Showing papers on "Ionization published in 1972"

The distribution of gains in uniformly multiplying avalanche photodiodes: Theory

Abstract: Expressions are derived for the probability P_{n,m} that a pulse initiated by n electrons (or holes) in a uniformly multiplying semiconductor diode will result in a total number of electrons (or holes) m , to give a gain m/n , and for the probability Q_{n,m} that the gain will be m/n or greater. It is shown that the distributions are far from Gaussian. The gain distribution P_{1,m} for a single photoelectron, for example, is shown to have a maximum value for m = 1 for any value of the average gain M=m/n . The derivations are valid for any electric field distribution and assume only that the hole ionization coefficient \beta(E ) can be approximated by the relation \beta(E) =k\alpha(E) , where \alpha(E) is the electron ionization coefficient and k is a constant. A method of determining an effective value of k , for cases where \beta=k\alpha is not a good approximation, is presented. The results can be used to calculate the average gain and the mean square deviation from the average, giving results in agreement with previously published relations [1], [2]. The implications of this theory on the use of avalanche diodes for low-level photodetection are discussed. It is shown that in the near infrared, cooled avalanche photodiodes can compare favorably with the best available photomultiplier when used either in a photon-counting mode, or for the reliable detection of low-level laser pulses.

Electromagnetic instabilities produced by neutral-particle ionization in interplanetary space

Abstract: This article examines the consequence of the ionization (e.g., photoionization) of a small population of neutral atoms (hydrogen or helium) in interplanetary space. It is found that, even if the density of the newly ionized particles is only a very small fraction of that of the solar wind, these particles can efficiently excite electromagnetic waves by means of a new collective instability. The instability is driven by an anisotropy in kinetic energy of the newly ionized particles. The typical linear growth rate is γ ≃ (ωin/2½) (υ0⊥/c) where ωin is the plasma frequency of the newly ionized ions, and υ0⊥ is the characteristic speed of the newly ionized ions perpendicular to the ambient magnetic field in the frame of the solar wind.

Heating of toroidal plasmas by neutral injection

Abstract: Presents a brief review of the physics of ion acceleration, charge exchange and ionization, trajectories for fast ions in toroidal magnetic fields, and fast-ion thermalization. The injection of fast atoms is found to be a highly competitive method both for heating present-day experimental toroidal plasmas and for bringing full-scale toroidal CTR plasmas to low-density ignition.

Electron Transmission Spectroscopy: Rare Gases

Abstract: A transmission experiment is used to observe structure in the total electron-impact cross section for He, Ne, Ar, Kr, and Xe below the respective ionization potentials, and also in the region of autoionizing states The positions of the resonances are tabulated and compared with the results of other investigators In both neon and argon, relatively large isolated resonances exist near the edge of the Rydberg series involving inner-shell autoionizing states, ie, near 475 and 29 eV, respectively

Influence of atom--atom collisions on the collisional-radiative ionization and recombination coefficients of helium plasmas.

Abstract: The collisional-radiative recombination and ionization coefficients have been calculated for hydrogen plasmas of low ionization degree. The method consists in solving a coupled system of rate equations which not only accounts for the electronic but also for the atomic collision processes.

Localized and Delocalized 1s Hole States of the O 2 + Molecular Ion

Abstract: The hole states of O 2 +, obtained by ionization of the oxygen molecule, have been examined theoretically in three approximations: (i) The frozen orbital approximation, which consists of single configuration calculations in terms of the Hartree‐Fock orbitals for the neutral O2 molecule; (ii) direct hole‐state calculations in which g or u inversion symmetry is imposed on each molecular orbital; (iii) direct hole‐state calculations without the restriction in (ii). For the 1s 4Σ− hole state the three approximations yield the following ionization potentials: (i) 563.5 eV; (ii) 554.4 eV; (iii) 542.0 eV. The experimental ionization potential is 543.1 eV, and it is concluded that the hole state is localized on one of the two oxygen atoms.

