Showing papers on "Ionization published in 1981"

Ion Product of Water Substance, 0-1000 C, 1-10,000 Bars. New International Formulation and Its Background,

Abstract: This paper is the background for a new international formulation for the ion product of water substance (May 1980) issued by the International Association for the Properties of Steam. The ion product of water (Kw) is represented by an equation, based on density and two quadratic functions of reciprocal absolute temperature, for use from 0 to 1000 °C and 1 to 10,000 bars pressure. The equation is believed to describe within ±0.01 units of log Kw* (where Kw* equals Kw/(mol kg−1)2) many of the measurements at saturated vapor pressure up to 200 °C, and to within ±0.02 units up to the critical temperature (374 °C). It also describes within the experimental uncertainty the several sets of measurements at high pressures and should provide values within ±0.05 and 0.30 units at low and high temperatures, respectively.

Energy-loss effect in inner-shell Coulomb ionization by heavy charged particles

Abstract: The influence of the energy loss of slow projectiles during inelastic collisions on the inner-shell ionization cross sections is treated analytically. The result agrees with available experimental cross sections for $K$-shell ionization by protons. Residual trends in the data may gauge the quality of wave functions employed in calculations of Coulomb ionization cross sections.

Electron scattering by ionized impurities in semiconductors

Abstract: Theories of electron scattering by ionized impurities in semiconductors are reviewed. The early foundations based on the Born approximation and their subsequent refinements are discussed thoroughly. The phase-shift method which is not restricted to the Born approximation is also presented. The situation in heavily doped semiconductors is described. The theories are then compared critically with experiments. Finally, conclusions are drawn and some plausible lines of future work are outlined.

Band-structure-dependent transport and impact ionization in GaAs

Abstract: We have performed a Monte Carlo simulation of high-field transport in GaAs including a realistic band structure to study the band-structure dependence of electron transport and impact ionization. The band structure has been calculated using the empirical pseudopotential method. Unlike previous theories of impact ionization, our method is capable of calculating various parameters, such as mean free path, from first principles. The calculated electron mean free path, drift velocity, and impact ionization rate are in reasonable agreement with the experimental data in spite of several simplifications of the model. Within statistical uncertainty we do not observe any orientation dependence of the ionization rate in contradiction to the interpretation of recently reported experimental results. We also find that the contribution of ballistic electrons to impact ionization is negligibly small. Based on the results of the calculation, a general discussion of impact ionization is given.

High density cascade effects

Abstract: The objective of this article is to review the effects of high density cascades in solids. Such cascades can be separated into those occurring in which “high density” refers to either the density of atomic collision events or to the density of ionization. Both types of cascade are discussed with reference to their effects in the bulk (i.e. damage production and inert gas detrapping) and on the surface properties (i.e. sputtering, secondary ion and electron emission). The appropriate experimental data are reviewed and discussed in relation to the various proposed spike models; i.e. displacement, thermal, plasticity and ionization spikes.

Empirical models of height integrated conductivities

Abstract: Two-dimensional distributions of the height-integrated Pedersen and Hall conductivities have been computed for latitudes poleward of 60/sup 0/ invariant representative of two activity levels: 0< or =K/sub p/< or =3/sub 0/ and 3/sub 0/22 keV obtained by the energetic particle detector of Isis 2 during 1971-1974 are used as input to a Rees-type computation. The assumption of equilibrium conditions and a recombination rate profile permit calculation of electron density profiles and conductivity profiles. Calculations are performed at 300 grid points, specifically 12 local times and 25 latitudes from 60/sup 0/ to 84/sup 0/ invariant latitude. The models include ionization due to galactic EUV and other background sources that produces base conductivities (S/sub P/approx.0.1, S/sub H/approx.0.2 mhos) as well as solar photon ionization through an empirical fit to Chatanika radar observations (Mehta, 1979). Substantial modulation of the conductivities is found to result from longtudinal variation of the magnitude of the earth's magnetic field. The results show considerable spatial gradients and are expected to be of use in numerical modeling.

