Showing papers on "Ionization published in 1986"

Cross Sections for Collisions of Electrons and Photons with Oxygen Molecules

Abstract: Data have been compiled on the cross sections for collisions of electrons and photons with oxygen molecules (O2). For electron collisions, the processes included are: total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational, and electronic states, dissociation, ionization, and attachment. Ionization and dissociation processes are considered for photon impact. Cross‐section data selected are presented graphically. Spectroscopic and other properties of the oxygen molecule are summarized for understanding of the collision processes. The literature was surveyed through August 1987, but some more recent data are included when available to the authors.

Tunnel Ionization Of Complex Atoms And Atomic Ions In Electromagnetic Field

Abstract: The expressions for tunnel ionization probability of the complex atoms and atomic ions from arbitrary states in electromagnetic field are obtained. These expressions correctly describe the experimental data on rare gases atoms ionization in an infrared electromagnetic field.

Ionization potentials and electron affinities in aqueous solution

Abstract: By combining gas-phase proton affinities and aqueous pK/sub a/ values, it is possible to obtain free energies of hydration for a large number of cations and anions. With some corrections these numbers can be used to convert gas-phase ionization potentials, I, and electron affinities, A, into the corresponding aqueous solution values, I' and A'. An assumption must be made about the free energy of hydration of H/sup +/, taken as -259.5 kcal/mol. The absolute potential of the hydrogen electrode is then 4.50 V, and values of I' and A' can be converted to standard oxidation or reduction potentials. The absolute electronegativity, (I' + A')/2, in solution is estimated and found to be little different from the gas-phase value, (I + A)/2. The absolute hardness, (I'-A')/2, is much smaller than the gas-phase value, but it is shown that the solution value has little utility. The gas-phase value should be used even in solution.

Mass spectrometry of inductively coupled plasmas

Abstract: The alliance of the ICP with MS shows promise of extending the sensitivity and selectivity of MS to elemental analysis of solutions. Some additional areas of research interest for ICP-MS include the following: its use as an element and isotope selective detector for chromatography, flow injection analysis, laser ablation, and other sample introduction techniques; fundamental studies of ionization other basic processes in the ICP; detection of halogens and perhaps other elements as negative ions; and auxiliary ionization of neutral analyte atoms formed from ICP ions that have recombined with electrons during the extraction process. The latter concept is particularly interesting because the present detection limits for ICP-MS are achieved even though only one analyte ion is detected for every 10/sup 5/ or 10/sup 6/ that would be going through the skimmer if no recombination occurred. 42 references, 6 figures, 2 tables.

Ion—Aerosol Attachment Coefficients and the Steady-State Charge Distribution on Aerosols in a Bipolar Ion Environment

Abstract: Calculations of the ion—aerosol attachment coefficients are carried out for Fuchs' theory (as corrected in this paper) and for a theory which includes three body trapping. The resulting charge distributions agree quite well for particles with radii greater than about 0.007 μm. For smaller particles three-body trapping becomes increasingly important. Comparison of theoretically predicted charge distributions with recently measured charge distributions at radii smaller than 0.02 μm show good agreement. Asymmetric charging due to differences in the physical properties of positive and negative ions can result in large differences in the number of positively and negatively charged particles, particularly at larger radii. The asymmetric charge distribution is also shown to depend on the ionization rate. For the case when aerosol concentrations are comparable to the ion concentrations the effect of polydispersity on the charge distribution is difficult to predict. It is shown that a dominant size particle can es...