Threshold Energies for Electron-Hole Pair Production by Impact Ionization in Semiconductors

Abstract: Threshold energies for energy- and momentum-conserving impact ionization of electron-hole pairs in actual semiconductors are determined by differential analysis of the energy-wave-vector relations of the participating charge carriers and phonons. A necessary condition for the initiating carrier to have minimum energy consistent with pair production is that the resultant carriers and all phonons involved have identical real-space velocities. This criterion allows calculations of ionization threshold energies for any semiconductor for which the one-electron-energy-wave-vector relationship is known. A step-by-step graphical procedure is presented for the calculation of threshold energies when the final particles are traveling along a principal axis of a semiconductor. Threshold energies resulting from the application of this procedure are presented for Si, Ge, GaAs, GaP, and InSb. Each of these materials exhibits numerous threshold energies for phononless ionization initiated by either type of carrier. The lowest thresholds for electron-initiated ionization without phonon assistance are 1.1, 0.8, 1.7, 2.6, and 0.2 eV relative to the conduction-band minima in Si, Ge, GaAs, GaP, and InSb, respectively. For ionization initiated by holes, the corresponding results are 1.8, 0.9, 1.4, 2.3, and 0.2 eV relative to the valence-band maximum.

Avalanche Ionization and the Limiting Diameter of Filaments Induced by Light Pulses in Transparent Media

Abstract: A limiting intensity is shown to exist for light propagation in transparent liquids and solids. In pure bulk materials it is determined by avalanche ionization over a wide range of pulse durations, wavelengths, and band gaps. The ionization rate per unit time is deduced from the thickness dependence of the dc breakdown. The negative real part of the index of refraction of the carriers stabilizes the size of self-focused filaments.

Ion Microprobe Mass Analyzer

Abstract: The quantitative analyses and other applications described in this article indicate a useful future for the ionmicroprobe mass analyzer in many areas of the science of solid materials. It should be possible to analyze all the elements quantitatively, but detection sensitivities will vary depending on the matrix, the element, and the polarity of the sputtered ion being studied. Most elements will have optimum yields in the spectrum of positive sputtered ions, and will be detected in concentrations of parts per million in micrometer-sized sampling areas. Electronegative elements will be detected with similar sensitivities in the spectrum of negative sputtered ions, but inert gases, which are ionized with difficulty and have small electron affinities, will be detected with considerably poorer sensitivities. In general, it will be possible to measure isotope ratios without chemical separation of the constituent elemrents of the sample. The precision of an ion microprobe isotope ratio measurement depends basically on the counting rates involved, and its accuracy can approach its precision if auxiliary standards are used. The isotope ratios of different elements can be compared readily because of the small mass-discrimination effects of the system. Surface layers can be quantitatively analyzed in depth with a resolution of tens of angstroms; hence, it should be possible to study the migration of atoms.

Thick-Target Processes and White-Light Flares

Abstract: Observations indicate that fast electrons in solar flares, which cause the hard X-ray burst and the impulsive microwave burst, lose energy predominantly by collisional processes. This requires a thick-target theory of the emission, for which the electron spectrum inferred from the X-ray spectrum becomes 1.5 powers steeper than in the usual thin-target theory. The low-energy end of this spectrum contains enough energy above about 5 keV to supply the white-light continuum emission occasionally observed in major flares. The penetration of the nonthermal electrons creates long-lived excess ionization which enhances the free-free and free-bound continuum in the heated medium. The emission will occur high above the photosphere at small optical depth in the visible continuum. Thus its spectrum will extend into the infrared and ultraviolet.

Ionization Cross Sections of Gaseous Atoms and Molecules for High-Energy Electrons and Positrons

Abstract: Ionization cross sections of forty gases have been measured for electrons of kinetic energies 0.1-2.7 MeV. The measurements are absolute and extensive precautions have been taken to minimize systematic and accidental errors. The energy dependence of the measured cross sections is accurately described by the Bethe asymptotic formula involving two parameters that represent important atomic properties. Comparisons have been made between ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ and between C${\mathrm{H}}_{4}$ and C${\mathrm{D}}_{4}$; the observed differences are of the order of 1% and too small to be resolved with certainty. A close comparison has been made between positrons and electrons in Ar at 0.67 and 1.1 MeV; the cross sections are observed to be equal within a probable error of 0.5%.