Electronically excited and ionized states of the chlorine molecule

Abstract: Potentials curves for the ground and excited states of the chlorine molecules and its positive and negative ions have been calculated by means of the MRD-CI method. The standard AO basis employed consists of 74 functions including two atomic d and one set of s and p bond species, and the results at the corresponding full CI level are estimated for each state via a perturbation correction. Special emphasis is placed upon the treatment of Rydberg-valence mixing in this system, which phenomenon is found to be essential to the understanding of Cl2 electronic absorption spectrum. All singlet states which correlate with the lowest dissociation limit plus many others which go to ionic Cl++Cl− or Rydberg Cl+Cl asymptotes are given explicit consideration. Among the triplet species of Cl2 which dissociate into the ground state atoms only the 3Πu state is not repulsive. The calculated D0 value for the ground state is 2.455 eV compared to the experimental value of 2.475 eV, while the vertical ionization energy and electron affinity are found to be 11.48 and 2.38 eV respectively, also in very good agreement with the corresponding measured data of 11.50 and 2.51 ± 0.1 eV. In addition to Cl2 laser line is confirmed to result from a 3Πg → 3Πu emission, whereby the calculated downward vertical transition energy of 4.86 eV fits in quite well with the known location of this line at 4.805 eV. The first two dipole-allowed transitions from the ground state of chlorine involve 1Σu+ and 1Πu states which are calculated to be nearly isoenergetic, and these results also match very well with the location of the first absorption band in this spectrum. Finally quite similarly as in O2 it is found that an avoided crossing between Rydberg and valences states produces a relatively steep potential well for an upper state (2 1Σu+), whose location concides with that of a second absorption band recently observed in synchrotron radiation studies.

A finite material temperature model for ion energy deposition in ion‐driven inertial confinement fusion targets

Abstract: We have developed a model for use in ion‐driven inertial confinement fusion (ICF) target design to describe the deposition of energy by an arbitrary ion traversing a material of arbitrary composition, density, and temperature. This model particularly emphasizes the deposition physics of light ions having specific energies of 3 MeV/amu or less. However, the model is also applicable to heavy ion fusion problems where there are specific energies in excess of 10 MeV/amu. We have found that an accurate description of the cold material stopping power must include both shell corrections to the Bethe theory as well as the alternative LSS (Linhard‐Scharff‐Schio/tt) model at low energies. We have incorporated finite temperature effects by scaling the relevant bound electron parameters with the degree of material ionization as well as by including the free‐electron stopping power. We discuss both the phenomenon of range shortening and range relengthening in heated material. Our preliminary calculations indicate that...

The formation of emission lines in quasars and Seyfert nuclei

Abstract: The photoionization and heating throughout a quasar emission-line cloud optically thick at the Lyman edge are calculated. Photoionization and collisional ionization from excited states of hydrogen are included, which maintain a substantial electron fraction after the exhaustion of Lyman continuum photons halts ground-state photoionization. Observed values are explained for Ly-alpha/H-beta, H-alpha/H-beta, P-alpha/H-alpha, He I 5876/H-beta, O I 8446/H-alpha, and Mg II 2798/H-beta. The dependence of line strengths on physical conditions is discussed, and plotting Fe II/4570/H-beta versus Balmer continuum/H-beta is suggested. Other observations are also suggested, and the degree of asymmetry is given between the forward and backward emission of lines from a finite slab to make possible the use of comparative line profile studies to elucidate cloud kinematics.

On the interpretation of molecular valence Auger spectra

Abstract: The interpretation of molecular valence Auger spectra by means of ab initio computational methods is discussed. As alternatives to self‐consistent‐field optimizations of the full Auger spectrum, the feasibility of a procedure based on single ionization potentials and of CI calculations using frozen orbitals is tested. Numerical calculations are performed for the CO, N2, NO, and CO2 molecules, which show especially structure‐rich Auger spectra. These spectra are analyzed in detail and they are found to contain three nonoverlapping regions of transitions corresponding to final state vacancies in outer–outer, outer–inner, and inner–inner valence orbitals. It is investigated whether this division of the transitions also is relevant with respect to the magnitude of dynamical relaxation errors in the single ionization potential procedure or with respect to the choice of molecular orbital basis in the CI calculations. CI effects are found to be important for intensities and energies of the Auger transitions in t...

A theoretical comparison between apparent and real vertical ionization drift velocities in the equatorial F region

Abstract: A theoretical analysis of the time dependent distribution of ionization in the low-latitude ionosphere is performed by using a realistic dynamic computer model of the equatorial F region, with special interest in the vertical motions of the electron density profile at the magnetic equator. The time dependence of the F2 peak height, at the magnetic equator, is governed primarily by the electromagnetic plasma drift velocity. It is shown that, for special time periods during sunset and evening hours, when the height of the F layer is above a threshold of 300 km, the apparent vertical displacement velocity of the F layer, inferred from ionosonde measurements, is the same as the vertical plasma drift velocity, as determined from incoherent backscatter radar measurements in the F region.