A Continuum Model of DC and RF Discharges

Abstract: A continuum model of direct current (dc) and 13.56-MHz radio-frequency (RF) discharges in a parallel-plate configuration is presented. The model consists of equations for electron and ion continuity, Poisson's equation for the self-consistent electric field, and an equation for electron energy. The equations are solved for a single set of conditions and a comparison is made between the structure of a dc and a 13.56-MHz RF discharge. One of the major structural differences is the emergence, under RF conditions, of large electron conduction currents in the quasi-neutral region. This results in substantially higher electron heating in the quasi-neutral region and a consequent shift in the peak ionization rate from the cathode sheath to the quasineutral region. In addition, the RF solutions suggest that equivalent circuit models and ambipolar diffusion models are promising ways to simplify predictions of discharge physics. The ultimate goal of this work is to provide an adequate description of the discharge physics so that discharge chemistry can be understood. The latter is essential in predicting the behavior of plasma film etching and deposition reactors used extensively in electronics materials processing.

Differential Cross Sections of Direct Single Electron Impact Ionization

Abstract: The experimental and theoretical situations for direct single ionization by electron impact are reviewed with special emphasis on triple differential cross sections for impact energies between several hundred eV and threshold. These cross sections can now be determined in absolute units, thus providing a powerful test of calculations. Only the target atoms helium and atomic hydrogen are considered, with quite simple wave functions, in order to focus attention on the correct description of the collision mechanism and to investigate the ranges of validity of various theoretical models. For the higher incident energies, processes with asymmetric energy partitioning and small momentum transfer are emphasized. For a long time these processes (which represent the predominant majority of all ionizing collisions) could not be calculated satisfactorily. Substantial progress in the theoretical treatment of the problem now allows the reproduction of the correct intensities and peak positions in the angular correlations of the outgoing electrons. It is now clear that double collisions, at least, have to be taken into account to describe the dynamics of these collisions. At lower energies both escaping electrons move rather slowly in the field of the ion core and exchange effects can no longer be neglected. Therefore, the theoretical situation is still rather involved. Close to threshold, recent triple differential cross section measurements permit a partial wave analysis, which should challenge theorists to explain these data by ab-initio calculations. The last chapter of this report deals with the progress in the determination of reliable absolute double differential cross sections.

Suppression of analyte signal by various concomitant salts in inductively coupled plasma mass spectrometry

Abstract: The interference on the ionization of cobalt by five salts, NaCl, MgCl/sub 2/, NH/sub 4/I, NH/sub 4/Br, and NH/sub 4/Cl, in an inductively coupled plasma (ICP) is first looked at theoretically, and subsequently the theoretical trends are established experimentally by mass spectrometry (ICP-MS). The interference trends are found to be in the order of the most easily ionized element in the matrix salt, i.e., Na > Mg > I > Br > Cl. Quantitatively the theoretical values for the amount of salt needed to produce a particular interference are 1-2 orders of magnitude higher than the experimentally determined values. The results reported here indicate that ICP-MS is somewhat more susceptible to ionization suppression effects than ICP atomic emission spectrometry. It is also found that the most easily ionized element in the salt dominates the matrix ion spectrum observed from the ICP in the order mentioned above. Total ion current measurements by ICP-MC at solute levels above 1% are complicated by orifice plugging and transport loss of the salt and analyte in the desolvation system for the ultrasonic nebulizer used. 27 references, 7 figures, 3 tables.

Transverse acceleration of oxygen ions by electromagnetic ion cyclotron resonance with broad band left‐hand polarized waves

Abstract: Central plasma sheet (CPS) ion conics are oxygen-dominated, with peak energies ranging from tens to hundreds of eV centered around pitch-angles between 115 and 130 degrees. Because of the lack of correlation between the CPS conics and the observed currents and/or electron beam-like structures, it is not likely that all of these conics are generated by interactions with electrostatic ion cyclotron waves or lower hybrid waves. Instead, it is suggested that the observed intense broad band electric field fluctuations in the frequency range between 0 and 100 Hz can be responsible for the transverse energization of the ions through cyclotron resonance heating with the left-hand polarized electromagnetic waves. This process is much more efficient for heating the oxygen ions than hydrogen ions, thus providing a plausible explanation of the oxygen dominance in CPS conics. Simple algebraic expressions are given from which estimates of conic energy and pitch angle can be easily calculated. This suggested mechanism can also provide some preheating of the oxygen ions in the boundary plasma sheet (BPS) where discrete aurorae form.