Free Radicals by Mass Spectrometry. XLV. Ionization Potentials and Heats of Formation of C3H3, C3H5, and C4H7 Radicals and Ions

Abstract: Using an energy-resolved electron beam, appearance potentials for,, and fragment ions from hydrocarbons of formula C3H4, C3H6, C4H6, C4H8, and C5H10 have been measured. In each case the fragment ap...

X‐Ray Photoelectron Spectroscopy of Alkali Halides

Abstract: We have measured ionization potentials for the removal of various electrons from the alkali chlorides and the sodium and cesium halides. These binding energies are compared with the predictions of a simple point charge model with and without corrections for polarization and repulsion. The simple model predicts and the data show that the spacings of the energy levels for a given ion are independent of what crystal it is in and are the same as for the free ion. The point charge model also allows us to calculate the difference between cation and anion energy levels in the same crystal. There is, however, a disagreement between the predicted and experimental values of this difference that ranges from about 1.8 eV for LiCl to −0.2 eV for RbCl. This discrepancy is markedly reduced by inclusion of polarization effects. The point charge model with polarization and repulsion corrections predicts absolute ionization potentials for the alkali and halide ions that differ in a systematic way from those observed. For the sodium halides, the difference between calculated and experimental energies decreases monotonically from about −4 eV for NaF to about 0.9 eV for NaI. The origin of these discrepancies is apparently due to charging of the samples and their trend is directly attributable to the size of the respective bandgaps. We show that the electrostatic model can be used to provide a comparison between experimental binding energies for inner electrons in a crystal and Hartree‐Fock calculations for binding energies of inner electrons in free atoms. Finally, using the point charge model we show that the binding energy of an electron on a highly charged ion in an ionic crystal is only slightly different from the binding energy of the same electron in the neutral atom.

The π-Orbital Energies of the Acenes

Abstract: The ‘observed’ π-orbital energies ϵv,j = −Iv,j derived from the vertical ionization potentials obtained by a photoelectron spectroscopic investigation of the acenes benzene A(1), naphthalene A(2), anthracene A(3), naphthacene A(4) and pentacene A(5) have been compared with π-orbital energies calculated by three different approximations: (a) the standard Huckel HMO model; (b) a first-order perturbation treatment, based on (a), that takes into account bond length changes which follow the ionization process; (c) a SCF π-electron model of the type proposed by Pople and by Pariser & Parr. In agreement with previous experience it is found that model (b) yields the most satisfactory parametrization of the experimental data.

Activated Reactive Evaporation Process for High Rate Deposition of Compounds

Abstract: Activated reactive evaporation is a new process developed for the high rate deposition of compounds. It circumvents problems associated with direct evaporation of compounds. Metal vapors from high rate evaporation sources are reacted with a gas in the vapor phase to form and deposit compounds. The vapor phase reaction has to be activated by ionization of the reacting species, i.e., metal vapor and gas in the reaction zone. The steps of compound synthesis and compound deposition are thus separated. Control of deposit stoichiometry, e.g., [C/M] ratio for TiC synthesis is readily achieved by controlling the relative amounts of reactants. Results on the synthesis of carbides, oxides, and nitrides are presented.

