Volume 20, Issue 3

At constant magnetic field the resonance frequencies of nuclei in ionic salts increase linearly with temperature in both the crystalline and molten states. The linear temperature-dependence of the chemical shift is ascribed to the excitation of higher paramagnetic electronic states which accompanies the increasing overlap of cation and anion wave functions; both are proportional to the linear increase in the mean squared vibrational amplitude of ions with temperature. The magnitude of the break in the chemical shift at the melting point, coupled with a knowledge of the change in the coordination number and the variation in the wave function overlap with internuclear separation leads to an estimate of the distance of closest approach of neighboring ions in the molten state. For an assumed value of F=0.9, where F= (ZL/Zs)/(ΔEʟ/ΔE S ), the distance of closest cation-anion approach decreases by 0.18 Å as crystalline TlCl melts to form the liquid salt. The overlap of Tl + and Cl - ions in crystalline TlCl at room temperature is 0.012 on the basis of the NMR frequency of 205 Τl + and is estimated to be 0.017 from machine calculations based on analytic wave functions for the ground states of the ions.

The x-ray scattering from liquid copper, silver, tin, and mercury was measured at temperatures of 1125°C, 1050°C, 335°C, and 28°C, respectively, from the open surface of horizontal samples using a focusing theta-theta diffractometer, quartz crystal monochromator positioned in the diffracted beam, scintillation detector, and pulse height discriminator. The effect on the measured intensities of the positioning of the sample with respect to the diffractometer axis and the meniscus of the liquid were considered. Calibration of the primary beam intensity by measurements on liquid mercury provided an alternate check of the standard normalization procedures for copper, silver, and tin. After calculation of the interference functions, atomic and radial distribution functions were evaluated from which interatomic distances and coordination numbers were obtained. The interatomic distances in the liquid were in good agreement with the GOLDSCHMIDT diameters of the respective elements.

If through an element of area of a continuum there acts not only a force but also a couple, we have to introduce besides the force-stresses the so-called couple-stresses. In this article we emphasize the importance of couple-stresses in dislocated solids.—§ 2 gives a short review of the present state of the theory of couple-stresses. In classical elasticity couple-stresses are to be interpreted as a non-local effect intimately connected with the range of the atomic forces. The couplestresses are of a higher order in this range than force-stresses and can therefore usually be neglected. In the field theory of dislocations couple-stresses generally are of the same order of magnitude as force-stresses, however. Hence they cause considerable effects. In § 3 we determine the macroscopic observable couple-stresses of homogeneously distributed screw and edge dislocations through averaging over their microscopic fluctuating stress field. With the PEIERLS model we show in § 4 that the core of a dislocation produces an asymmetric state of stress and for that reason also couple-stresses, which are negligibly small under certain circumstances. Introducing a simple polycrystal model we derive in § 5 the constitutive relations for couple-stresses and dislocation density in an isotropic form. The results are discussed in § 6.

Die Bindung in Festkörpern vom Typ A Z B 8-z wird als Überlagerung rein kovalenter und rein ionogener Zustände beschrieben, wobei der relative Anteil der einzelnen Zustände durch Wahrscheinlichkeiten gegeben ist, die von der Energie des jeweiligen Zustandes abhängen. Mit diesen Wahrscheinlichkeiten werden effektive Ladungen berechnet und den nach anderen Methoden erhaltenen gegenübergestellt.

The dynamical theory of electron diffraction for a finite crystal is developed by using higher BORN approximations. The general formula obtained here is applied to wedge-shaped and spherical crystals. In the latter case, the intensity, integrated intensity and line breadth of the DEBYE-SCHERRER ring are calculated. The results indicate an anomaly of line breadth.

Die von VOLKIN angegebene Erweiterung der Theorie von ZEMACH und GLAUBER für die Streuung niederenergetischer Neutronen an freien Molekülen wird vereinfacht und abgeändert. Eine andere Behandlung einer Operatorsumme gestattet es, eine wesentlich bessere Ausgangsnäherung zu finden, die Ähnlichkeit mit der KRIEGER—NELKIN-Näherung zeigt. Korrekturen zu dieser Ausgangsnäherung werden angegeben. Die Ergebnisse einer Rechnung für Wasser in der Gasphase nach dem VOLKIN· sehen und dem hier entwickelten Verfahren werden mit Meßwerten von HOFMEYER an Wasserdampf verglichen. Es zeigt sich eine bessere Übereinstimmung zwischen Theorie und Experiment mit dem hier angegebenen Verfahren als nach der VOLKINSchen Methode.