Relativistic contributions to the low‐lying excitation energies and ionization potentials of the transition metals

Abstract: Numerical Hartree–Fock calculations have been performed for the lowest terms of the dns2, dn+1s, and dn+2 configurations of the neutral transition metal atoms and the dns and dn+1 configurations of the ions. The excitation energies and ionization potentials are compared with their relativistic counterparts calculated according to the approximate scheme of Cowan and Griffin. The relativistic corrections are large, surprisingly so even for the first transition series, and act to stabilize ’’s‐electron rich’’ configurations. They are thus of opposite sign to the electron correlation contributions, which generally tend to stabilize ’’d‐electron rich’’ configurations. These correlation effects are particularly severe towards the end of the first transition series. Estimates for relativistic corrections from perturbation theory agree well with relativistic SCF calculations for the first and second transition series but depart substantially for the third series. The excitation and ionization energies from the re...

Physics of ultra-pure germanium

Abstract: A broad review of the physics of point defects, i.e. electrically active and neutral impurities and impurity complexes is given. Basic material science which is crucial for understanding the physics is summarized in an introductory section. It is followed by the detailed description of the novel measurement techniques—photothermal ionization spectroscopy, high-Q electron paramagnetic resonance and deep level spectroscopy. These spectroscopic techniques have had a profound impact on the investigations of ultra-pure germanium. The major part of this work deals with the physics of point defects in ultra-pure germanium. The high purity of this semiconductor allows the undisturbed observation of highly excited bound states of shallow donors and acceptors. Many new previously unknown acceptor and donor centres have been discovered in ultra-pure germanium. The nature of several of these centres can be understood in terms of a complex consisting of two impurities—a light interstitial atom (for example, h...

A theoretical study of the high-latitude winter f region at solar minimum for low magnetic activity

Abstract: A simple plasma convection model is combined with an ionospheric-atmospheric composition model in order to study the high-latitude winter F region at the solar minimum for low magnetic activity. The high latitude ionospheric features, such as the main trough, the ionization hole, the tongue of ionization, the aurorally produced ionization peaks, and the universal time effects are a natural consequence of the competition between the various chemical and transport processes known to be operating in the high-latitude ionosphere. In the polar hole, the F region peak electron density is below 300 km, and the dominant process at 300 km for NO(+) ions is diffusion.

Two‐color photoionization of naphthalene and benzene at threshold

Abstract: We observe the resonant two‐photon ionization (R2PI) spectrum of naphthalene and benzene at and just above the first ionization potential in a supersonic molecular beam. For naphthalene, the spectrum consists of direct ionization step functions indicating the relative Franck–Condon factors for transitions into the ground state ion. The spacings observed between these steps measure the ν8 vibrational frequency in the naphthalene ion. The electric field dependence of these steps is found to be consistent with a field ionization red shift of the thresholds. In benzene we observe similar direct ionization. Superimposed on the threshold steps, however, are broad features (∼100 cm−1) due to vibrational autoionization of Rydberg states at this same energy. Implications of this data on the observation of high Rydbergs in such large organic molecules are discussed.

Multi-photon ionization in the mass spectrometry of polyatomic molecules: Cross sections

Abstract: Multi-photon ionization (MPI) with tunable visible/UV laser light is shown to be a sensitive tool for analysis of traces in gas mixtures when combined with a mass spectrometer. Mass spectra of six different organic molecules, obtained with low intensity laser light, are presented and demonstrate the facility of ionization without fragmentation (soft ionization) under proper experimental conditions. Quantitative values for the cross sections for both two photon steps are obtained from the measured intensity dependence and the absolute ion numbers. Such quantitative data help in the evaluation and definition of this new ionization technique in mass spectrometry. Efficiencies of ionization for some molecules are as high as 25% leading to 106 ions in a single pulse from the dye laser (1 kW). Detectability as low as 2 parts in 109 is thus predicted.