Supersonic cluster beams of III–V semiconductors: GaxAsy

Abstract: Supersonic beams of semiconductor clusters with the formula GaxAsy were generated by laser vaporization of a disc of pure GaAs mounted on the side of a pulsed supersonic nozzle. These cluster beams were characterized by laser photoionization with various fixed-frequency lasers followed by time-of-flight mass spectrometry. Mass analysis of the clusters with x+y>10 showed all clusters in the composition range from Gax+y through GaxAsy to Asx+y to be present in roughly the amount expected from a binomial distribution. In the smaller clusters strong variations were observed from this expected binomial distirbution as a result of kinetic effects in the cluster formation process. Photoionization with an ArF excimer laser at very low pulse energy revealed a pronounced even/odd alternation in the photoionization cross section of the GaxAsy clusters, depending only on the total number of atoms in the cluster. Clusters in the 5–21 atom range with an odd number of atoms were one-photon ionized by the 6.4 eV ArF excimer laser photons. This even/odd alternation in ionization properties of the clusters supports the view that the even clusters have fully paired singlet ground states with no dangling bonds. At higher ArF excimer laser fluences, the observed mass spectrum became increasingly affected by fragmentation. As is true with bulk GaAs surfaces, these GaxAsy clusters evaporate largely by the loss of arsenic (probably As2) when heated by the laser, leaving behind clusters which are richer in gallium.

Electron bombardment fragmentation of Ar van der Waals clusters by scattering analysis

Abstract: Arn clusters generated in a supersonic nozzle beam are investigated by exploiting their different kinematical behavior in a scattering experiment. In a crossed molecular beam experiment the Arn clusters are scattered from He atoms at E=69.7 meV. By measuring the angular and velocity distributions of the scattered clusters, the different species can be separated from each other. In this way fragmentation probabilities of small Arn clusters up to n=6 by electron impact ionization are measured. The influence of collisional dissociation of the dimer is accounted for by classical trajectory calculations. The dimer fragmentation is found to be f21=0.40±0.05, while the trimer totally fragments to dimers ( f32=0.7±0.3) and monomers ( f31=0.3). All clusters with n from 4 to 6 appear with a probability of 0.95 on the dimer ion mass.

Coupled electron energy and vibrational distribution functions in stationary N2 discharges

Abstract: The electron energy distribution function in N2 and the vibrational distribution function of N2 molecules in the electronic ground state were calculated self-consistently for steady state conditions by solving the Boltzmann equation coupled to a system of rate balance equations for the fibration levels including e-V, V-V, and V-T energy exchange processes. This formulation provides a relationship between the degree of vibrational excitation, as measured by a characteristic vibrational temperature theta , the ratio of the electric field to the gas density E/N, and the degree of ionisation ne/N. The values of theta have only a small dependence on E/N in the range 3*10-16-3*10-15 V cm2 and increase with ne/N from about 2*103K to 6*103K in the range 10-11

Ion Bombardment Secondaty Electron Maintenance of Steady RF Discharge

Abstract: The results of both theoretical and experimental study of a steady E-type RF discharge, in which ionization is controlled by fast electrons generated in RF sheaths near the electrodes (y regime), are presented. It is shown that the transition of an RF discharge into the ? regime is of a threshold nature and is accompanied by a sharp increase in plasma density and a decrease in electron temperature. The suggested theoretical model of an RF discharge in the ? regime satisfactorily describes experiments carried out in a wide range of frequencies, RF voltages, and gas pressures.