Für kubisch-raumzentrierte Vanadiumeinkristalle wurde experimentell bestätigt, daß die Kernresonanzfluoreszenzausbeute von der Orientierung des Kristalls abhängt. Dies ist nur möglich, wenn die Abbremsung von Chrom-Rückstoßkernen mit Energien im Bereich von 20 — 90 eV im Einkristall anisotrop ist. Es ergaben sich Ausbeuteunterschiede von 10%. Da die Messungen nur integrale Aussagen über alle Abbremsvorgänge innerhalb eines Kegels von 122° Öffnungswinkel gestatten, muß man annehmen, daß die Abbremsung sehr stark von der anfänglichen Flugrichtung der Kerne abhängt. Es wird eine Formel abgeleitet für die Kernresonanzfluoreszenzausbeute in Einkristallen unter Berücksichtigung der anisotropen Abbremsung der Kerne.

Mit Hilfe der Kernresonanzfluoreszenz wurden Abbremswege von Cr-Atomen in verschiedenen Verbindungen im Energiebereich von 20 — 90 eV bestimmt. Die Abbremswege betragen in Vanadiummetall ca. 2 Å und steigen bei Verbindungen und Lösungen bis auf 6 Å an.

In order to study the bulk-conductivity in organic liquids measurements with blocking quartzelectrodes were made. When applying a constant field, a bulk-current through the dielectric liquid can be observed, which is decreasing exponentially j d (t)=j 0 _ exp {—t/R_C}+j 0 exp {—t/R + C}. R - and R + are the bulk-resistivities for negative and positive charge-carriers, from which the bulk-conductivity of the liquid can be calculated. It is identical with the dc.-conductivity measured with conducting electrodes, that is it obeys the law: σ=σ 01 exp { —E 1 /k T} +σ 02 exp {—E. 2 /k T}. Therefore the generation of charge-carriers must be independent of the electrodes and the activation-energies E 1 and E 2 must correlate with ionisation within the liquid. The mobility of negative carriers is about two or three times that of positive ones. This fact leads to conclusions concerning the nature of the carriers. A tunnel-process is proposed to explain the discharging of positive ions at a metallic cathode.

Investigations were made of energy-transfer in liquid scintillators under UV- and β-excitation. The influences of n-hexane and methanol as diluting solvents on the scintillation process in p- terphenyl-toluene were measured. Differences of energy transfer under β - and UV-irradiation are discussed. Radiationless energy-transfer occurs over distances from 15 to 32 Angström units. Quenching effects of phenol on p-terphenyl, 2,5-diphenyl-oxazole, and 2,4 (or 5) -diphenyl-imidazole were studied. Oxygen-quenching in liquid scintillators containing hydroxy-benzenes was investigated. The results are in correspondence with the KALLMANN-mechanism of radiationless energy-transfer. — Further investigations were made of the connection between the chemical structure of aryl-imidazoles and their scintillation properties. Scintillation properties depend on spectral data. The results aprove HELLERS postulates, concerning structural requirements for good scintillation properties of organic liquid scintillators.

It is shown that the fluorescence of various Eu-chelates (dibenzoylmethide, benzoylacetonate, acetylacetonate and β-naphthoylacetonate) is much enhanced by additional coordination of ophenanthroline or α.α'-dipyridil. Complexes of the composition Eu (β-diketone) 3 -Phen, Eu(β- diketone) 3 -Dip etc. could be isolated and the absorption and the fluorescence in various solvents was studied. Compared to the trischelates the temperature dependence of the additional coordinated chelates is remarkably reduced.

Electrophotoluminescence (GUDDEN—PoHL-Effect) was observed with ZnS: Al, Zn single crystals. The effect is restricted to a narrow range near the anode, in agreement with previous assumptions. The dependence of the effect on bias voltage, temperature, duration of dark interval and excitation is investigated and discussed.

Ausgehend von der bekannten Tatsache, daß die Lösung der ScHRÖDISGER-Gleichung für ein Elektron äquivalent ist zur Ermittlung der zum niedrigsten Wert der Energie gehörenden Testfunktion bei gleichzeitiger Orthogonalität der zu verschiedenen Zuständen gehörenden Funktionen, wird der Erwartungswert der Energie für nicht-analytische, in einem zwei- bzw. dreidimensionalen Raster definierte Funktionen formuliert. Aus diesem Ausdruck wird ein Kriterium entwickelt, das es gestattet, aus einer auf einfache Weise vorgegebenen Testfunktion durch iterative Punkt-für-Punkt-Variation mit einem Digitalrechner die innerhalb vorgegebener Grenzen jeweils energieärmste und daher beste Funktion aufzubauen. Die Varianz * der endgültigen Funktion wird berechnet, aber nicht zur Steuerung des Rechenvorganges benutzt. Ein FORTRAN II-Programm zur Berechnung zweidimensionaler Wellenfunktionen wird skizziert. Dieses wurde an der Rechenanlage IBM 7090 (mit 32 768 Kernspeicherplätzen) des Deutschen Rechenzentrums, Darmstadt, an vielen Beispielen erprobt.

A simple treatment of spin densities and ESR-coupling constants of substituted alkylradicals is developed. It is shown that the spin densities ρ a in the α-carbon 2 p z -functions may be calculated by use of a product rule from substituent parameters 1 (X i ). Furtheron it is found, that the parameters Q in the well known relations a = Q-ρ a are unique constants for rotating methylgroups, whereas for α-protons they depend on the electron withdrawing forces of the substituents. Values of Δ (X i ) and QH H CH are tabulated and discussed. The ESR-coupling constants of four new transient alkyl radicals are also given.