Applications of scaling to problems in high-field electronic transport

Abstract: Utilizing changes in the carrier distribution function by magnitude and momentum scaling of scattering rates, the author a number of interesting results concerning ionization coefficients and transient drift and diffusion. Starting with a general definition of the ionization coefficient which includes nonlocal effects, the behavior of this coefficient under scaling is determined and used to find a simple analytical expression in terms of physical parameters valid for all field strengths. When fit to data for silicon, surprisingly large but consistent high‐field, effective ionization energies are found for electrons (3.6 eV) and holes (5.0 eV). This expression can also relate ionization near an interface to that in the bulk. Rate scaling is also used to predict changes in velocity overshoot and diffusion‐limited rise times between bulk and interface behavior. These comparisons are facilitated by a novel relationship between the time dependence of the spacial diffusion of a carrier pulse and it spacial displacement in an applied field. A classification of scattering rates suggested by momentum scaling as well as alternative scaling procedures applicable in special cases are briefly included. The former provides information on the behavior under momentum scaling of the positional‐displacement correlation factor of surface‐roughness scattering. The latter shows how scattering‐rate scaling and momentum scaling can be made to transform into each another.

Experimental study of the ionisation of atomic hydrogen by fast H+ and He2+ ions

Abstract: Cross sections for the ionisation of ground state hydrogen atoms by 38-1500 keV H+ and 125-2200 keV He2+ ions have been determined using a crossed-beam technique. Collision products produced in the intersection of the primary ion beam with a highly dissociated thermal energy beam of hydrogen are identified by time-of-flight spectroscopy and counted using a coincidence technique. Cross sections sigma (H+) for proton impact, which involve much smaller uncertainties and cover a wider energy range than previous measurements, now permit an accurate assessment of the validity of the various theoretical predictions. Cross sections sigma (He2+) for He2+ impact, the first such measurements, are also compared with theory and measured cross section ratios sigma (He2+)/ sigma (H+) are considered in terms of the Z2 scaling predicted by the Born approximation.

Velocity Dependence of the Ionization Probability of Sputtered Atoms

Abstract: The velocity dependence of the ionization probability of sputtered ${\mathrm{O}}^{\ensuremath{-}}$ from chemisorbed oxygen layers on V and Nb is studied. The ionization probability is found to depend on the normal component of the emission velocity, which suggests that the ionization process is an ion-surface interaction and not an ion-atom binary interaction. For ${v}_{\ensuremath{\perp}}g1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ cm/sec, the ionization probability shows an exponential dependence on ${v}_{\ensuremath{\perp}}$. However, this velocity dependence fails at lower velocities.

Ga 1-x Al x Sb avalanche photodiodes: Resonant impact ionization with very high ratio of ionization coefficients

Abstract: The liquid-phase epitaxy and device fabrication of p-n and p-i-n Ga 1-x Al x Sb avalanche photodiodes is described. Breakdown voltages up to 95 V and dark currents of 10-4A/cm2have been obtained. With p-i-n diodes we have measured the impact ionization coefficients α (electrons) and β (holes) with different composition and temperature. A resonant enhancement of the hole ionization coefficient is found for x = 0.065 (300 K) where the ratio \beta/\alpha exceeds values of 20. This effect is attributed to impact ionization initiated by holes from the split-off valence band: if the spin orbit splitting Δ is equal to the bandgap energy E g , the threshold energy for hole initiated impact ionization reaches the smallest possible value ( E_{i} = E_{g} ) and the ionization process occurs with zero momentum. This leads to a strong increase of β at \Delta/E_{g} = 1 . The experimentally determined dependence of ionization coefficients on threshold energy is compared with theoretical expectations.

Electron impact ionization cross sections and rates for highly ionized atoms.

Abstract: Electron impact ionization cross sections and rates for hydrogen-, helium- and lithium-like ions are presented in simple analytical form, based on a set of parameters derived from distorted wave exchange cross section calculations. Isoelectronic fits to the parameters used in the formulate allow rapid generation of cross sections and rates for any ion in these sequences with a minimum of computational effort. The use of accurate Bethe slopes in the cross section-fitting algorithm yields good accuracy both in the near threshold and high energy regions. The rate coefficient is given in the form of a correction to the commonly used Seaton formula and is thus readily adaptable to existing plasma modeling computer codes. Comparisons of the present results with other theoretical and experimental data are made.

