Negative Ions in a Glow Discharge of Tetrafluoromethane

The Langmuir — probe measurements in a CF₄ — glow discharge reveal a large number of negative ions (10¹⁰ cm^?3) in the plasma between the planar C-electrodes. Under special conditions the density of them exceeds the electron density at several orders of magnitude and sustains a quasi — electron free plasma. The ratio depends on the residence time of the molecular gas in the discharge reactor. It is due to dissociative electron attachment to highly molecular gas components arising from the plasmachemical conversion of CF₄. F^? and CF₃^? are the most important negative ions.     

Radiative transition of hydrogen-like ions in quantum plasma

At fusion plasma electron temperature and number density regimes of 1?×?10³–1?×?10⁷?K and 1?×?10²⁸–1?×?10³¹/m³, respectively, the excited states and radiative transition of hydrogen-like ions in fusion plasmas are studied. The results show that quantum plasma model is more suitable to describe the fusion plasma than the Debye screening model. Relativistic correction to bound-state energies of the low-Z hydrogen-like ions is so small that it can be ignored. The transition probability decreases with plasma density, but the transition probabilities have the same order of magnitude in the same number density regime.     

Evaluation of the parameters of electron transport in liquid hydrocarbons using the photoconductivity technique

The ballistic model of electron transport proposed by Mozumder (Chem. Phys. Lett. 1993. V. 207. P. 245) is applied to interpret experimental data on the photoconductivity of solutions in three hydrocarbon liquids: methylcyclohexane, isooctane, and squalane. For methylcyclohexane, the values of the quasi-free electron mobility 100 cm² V^?1 s^?1 and electron trap density 3 · 10¹⁹ cm^?3 agree with experimental data. For isooctane, the motion of a quasi-free electron is nearly diffusive, whereas the trap density is ～0.8 · 10¹⁸ cm^?3. As to squalane, the two-state model turned out to be inapplicable to describing electron transport. The results obtained are used to discuss initial stages of ionization in liquids.     

Langmuir Probe Diagnostics of a Low Temperature Non-Isothermal Plasma in a Weak Magnetic Field

This article presents measurements by a cylindrical Langmuir probe in the plasma of a DC cylindrical magnetron discharge át the pressure 1.5 Pa that aim at the experimental assessment of the influence of a weak magnetic field to the estimation of the electron density when using conventional methods of probe data interpretation. The probe data was obtained under the presence of a weak magnetic field in the range 1.10^?2?5.10^?2 T. The influence of the magnetic field on the electron probe current is experimentally assessed for two cylindrical probes with different radii, 50 μm and 21 μm. This assessment is based on comparison of the values of the electron density estimated from the electron current part with the values of the positive ion density estimated from the positive ion current part of the probe characteristic respectively by assuming that at the magnetic field strengths used in the present study the probe positive ion currents are possible to be assumed as uninfluenced by the magnetic field. For interpretation of the probe positive ion current two theories are used and compared to each other: the radial motion model by Allen, Boyd and Reynolds [10] and Chen [11] and the model that accounts for the collisions of positive ions with neutrals in the probe space charge sheath that we call Chen-Talbot model [8]. At lower magnetic field 3 · 10^?2 T the positive ion density values interpreted by using the Chen-Talbot model [8] are in better agreement with the values of electron density compared to those obtained by using the theory [10,11]; therefore the model [8] is used for calculation of the positive ion density from the probe data at higher magnetic fields. The comparison of the positive ion and electron density values calculated from the same probe data at higher magnetic fields shows that up to the magnetic field strength 4 . 10^?2 T with the probe 100 μm and up to 5 . 10^?2 T with the probe 42 μm in diameter respectively the decrease of the magnitude of the electron current at the space potential due to the magnetic field does not exceed the error limits that are usual for Langmuir probe measurements (absolute error ±20%).     

Identification of dislocations and their influence on the recombination of charge carriers in gallium nitride

The relation between the density of etch pits revealed in GaN by etching in a KOH/NaOH eutectic and the density of dislocations determined by transmission electron microscopy (TEM) is studied along with the relation between the density of dislocations and the density of dark spot defects observed in GaN by microcathodoluminescence (MCL) and electron-beam-induced current (EBIC). It is demonstrated that selective etching is a reliable rapid method for the determination of the type and density of dislocations in GaN in the range 10⁶–10⁸ cm^?2, while MCL and EBIC can be used for the rapid nondestructive determination of the density of dislocations in the range 10⁶–10⁸ cm^?2. It is also found that some deep electron and hole traps are related to dislocations.     