Electron Spectroscopy of Open‐Shell Systems: Spectra of Ni(C5H5)2, Fe(C5H5)2, Mn(C5H5)2, and Cr(C5H5)2

Abstract: The high resolution HeI electron spectra of Ni(C5H5)2, Fe(C5H5)2, Mn(C5H5)2, and Cr(C5H5)2 have been recorded and analyzed in terms of a molecular orbital description of the electronic structure. The ground state electronic configurations have been assigned by considering the feasible ground state configurations, determining the number and type of ionic states obtained from ionization of these configurations, and then comparing the predicted transitions with those observed experimentally. The ground state configuration and adiabatic first ionization potential of these molecules are: Cr(C5H5)2, ··· (e2g)3(a1g)1, 3E2g, I.P.=5.50 eV; Mn(C5H5)2, ··· (e2g)4 (a1g)1, 2A1g, I.P.=6.55 eV; Fe(C5H5)2, ··· (a1g)2 (e2g)4, 1A1g, I.P.=6.72; Ni(C5H5)2, ··· (a1g)2 (e2g)4 (e1g)2, 3A2g, I.P.=6.2 eV. Vibrational structure has been observed in the spectrum of ferrocene and is assigned to progressions in ν4, the symmetric ring‐metal stretching mode.

Theoretical Study of Ionization of Air by Intense Laser Pulses

Abstract: A detailed theoretical study of cascade ionization of air by rf fields and by laser beams is given. Experimental rate constants for energy loss, ionization, and attachment are used. The Boltzmann equation for the electrons is solved in both classical and quantum form. Provision is made for both single-photon and multiphoton ionization and detachment processes. The latter processes have been incorporated parametrically in our calculations owing to the lack of a quantitative description of multiphoton absorption. Possible anomalies in comparison with available experiments are noted for photons in the 1-2-eV range when the illuminated volume is large.

Electron Impact Cross Sections for Argon

Abstract: A reasonably complete set of phenomenological discrete and ionization cross sections for argon is given using combinations of data and theoretically meaningful extrapolations of the generalized oscillator strengths. After including estimates of inner‐shell cross sections, the degradation of electrons in Ar is considered. Calculations of the final populations for each excited state are given as a function of incident energy and when applied to ionization, results in electron volts per ion pair values near 29 eV. Inner shells in our work appear to contribute by adding on the order of 5% to the loss function at energies above a kilovolt.

Energy-Conservation Considerations in the Characterization of Impact Ionization in Semiconductors

Abstract: A simple expression for the ionization coefficient of charged carriers in a semiconductor as a function of electric field and lattice temperature has been developed by simultaneously fitting three physical asymptotic cases to Baraff's result. These cases are for low field (Shockley), high field (Wolff), and limitations imposed by energy conservation at high electric field or when the energy loss by phonon scattering is negligible. Given the threshold energy for ionization and the optical-phonon energy, our expression requires a single additional parameter to predict experimental results. Although the final expression is thus essentially a one-point fitting, it reproduces experimental data over as much as four decades of ionization coefficient with better accuracy than frequently used empirical two-parameter expressions. Excellent fits with much of the existing electric field dependence of the ionization coefficients for electrons and holes in Ge, Si, GaAs, and GaP were obtained. The temperature dependence was examined in the cases of GaAs and Si and excellent agreement was obtained in the case of GaAs. Some data on Si were found to be in considerable error in the sense that the data do not appear to be consistent with energy conservation.

Radial diffusion of ionized helium and protons: A probe for magnetospheric dynamics

Abstract: The comparison of radial diffusion of magnetospheric helium ions and protons is a sensitive test of the diffusion process assumed (magnetic or electrostatic fluctuations) and thus valuable as a probe of magnetospheric dynamics for protons, electrons, and heavy ions, as well as for the helium ions themselves. In this paper radial profiles of ionized helium flux and protons are calculated, and certain physical effects ignored in previous treatments are taken into account. These effects include (a) charge-exchange processes (He++ → He+ → He or He++), which provide a loss mechanism as well as a way of converting solar-wind alpha particles to He+, and (b) contribution of fluctuating electrostatic fields to the radial-diffusion coefficient. Like nearly all other treatments of radial diffusion, other processes such as pitch-angle scattering have been omitted for lack of quantitative knowledge. The combination of a solar-wind source, of losses dominated by charge exchange, and of electrostatic radial diffusion (with a plausible power spectrum for the fluctuations) leads to (a) fast conversion of alphas to He+, and (b) a radial-diffusion coefficient proportional, at fixed energy per nucleon, to (Z/A)². Thus helium and other heavier ions (C, N, O) diffuse much more slowly than protons in the magnetosphere, and this slow diffusion can explain the very small α/p ratios observed near L = 3.

