheric structures in the polar cap: Their origin and relation to 250‐MHz scintillation

Abstract: An investigation of the polar cap ionosphere near the peak of the last solar cycle identified polar cap F layer arcs and ionization patches as unique features of the polar cap ionosphere, and as sources of severe scintillations observed on 250-MHz satellite beacon signals. The continuing investigations in January and December 1983 and January 1984 have shown that arcs and patches persist as the dominant features of the winter polar cap ionosphere during periods of low sunspot numbers. Improved ionospheric soundings made at Thule, Greenland (86°CGL), showed a clear diurnal variation for the occurrence of the patch-type ionization. Discussion of various possible mechanisms producing the observed ionization patches leads to the conclusion that the solar produced ionosphere equatorward of the dayside cusp is the source region of the ionization patches. Polar plasma convection transports this ionization across the cusp and the central polar cap. The local time dependence of the occurrence of the patches at Thule is shown to be a manifestation of the well-known universal time control of the polar cap F region. A strong positive solar cycle dependence of the scintillations was measured during three extended campaigns and confirms earlier measurements. The diurnal variation of scintillations is almost flat at solar maximum and has a local time variation very similar to that of the patch type ionization at solar minimum. Both arcs and patches contribute to substantial scintillations around solar maximum, while only the patches are responsible for the considerably weaker scintillations during solar minimum.

Multiphoton ionization of ammonia clusters and the dissociation dynamics of protonated cluster ions

Abstract: Ammonia clusters, produced via supersonic expansion, are subjected to multiphoton ionization using a tunable pulsed laser. The products of ionization and subsequent dissociation processes are investigated using a time‐of‐flight mass spectrometer equipped with a reflectron (reflecting electric field). Cluster ions containing more than 65 ammonia molecules were formed via internal ion molecule reactions taking place within the initially ionized aggregate. The proton transfer reaction which ensues leads to substantial excess internal energy and subsequent cluster decomposition. Dissociation due to both collisional and unimolecular processes was identified in the field‐free region. Studies of the dynamics of dissociation were made for species up to the protonated 16‐mer; the loss of up to at least five monomer units following cluster ion formation was observed. Results of an investigation of the influence of laser fluence and the energy of the ionizing photons are also presented.

The Charge-Energy-Mass Spectrometer for 0.3-300 keV/e Ions on the AMPTE CCE

Abstract: The CHEM spectrometer on the CCE spacecraft is designed to measure the mass and charge-state compositions as well as the energy spectra and pitch-angle distributions of all major ions from H through Fe with energies from 0.3 to 300 keV/charge and a time resolution of less than 1 min in the Earth's magnetosphere and magnetosheath. It has the sensitivity and resolution to detect artificially injected Li ions. Complementing the hot-plasma composition experiment and the medium-energy particle analyzer, this experiment will provide essential information on outstanding problems related to dynamical processes of space plasmas and of suprathermal ions. The instrument uses a combination of electrostatic deflection, post acceleration, and time of flight versus energy measurements to determine the ionization state Q, mass M, and energy E of the ambient-ion population. Pitch angle and anisotropy measurements are made utilizing the spinning motion of the CCE spacecraft. Isotopes of hydrogen and helium are resolved as are individual elements up to neon and dominant elements up to iron. Because of the intrinsically low instrument background achieved by using fast coincidence techniques combined with electrostatic deflection, the instrument has a large dynamic range and can identify rare elements and ions even in the presence of high-intensity radiation background. To increase significantly the information returned from the experiment within the allocated telemetry, an intelligent on-board data system which is part of the CHEM instrument performs fast M versus M/Q classifications.

Threshold shift and above-threshold multiphoton ionization of atomic hydrogen in intense laser fields.

Abstract: Accurate ab initio nonperturbative L/sup 2/ non-Hermitian Floquet calculations for intensity-dependent threshold shifts and ground-state total ionization widths (rates) for one-, two-, and three-photon-dominant intense-field ionization of atomic hydrogen are presented. The results show the importance of both the ac Stark shift and the pondermotive potential in the determination of the net threshold shift. In addition, branching ratios to individual continua have been estimated, yielding physical insights regarding the general features and mechanisms of the frequency- and intensity-dependent continuum-continuum transitions and ''peak switching'' phenomena in the above-threshold ionization processes.