On the manifestation of electronic structure effects in metal clusters

Abstract: Potasium (K x x<40) and sodium(Na x , x<66) clusters were generated in molecular beams and probed by photoionizationmass spectroscopy. Results obtained include measurements of neutral cluster abundances and determinations of ionization potentials. The I. P. values can be rationalized in terms of a global electrostatic model which extrapolates to the bulk work function. This model also applies to transition metals as can be shown by comparng it to the limited experimental data sets available. The I. P.’s of small clusters exhibit ‘‘quantum size’’ effects which can be understood by individual quantum chemical calculations. As previously found for sodium clusters, photoionization mass spectra obtained for potassium, mixed potassium/sodium and potassium/lithium exhibit abundance maxima at M+ 8 and M+ 20, where M is an alkali metal. This has been interpreted in terms of increased thermodynamic stability of the corresponding neutrals relative to neighboring clusters. We present data which show that a spherical jellium model, while providing a set of numbers correlating well with those of preferred stability in alkali clusters, is less successful in explaining other properties.

UV laser excited fluorescence spectroscopy of the jet‐cooled copper dimer

Abstract: UV laser excited fluorescence spectra of the jet‐cooled copper dimer are reported. Cu2 is produced via laser vaporization of the metal inside the throat of a pulsed, supersonic nozzle. Strong fluorescence is observed upon excitation of 63Cu2 in the range 287–324 nm, corresponding to several v‘=0 bands of the II–X transition recently found in excitation using resonant two‐photon ionization [Smalley et al., J. Chem. Phys. 78, 2866 (1983)]. The II–X emission spectra are characterized by extremely long progressions in the ground state vibration, enabling us to measure G(v) and ΔGv+1/2 values over the range v=0–72. This data has been fit to yield improved vibrational constants for 63Cu2 X 1Σ+g: ωe =266.43±0.59 cm−1, ωexe =1.035±0.030 cm−1, ωeye =+(1.70±0.58)×10−3 cm−1, and ωeze =−(1.78±0.37)×10−5 cm−1. The G(v) data has also been fit to a near‐dissociation expansion which incorporates the correct limiting behavior of G(v), determined by the asymptotic inverse power contribution to the potential. This fit gives...

Highly ionized atoms in cooling gas

Abstract: The ionization of low density gas cooling from a high temperature was calculated. The evolution during the cooling is assumed to be isochoric, isobaric, or a combination of these cases. The calculations are used to predict the column densities and ultraviolet line luminosities of highly ionized atoms in cooling gas. In a model for cooling of a hot galactic corona, it is shown that the observed value of N(N V) can be produced in the cooling gas, while the predicted value of N(Si IV) falls short of the observed value by a factor of about 5. The same model predicts fluxes of ultraviolet emission lines that are a factor of 10 lower than the claimed detections of Feldman, Brune, and Henry. Predictions are made for ultraviolet lines in cooling flows in early-type galaxies and clusters of galaxies. It is shown that the column densities of interest vary over a fairly narrow range, while the emission line luminosities are simply proportional to the mass inflow rate.

Comet giacobini-zinner: in situ observations of energetic heavy ions.

Abstract: Conclusive evidence is presented for the existence of heavy energetic, singly charged cometary ions within 1.5 million km of comet Giacobini-Zinner. The most direct evidence for establishing the mass of these ions was obtained from an analysis of the energy signals in one of the solid state detector on board the International Cometary Explorer. Maximum fluxes were recorded approximately one hour before and after closest approach to the cometary nucleus. Transformation of the particle angular distribution observed at approximately 50,000 km radial distance from the comet, during the inbound pass into a rest frame in which the distributions are nearly isotropic, requires a transformation velocity that is consistent with the local solar wind velocity (assuming these particles are primarily singly ionized with a mass of 18 + or - 6 amu). It is implied that these water-group ions underwent strong pitch angle scattering subsequent to ionization and that they were accelerated or heated. The derived ion density, approximately 0.1/cu cm, is consistent with a crude model for the production and transport of pickup ions.

Electrical breakdown at semiconductor grain boundaries.