Calculation of vibrational preionization in H2 by multichannel quantum defect theory: Total and partial cross sections and photoelectron angular distributions

Abstract: Multichannel quantum defect theory has been used to calculate the effect of vibrational preionization on the total and partial oscillator strength distributions and photoelectron angular distribution in H2 for excitation between 790 and 760 A. The total oscillator‐strength distribution obtained agrees well with the high‐resolution photoionization data of Dehmer and Chupka. The partial oscillator strength resonance profiles are predicted to have different shapes in different vibrational ionization channels, while their widths change little with channel. The preionization resonances are also predicted to affect the angular distribution asymmetry parameters b over a broader range than they affect the oscillator strength distribution. The gross features of the preionization resonances are discussed in terms of approximate solutions of the MQD equations.

Selective electron capture into highly stripped Ne and N target atoms after heavy-ion impact

Abstract: Auger electron and X-ray spectra from Ne and N gas targets excited with 1.4 MeV amu-1 Ar12+, Kr15+, Xe24+, and Pb36+ ions are measured, varying the target pressure and mixing other gases into the target volume. A dramatic change of line intensities from outer-shell configurations having a KL two-electron core and a third electron in the n=4, 5, 6 shell is observed, depending on the target pressure and systematically on the target ionisation potential. This effect is explained by highly selective electron capture from neutral target atoms or molecules into outer-shell orbitals of slowly (Er<10 eV) recoiling and highly stripped target atoms with metastable core states. It turns out that selective electron capture is a dominant mechanism for the population of H-, He- and Li-like states of low atomic number (Zt

Elastic peak electron spectroscopy for auger electron spectroscopy and electron energy loss spectroscopy

Abstract: Elastic peak electron spectroscopy deals with the spectra of secondary electrons in the neighbourhood of Ep primary energy and determines them in absolute units based on the Ne percentage of elastically reflected electrons. A procedure is described for evaluating elastic peak spectra. Experimental results on graphite, Si, Ge, stainless steel, Mo, W and Au polycrystalline samples are presented in the medium energy (Ep = 1–3 ke V) range. Spectra were measured with a cylindrical mirror analyser operated in the direct current mode. For elements (Si, Mo, etc.) Ne is determined mainly by the atomic number Z, Ne(Z)∼(Z – 26)0.65 was found for Z ≥ 26 and Ep = 3.1 ke V. The Ep dependence of Ne can be approximated by a power law Ne(Ep)∼Ep−β(Z), with β(Z) varying between 2 and 0.4 for the Z = 4–79 atomic number range. The elastic peak is composed of electrons escaping from a surface layer determined by the inelastic mean free path of primary electrons. Using Seah's values and from the Ne experimental data the backscattering cross-sections were determined. They varied between 5 × 10−20(C)−2.8 × 10−18 cm2 (W, Au). Some new results are presented on the volume plasmon losses. In Si and Ge the mean free path of medium energy electrons is determined by the first volume plasmon loss, whereas in metals the contributions of other loss processes are important. It is practical to use the elastic peak and Ne as references for Auger and loss peaks allowing the estimation of ionization cross-sections in Auger electron spectroscopy.

Molecular beam photoionization study of CO, N2, and NO dimers and clusters

Abstract: Photoionization efficiency (PIE) data for (CO)+2, (N2)+2, and (NO)+2 have been obtained near the thresholds using the molecular beam method. The ionization energies (IE) for (CO)2, (N2)2, and (NO)2 were measured to be 13.05±0.04 eV (950±3 A), 14.69±0.05 eV (844±3 A), and 8.736±0.002 eV (1419.2±0.3 A), respectively. Using these values, the known IE’s for CO, N2, and NO, and the estimated binding energies for (CO)2, (N2)2, and (NO)2, the bond dissocations energies for CO+⋅CO, N+2⋅N2, and NO+⋅NO were deduced to be 0.97±0.04, 0.90±0.05, and 0.598±0.006 eV, respectively. From the analysis of the PIE curve for (NO)+2, the IE’s of (NO)2 to NO+(?1S+, v = 1)⋅NO and NO+(?1S+,v = 2)⋅NO were determined to be 8.997±0.007 eV (1 378±1 A°) and 9.253±0.013 eV (1 340±2 A°), respectively. This measurement supports the conclusion that the bonding of NO+ in NO+⋅NO is stronger than that of NO, but weaker than that of NO+. The IE’s for (CO)3, (N2)3, and (NO)n = 3–6 were also measured.