Study of defect evolution by TEM with in situ ion irradiation and coordinated modeling

The paper describes a novel transmission electron microscopy (TEM) experiment with in situ ion irradiation designed to improve and validate a computer model. TEM thin foils of molybdenum were irradiated in situ by 1?MeV Kr ions up to ～0.045 displacements per atom (dpa) at 80°C at three dose rates ?5?×?10^?6, 5?×?10^?5, and 5?×?10^?4?dpa/s – at the Argonne IVEM-Tandem Facility. The low-dose experiments produced visible defect structure in dislocation loops, allowing accurate, quantitative measurements of defect number density and size distribution. Weak beam dark-field plane-view images were used to obtain defect density and size distribution as functions of foil thickness, dose, and dose rate. Diffraction contrast electron tomography was performed to image defect clusters through the foil thickness and measure their depth distribution. A spatially dependent cluster dynamic model was developed explicitly to model the damage by 1?MeV Kr ion irradiation in an Mo thin foil with temporal and spatial dependence of defect distribution. The set of quantitative data of visible defects was used to improve and validate the computer model. It was shown that the thin foil thickness is an important variable in determining the defect distribution. This additional spatial dimension allowed direct comparison between the model and experiments of defect structures. The defect loss to the surfaces in an irradiated thin foil was modeled successfully. TEM with in situ ion irradiation of Mo thin foils was also explicitly designed to compare with neutron irradiation data of the identical material that will be used to validate the model developed for thin foils.     

Die magnetische Suszeptibilität der Defektelektronen in Silizium und Germanium

For semiconductors it is possible, to determine the contribution of the free carriers to the magnetic susceptibility. Holes in silicon (density 10¹⁸ to 10¹⁹cm^?3) have at 297 and 141° K a small paramagnetic susceptibility. In contrast, one computes from cyclotron mass parameters for 4° K a strong diamagnetic susceptibility. Holes in germanium (density 4 · 10¹⁷ and 4 · 10¹⁹ cm^?3) have a diamagnetic susceptibility, which is much smaller, than one would expect from cyclotron resonance masses. — Neutral boron atoms contribute in small concentration at 141 °K to the susceptibility.     

Physical properties of hot,dense matter: The general case

The thermodynamic properties of hot, dense matter are examined in the density range 10^?5 fm^?3 ? n ? 0.35 fm^?3 and the temperature range 0 ? T ? 21 MeV, for fixed lepton fractions Y_? = 0.4, 0.3 and 0.2 and for matter in β-equilibrium with no neutrinos. Three phases of the matter are considered: the nuclei phase is assumed to consist of Wigner-Seitz cells with central nuclei surrounded by a nucleon vapor containing also α-particles; in the bubbles phase the cell contains a central spherical bubble of nucleon vapor surrounded by dense nuclear matter; the third phase is that of uniform nuclear matter. All are immersed in a sea of leptons (electrons and neutrinos) and photons. The nuclei and bubbles are described by a compressible liquid drop model which is self-consistent in the sense that all of the constituent properties — bulk, surface, Coulomb energies and other minor contributions — are calculated from the same nuclear effective hamiltonian, in this case the Skyrme 1' interaction. The temperature dependence of all of these energies is included, for bulk and surface energies by direct calculation, for the Coulomb energy by combining in a plausible way the usual electrostatic energy and the numerical results pertaining to a hot Coulomb plasma. Lattice contributions to the Coulomb energy are an essential ingredient, and lattice modifications to the nuclear translational energy are included. A term is constructed to allow also for the reduced density of excited states of light nuclei. All of these modifications incorporate necessary physical effects which modify significantly the matter properties in some regions.     

Many-body effects in the first excited subband of the n-inversion layer of Si

We have calculated the self-energies of electrons in the lowest and first excited sub-bands of Si inversion layers. The self-consistent wavefunctions calculated in the Hartree approximation were used, and dynamic screening was approximated by the Lundqvist-Overhauser model. The correlation energy of an electron in the excited band is quite large: about ?10 meV at an inversion layer density of 10¹¹ cm^?2 to about ?16 meV at 3 × 10¹² cm^?2. The calculated separation between subbands is in very good agreement with available experimental measurements. An exciton is predicted with a binding energy of 0.9 meV at N_inv = 10¹² cm^?2 calculated in the static approximation.     