The abundance of atomic carbon near the ionization fronts in M17 and S140.

Abstract: We have observed the 492 GHz ground-state line of atomic carbon in the edge-on ionization fronts in M17 and S140. We find that, contrary to expectation, the C_I emission peaks farther into the molecular cloud from the ionization front than does the CO. In fact the peak C_I abundance in M17 occurs more than 60 mag of visual extinction into the cloud from the ionization front. Calculations of the ratio of C_I to CO column densities yield values of 0.1-0.2. These observations do not support chemical models which predict that neutral atomic carbon should be found only near the edges of molecular clouds. Other models are discussed which may explain the observations.

Photodissociation and photoionization

Abstract: Multiphoton Ionization of Gaseous Molecules Laser Isotope Separation by the Selective Multiphoton Decomposition Process Photoionization in Non-Polar Liquids Photoelectron Spectroscopy of Excited States Predissociation of Polyatomic van der Waals Molecules Ion Photofragment Spectroscopy the Franck-Condon Principle in Bound-Free Transitions Theoretical Aspects of Photodissociation and Intramolecular Dynamics Quantum Theory of Molecular Photodissociation Index.

Theory of resonantly enhanced multiphoton processes in molecules

Abstract: In this paper we formulate a theory for the analysis of resonant enhanced multiphoton ionization processes in molecules. Our approach consists of viewing the (n+m) photon ionization process from an isotropic initial state as m‐photon ionization out of an oriented, excited state. The orientation in this resonant state, which is reached by n‐photon excitation from the initial state, is nonisotropic, and is characteristic of this absorption process. The ionization simply probes this anisotropic population. The calculation of the REMPI process thus consists of determining the anisotropy created in the resonant state and then coupling this anisotropic population to ionization out of it. While the former is accomplished by the solution of appropriate density matrix equations, the latter is done by coupling these density matrix elements to angle‐resolved ionization rates out of this state. An attractive feature of this approach is that the influence of saturation effects, and other interactions, such as collisions, on the photoelectron properties is easily understood and incorporated. General expressions are derived for photoelectron angular distributions. Based on these, several properties of the angular distributions that follow purely on symmetry considerations are discussed. One of the new features that emerge out of this work is the saturation induced anisotropy in REMPI. In this effect higher order contributions to the angular distributions appear since saturation influences different ionization channels differently thereby creating an additional anisotropy in the excited state.

Cross sections for ionization of water vapor by 7-4000-keV protons.

Abstract: Cross sections for production of electrons and positive ions by proton impact on water vapor have been measured from 7\char21{}4000 keV by the transverse-field method.

Physics of small metal clusters: Topology, magnetism, and electronic structure.

Abstract: The electronic structure of small clusters of lithium atoms has been calculated using the self-consistent-field, molecular-orbital method. The exchange interaction is treated at the unrestricted Hartree-Fock level whereas the correlation is treated perturbatively up to second order by including pair excitations. This is done in two steps, one involving only the valence electrons and the other including all the electrons. A configuration-interaction calculation has also been done with all possible pair excitations. The equilibrium geometries of both the neutral and ionized clusters have been obtained by starting from random configurations and using the Hellmann-Feynman forces to follow the path of steepest descent to a minimum of the energy surface. The clusters of Li atoms each containing one to five atoms are found to be planar. The equilibrium geometry of a cluster is found to be intimately related to its electronic structure. The preferred spin configuration of a cluster has been found by minimizing the total energy of the cluster with respect to various spin assignments. The planar clusters are found to be less magnetic than expected by Hund's-rule coupling. For three-dimensional clusters, however, the magnetism is governed by Hund's rule. The effect of correlation has been found to have decisive influence on the equilibrium topology and magnetism of the clusters. The binding energy per atom, the energy of dissociation, and the ionization potential of the clusters are compared with experiment and with previous calculations. The physical origin of the magic numbers and the effect of the basis functions on the calculated properties have also been investigated.