Abstract: Carrier transport through electrically active grain boundaries is studied under high-electric-field conditions. Electrons trapped at the interface and screened by ionized shallow and deep bulk defects are responsible for the formation of double Schottky barriers which reduce the carrier flow by several orders of magnitude. At large applied bias, electric fields up to 1 MV/cm can build up near the interface, leading to the generation of hot electrons and to the subsequent production of holes by impact ionization. This process is studied in a realistic model, taking the inhomogeneous field as well as longitudinal-optic and -acoustic phonon scattering into account. The hot-electron distribution function is calculated as the solution of a Fokker-Planck equation in energy space. With a reasonable estimate for the pair-creation rate we determine the yield for hole production near the interface. These minority carriers are swept back to the grain boundary where they serve as an additional screening charge for the electrons. We determine the steady-state and dynamic behavior of the barrier, including the holes in our calculations. The accumulation of holes at the interface can lead to the breakdown of the barrier which, in its most dramatic form, can even result in a bistability. The dynamic behavior of the barrier reflects the presence of holes through the development of a negative capacitance, in agreement with experimental observations. The negative capacitance is explained by the finite recombination time of the holes at the interface. In a model where the recombination is substituted by hole emission, a realistic current-voltage characteristic but no negative capacitance is found.

New quasi-Landau structure of highly excited atoms: The hydrogen atom

Abstract: A basically new structure of quasi-Landau resonances is discovered with the magnetized hydrogen atom around the ionization threshold. The resonances are theoretically rationalized by classical trajectory calculation. In energy spacing and corresponding resonance time they form a highly regularly organized series with, classically, an infinite number of members.

Determination of population and alignment of the ground state using two‐photon nonresonant excitation

Abstract: A method is presented for determining the population A(0)0, the quadrupole alignment factors A(2)0, A(2)1, A(2)2, and the hexadecapole alignment factors A(4)0, A(4)1, A(4)2, A(4)3, A(4)4 for a (v,J) ground state distribution of a diatomic molecule probed by linearly polarized two‐photon nonresonant excitation. General expressions are developed for the O, P, Q, R, and S branch transitions as a function of the rotational quantum number J. This treatment assumes that the resonant state reached by the two‐photon transition is subsequently detected independent of its alignment. This can be achieved by 2+n multiphoton ionization in which the ionization steps are saturated, or by 2+1 laser induced fluorescence in which the fluorescence is collected independent of its polarization and spatial anisotropy. To extract the population and the eight alignment parameters the line intensities must be measured for several polarization settings of the laser beam. However, when the ground state distribution has cylindrical ...

Enhanced stability of ion–clathrate structures for magic number water clusters

Abstract: The near threshold vacuum‐UV photoionization of water clusters has been performed by using a resonance line emission of argon at 1183 eV The well‐known intensity anomaly at the cluster ion (H2O)21H+ is observed even in this threshold photoionization, for the first time, with very small excess energy Structures for the water cluster ions (H2O)21H+ and (H2O)28H+ which exhibit enhanced structural stability (magic number), are presented based on Monte Carlo simulations as well as on the analogy of our previous study on the stability of the (H2O)20NH+4 ion The Monte Carlo calculations are carried out at the temperatures of 200, 150, 100, and 50 K for the ionized water clusters (H2O)nH+ around n=21 and also around n=28, which includes the ionic hydrogen‐bond interactions between an H3O+ ion and neutral H2O molecules The clusters (H2O)21H+ and (H2O)28H+ have greater binding energies per molecule than their neighbors although the enhancement of the latter is somewhat temperature dependent The calculations s

Classical ionization and charge-transfer cross sections for H+ + He and H+ + Li+ collisions with consideration of model interactions.