Magnetooptical compression of atomic beams

We consider the propagation of an atomic beam in a quadrupole magnetic field under transverse irradiation by a cooling laser field. The cooling laser field was chosen in the form of a two-dimensional σ⁺-σ^? configuration. We show that the sub-Doppler resonance in the radiation force can be used to reduce the diameter of the atomic beam to a value on the order of 10 mm. We establish that the simultaneous transverse cooling and compression of the atomic beam allow its phase density to be increased to values of the order of 10^?4–10^?3. The dipole interaction of an atom with the cooling and compressing laser field in a quadrupole magnetic field is analyzed in terms of a simple (3 + 5)-level model atom.

     

Some tests of fragmentation models in exclusive channels inK − p reactions at 32 GeV/c

Experimental data for exclusive channels of multiplicity 4–8 in 32 GeV/cK ^? p reactions are compared with predictions of the Lund model. This leads to the following insights: — Evidence is obtained for azimuthal correlations betweenK ⁺ K ^? pairs and p pairs supporting the Lund fragmenttation scheme. — The disagreement of the observed behaviour of leading protons andK ^? with the model predictions indicates that the fate of the valence quarks is not well described by the model. The data support the idea of valence quark separation as advocated by dual models.     

EPR spectra of gable-type copper(II) porphyrin dimers in fluid solution: extraction of exchange interaction in weakly coupled doublet pairs

A comparison between EPR spectra of rigidly linked dicopper porphyrin dimers and those of the corresponding monocopper dimers (copper porphyrin-free base porphyrin dimers) in fluid solution reveals a very weak exchange interaction between the two copper spins. In these dimers, two porphyrin moieties are linked via an aromatic spacer such as benzene, naphthalene or phenanthrene in a gable-type geometry, with a distance of 10–13 Å. Although essentially all the spectra from the monocopper dimers are the same, exhibiting hyperfine (hf) structure due to the copper and nitrogen nuclei, the EPR spectral patterns of the dicopper dimers depend on the spacer molecule. Differences in hf patterns among the dicopper porphyrin dimers are ascribed to isotropic spin—spin coupling, i.e., exchange coupling between the two copper spins. This is because the anisotropic dipole—dipole interaction is averaged out due to random tumbling of the solute molecules in fluid solution. From the line shape analysis, the absolute value of the exchange interaction (|J|) is found to be 4 × 10^?4 cm^?1 ≦|J| < 3 × 10^?3 cm^?1 for the benzene linked dicopper dimer (Cu—Bz—Cu) whereas |J| ～ 1 × 10^?4cm^?1 for the other two dimers (Cu—Np—Cu and Cu—Pn—Cu). These values are comparable with or much smaller than the dipole—dipole coupling, which is estimated as about 1–3 × 10^?3 cm^?1 from the centre-to-centre distance. Since Cu—Bz—Cu shows a significantly larger |J| than Cu—Pn—Cu, despite a slightly longer centre-to-centre distance, and since no correlation could be obtained between |J| and the separation of the two copper atoms, it is likely that the interaction via spacer molecules is dominant between the two halves.     

Correlation effects and the density of states in amorphous silicon

The evaluation of the density of gap states from the charge density measured in field-effect experiments is considered in terms of a model which incorporates electron correlation effects. Due to the positive Hubbard U a density of states is deduced which differs appreciably from the “field effect” densities published previously. Besides the lack of the E_x and E_y peaks the minimum density is found to be ～ 10¹⁵ ? 10¹⁶ cm^?3 eV^?1.     

The electron-hole-liquid in 15R-SiC

The observation of an electron-hole-liquid in a semiconductor with rhombohedral structure — the multivalleyed, polar material 15R-SiC — is reported from a study of excitation dependent and time delayed spectra. The EHL lifetime is found to be < 15 nsec from which a density > 1.8 × 10¹⁹ cm^?3 and a critical temperature > 60 K are estimated. The EHL binding energy is found to be 20±5 meV.     