The photoelectron spectra of methane, silane, germane and stannane

Abstract: The photoelectron spectra of the group iv hydrides, CH 4 , SiH 4 , GeH 4 and SnH 4 are shown to be due to ionization from valence ax and t 2 orbitals analogous to the s and p orbitals of the corresponding inert gases. The associated vibrational pattern of the ax bands is a simple progression of the breathing vibration. That of the t 2 systems shows the effect of Jahn-Teller splitting part of which converts to spin-orbit splitting for the heavier hydrides. The bond lengths and angles of the ionized states are evaluated approximately from the structures and. intensities of the band patterns. The experimental results give some support for theoretical calculations by Dixon which indicate that the triple degeneracy of the (t 2 ) -1 state is removed by distortion to a D 2d conformation, its low energy 2 B 2 component involving a fairly long progression in the v 2 vibration and, in the case of methane, progressions in v 1 containing several members. This agrees with our observations but our analysis of the spectra indicates that this state of CH 4 + takes up a conformation that is almost square coplanar with an extremely low inversion barrier. The 2 B 2 ionized states of SiH 4 , GeH 4 and SnH 4 though considerably distorted do not become coplanar.

Near Hartree‐Fock Calculations on the Ground State of the Water Molecule: Energies, Ionization Potentials, Geometry, Force Constants, and One‐Electron Properties

Abstract: Near Hartree‐Fock wavefunctions have been calculated for the ground state of the water molecule using both Slater and contracted Gaussian basis sets. Total energies of −76.063 hartree were obtained with a (5s4p1d/3s1p) Slater basis and a [6s5p2d/3s1p] contracted Gaussian basis derived from an (11s7p2d/5s1p) primitive set; these energies are estimated to be within 0.003±0.002 hartree of the Hartree‐Fock limit. The Hartree‐Fock wavefunctions account for ∼70% of the dissociation energy of water. The Hartree‐Fock vertical ionization potentials (in electron volts), 11.1(2B1), 13.3(2A1), and 17.6(2B2), are too low by 1–1.5 eV as expected. With the Gaussian basis set a potential surface was computed and the equilibrium geometry and harmonic force constants were calculated. The calculated bond length, 0.941 A, and bond angle 106.6°, are in good agreement with the experimental values, 0.957 A and 104.52°. In spite of the rather good agreement for the geometry, the force constants are in error by 15%–20%. This is a...

Photoelectron spectra of polycyclic aromatic hydrocarbons

Abstract: The high-resolution He(I) PE spectra of anthracene, phenanthrene, pentacene, perylene, chrysene, 1,2-benzanthracene, 1,2-benzpyrene, benzo[g, h, i]perylene and ovalene are presented and their essential features briefly discussed on the basis of HMO, Pariser–Parr–Pople, extended Huckel and MINDO/2 calculations. With the exception of a few closely spaced orbitals in the larger hydrocarbons, the four theoretical treatments yield the same π level ordering, thus allowing a tentative assignment of 56 observed ionization potentials in the 6–11 eV region to computed π levels. Increasing the size of the hydrocarbon shifts the onset of the σ bands only slowly to lower ionization potentials.

Angular Distribution of Auger Electrons Following Photoionization

Abstract: It is shown that the photoionization process of inner-shell electrons by unpolarized radiation leads to an alignment of the ionized atoms. From the nonisotropic angular distribution of the following Auger electrons the relative partial photoionization cross sections can be determined.

Ionization potentials of ferrocene and Koopmans' theorem. An ab initio LCAO-MO-SCF calculation

Abstract: The first ionization potentials of ferrocene have been computed in the LCAO-MO-SCF scheme as the difference of the total energy for the neutral molecule and the positive ion. The corresponding sequence of ionization potentials is found to be IP.(e 2g )