Abstract: The three-body system is analyzed in relation to the calculation of atomic scattering cross sections. A method is presented to generate the initial electronic conditions for the classical-trajectory Monte Carlo method in the case where the active electron is subject to non-Coulomb interactions. The method is then applied to study the collisions of H/sup +/ with He and Li/sup +/ targets in the intermediate- to high-energy range. Single-electron capture and single-ionization total cross sections are presented for both collision systems. In the case of He targets, total cross sections for double ionization and singly differential cross sections for free-electron production are also calculated. Cross sections and initial electronic distributions are obtained with both Coulomb and model interactions and compared. Good agreement is found between theoretical and experimental results, except for the double ionization of He.

A Survey of the Electron and Ion Transport Properties of SF6

Abstract: A compilation of the available data on some of the basic transport properties of sulfur hexafluoride (SF6) is presented. The properties considered are a) electron ionization, attachment, detachment and diffusion coefficients, and electron drift velocity; b) positive and negative ion mobilities, negative ion diffusion, and ion-ion recombination coefficients; and c) secondary ionization coefficients. Approximate analytical representations of the data are also given.

Photoionization thresholds of rare‐earth impurity ions. Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2

Abstract: The spectral dependence of the photoionization energy of Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2 systems have been measured and thresholds experimentally determined and compared with theoretical values calculated from electrostatic models. It is shown that the excited state absorption transitions or the persistent hole burning observed by other authors occur above the threshold energy of photoionization of the impurities and that the states of the crystal which form the bottom of the conduction band may play an important role in the strong probability of these processes. A review of thresholds now known is also given.

Theoretical investigation of spectra of multicharged ions of F-like and Ne-like isoelectronic sequences

Abstract: Relativistic perturbation theory was used for the total interaction between electrons to calculate distances between the levels 2p 3 2 , 2p 1 2 , 2s 1 2 in an F-like ion. Using Ne-like ions as an example, a new method has been developed to determine ionization potentials from experimentally found energies of 2–3 transitions. Relativistic perturbation theory with the zeroth approximationof the model was used to calculate energy levels for the configurations 1s22s22p5 3s, 3p, 3d, 1s22s2p6 3s, 3p, 3d in Ne-like ions. All calculations refer to nuclear charges (Z) of 16–80. Peculiarities of states with core vacancies are noted. Comparisons have been made between experimental and theoretical results for both the X-ray and the vacuum ultraviolet spectral regions.

A Model for the Bulk Plasma in an RF Chlorne Discharge

Abstract: A model has been constructed to describe the electrical characteristics of the central bulk plasma region in a 13.56-MHz parallel-plate discharge in chlorine at pressures of about 1 torr. This region is modeled as a volume-controlled plasma with the electron balance dominated by single-step electron-impact ionization and attachment and with the electron energy distribution function in equilibrium with the local instantaneous electric field. Relationships between the ionization frequency, the attachment frequency, the electron drift velocity, and the electric field are provided by solutions of the Boltzmann equation for mixtures of Cl2 and Cl which result from Cl2 dissociation. From a measured current waveform and Cl2/Cl density ratio, the model generates the local electric-field waveform, the time-varying electron density, and the power density in the central portion of the bulk plasma. The calculated time-averaged power input per unit discharge length compares well with experimentally determined values.

Pulsed laser desorption with resonant two-photon ionization detection in supersonic beam mass spectrometry

Abstract: Laser desorption is used as a means of volatilizing high-melting-point or thermally labile compounds into a supersonic jet expansion. The neutral desorbed species are carried into the acceleration region of a time-of-flight mass spectrometer (TOFMS) where they are ionized by resonant two-photon ionization with an ultraviolet laser. In this soft ionization process only the molecular ion is detected in the TOFMS for several large polynuclear aromatic hydrocarbons and nonvolatiles such as adenine and cytosine. The desorption process in this work uses a shock mechanism in thick samples in which approximately a monolayer is desorbed per laser pulse so that extended repetitive desorption can be obtained for each sample. The effect of different wavelengths for producing neutral molecules without extensive ionization-fragmentation processes occurring is also investigated. In addition, the translational cooling of desorbed neutrals in the supersonic expansion is investigated based upon the ion packed width detected in the TOFMS.