Ion-molecule collisions in gaseous111InCl and111InI systems

Using gaseous sources of¹¹¹InCl and¹¹¹InI with densities between 1.0·10¹⁷ cm^?3 and 4.4·10¹⁹ cm^?3 the perturbation of the 171.3–245.4 keVγ-γ cascade was measured as a function of time and density by the time-differential perturbed angular correlation (TDPAC) method. The anisotropy shows a strong dependence on the density. By means of an extended model based on a stochastic model of Bosch and Spehl collision cross sections for charge transfer and deorientation could be determined. The cross sections for charge transfer collisions were within the region from 2·10^?15 cm² to 26·10^?15 cm² and for deorientation collisions between 1·10^?15 cm² and 100·10^?15 cm².     

High resolution infrared study of the parallel band ν3 of chloroform CH35 Cl3

The infrared spectrum of chloroform in the region of the parallel fundamental band ν₃ around 367 cm^?1 has been measured with a Fourier spectrometer at a resolution of 0.001 cm^?1. An isotopically pure sample of CH³⁵Cl₃ was used. More than 5000 lines were assigned in the ν₃ band. A reanalysis of the ground state constants was performed by combining 1671 combination differences from this work, 712 differences from a previous study of the ν₂ band, and 80 millimetre wave lines from the literature. In the analysis of the ν₃ band, a model of an unperturbed symmetric top band was applied. The data were fitted with a standard deviation of 0.18 × 10^?3 cm^?1, and the following leading parameters were obtained: ν₀ = 367.295 550(8) cm^?1, B ₃—B ₀ = ?77.058(4) × 10^?6 cm^?1 and C ₃—C ₀ = ?18.600(11) × 10^?6 cm^?1. In addition, several hot bands have been studied. The isotopic effects were studied also by analysing spectra of the isotopically natural sample.     

The weight,shape, and speed of the universe

Present astronomical data indicate an unbound universe with density ～1.6 × 10^?31 g cm^?3 in which galaxies could not have formed gravitationally. We show how magnetohydrodynamic (MHD) processes allow galaxy formation in an open anisotropic MHD universe with shear, rotation, and fluid flow. The dipole anisotropy of the microwave background radiation sets their respective first-order values at 3.7×10^?15 yr^?1, 10^?14 yr^?1, and 5×10^?4 c. Second-order effects of Maxwell and Reynolds stresses require that the magnetic field, shear, and Hubble expansion be 10^?8 G, 3.7×10^?15 yr^?1, and 10^?10 yr^?1 (100 km sec^?1 Mpc^?1). The model is rigidly self-consistent, predicting both the recent value of the Hubble expansion above and of the shear (? 9×10^?15 yr^?1) given by the microwave background's recently measured quadrupole anisotropy.     

Eine Bestimmung der effektiven Zustandsdichte des Bandes für überschüssige Defektelektronen in Anthrazen

Steady-state space-charge-limited current measurements were used to obtain the effective density of states in the band appropriate to excess hole motion in anthracene. Since the energetic trap density decreases exponentially with the energy distance from the band, the traps are filled by the space charge up to a certain quasi Fermi level. The thermal activation energy of the current should be a kind of spatial avarage of this level. This energy was measured and used to calculate the effective density of states. The result, 4·10²¹ cm^?3, is close to the double molecular density which is 8,4·10²¹ cm^?3. Further, a discussion on traps is given in an appendix.     

Exciton lifetime in quantum well with vicinal high-density excitons

Radiative lifetime of an exciton in a GaAs quantum well (QW) is controlled by high-density excitons, which restrict the exciton coherence through scattering. In order to circumvent the phase space filling effect of high-density excitons, we have prepared a QW structure in such a way that a reservoir for high-density excitons is separated from the QW. The lifetime increases (up to 30%) with the exciton density in the reservoir and saturates at 1×10¹⁷/cm³. The upper bound lifetime is determined by the excitonic relative motion.     

Computer simulation of graphite target ablation under the action of a nanosecond laser pulse

A quasi-gasdynamic approach is used for computer simulation of plasma expansion from a graphite plate subjected to a nanosecond laser pulse. A one-component plasma consisting of carbon molecules alone is considered. This simplifies the experimental conditions used previously to study the dynamics of the gas resulting from evaporation. The results of computer experiment conducted for different initial temperatures and pressures of the plasma are in good qualitative agreement with the real experimental data including in the time instant the density of the expanding gas reaches a maximum. It is shown that high-density clusters are likely to appear in front of the main plasma flux. The results of the computer simulation are compared with the Singh approximation of pressure, velocity, and density of the gas flow. It is concluded that this approximation is valid only within a short (compared with the entire expansion length) plasma expansion interval existing during the initial spread for t = 4 × 10^?9 s.