Separations by thermal diffusion in the mixtures 14 N 15 N/ 14 N 2 and 15 N 2 / 14 N 2 have been made at average temperatures between 152°K and 470°K. The results are consistent with the CHAPMAN-ENSKOG theory in sharp contrast to separations in isotopic CO mixtures 1 . From the difference in behaviour between CO and N 2 it is concluded that energy transfer between translation and rotation of the molecules is not responsible for the influence of isotopic substitution on the thermal diffusion factor.

Primary dissociation processes on excited cyclohexane have been investigated in d. c. and a. c. (50 cycles and 535 cycles) glow discharges. The C 2 -, C 3 - and C 4 -hydrocarbons formed, have been analysed by gaschromatography. These dissociation products can be explained quantitatively by a simple reaction scheme, which is based on C 3 : C 3⁻ , C 4 : C 2⁻ and C 2 : C 2 : C 2⁻ cleavages of the cyclohexane ring and the following intramolecular disproportionation of the biradicals formed. The composition of the dissociation products is more or less identical for the two different a. c. investigations. Different results are observed for a. c. and d. c. glow discharges. The results indicate that the differences in the probabilities for the dissociation processes between a. c. and d. c. glow discharges are caused by the electrical property of the stationary and the moving striations of the positive column.

Bei der Bestrahlung von N 2 O mit der 1470 Å-Xenonlinie und den Kryptonlinien 1236 A und 1165 A wurde die Fluoreszenz von NO (β) - und NO (γ) -Banden nachgewiesen. Die relative Intensität beider Systeme ist sowohl bei 1470 A als auch bei 1236 A und 1165 A stark von der Strömungsgeschwindigkeit des N 2 O, dem N 2 O-Druck und dem Zusatz von NO abhängig. Die Bildung von NO (A 2 ∑ + ) - und NO (B 2 π) -Molekülen muß bei 1470 Å ausschließlich durch Sekundärreaktionen über metastabile Teilchen, wahrscheinlich über O ( 1 S) und/oder N ( 2 D, 2 P), erfolgen. Es ist zu vermuten, daß auch bei den Kryptonlinien, bei denen energetisch die Bildung von NO (A 2 ∑ + ) - und NO (B 2 π) bei der N 2 O-Dissoziation möglich ist, mindestens ein Teil der β - und γ-Fluoreszenz durch Chemilumineszenz-Reaktionen entsteht.

A powerfull P. I. G. reflex arc in lithium vapour has been studied using spectrographic, mass spectrometric and probe measuring techniques. The Li-vapour has been produced in a special Li-oven; the vapour entered through a small tube directly into the hollow anode cylinder. The residual gas pressure was 10 -7 mm Hg with the arc in operation, the density in the plasma column was about 10 13 cm -3 . The discharge showed two different modes of operation. We observed that at a fixed flow of Li-vapour the discharge can be operated in a stable state (Mode I) only below a certain limit of the applied electrical power. An increase of electrical power (or a decrease of the flow of Li-vapour) causes a strong instability (Mode II). 1. Mode I showed the discharge column to be very well stabilized by the confining magnetic field. Electrostatic probe measurements showed no voltage fluctuations. The electron temperature ranged from 1.8 to 8 eV, depending on the electrical power applied. 2. Mode II showed the plasma column to become diffuse. Large voltage fluctuations of several volts in the megacycle region could be detected on the electrostatic probe. A powerfull flux of ions leaving the plasma column perpendicular to the magnetic lines of force and hitting the walls with energies up to 327 eV has been measured. These energetic ions are responsible for the strong light emission of the walls of the vacuum tube, this light intensity being much higher than the intensity of the plasma column. The experimental results are similar to those observed by NEIDIGH, WEAVER, NEZLIN and SOLNTSEV under different experimental conditions.

The object of this paper is an experimental investigation of the electron temperature dependence of the ambipolar diffusion coefficient as well as the electron-ion recombination coefficients in the afterglow of plasmas produced in neon. By means of a microwave-cavity technique the electron density was measured as a function of time during the afterglow period of a d. c. discharge while at the same time the electron energy was increased by the power level of a microwave signal. It is found that the ambipolar diffusion coefficient D a increases with increasing electron temperature T e following the well-known relation: D a ∞ (1 + T e /T g ) , while the recombination coefficient a decreases with increasing electron temperature. The dependence of a on electron temperature is found to be: α ∞ T e -0,4 from 900 ° K to 2400 °K , while a weaker dependence in the range of α ∞ T e -0,25 was measured in the lower temperature region from 300 °K to 600 °K . At 300 °K the recombination coefficient in neon is found to be α = 2.0 · 10 -7 cm3 sec -1 . All measurements were done at electron densities of ~ 10 9 /cc and gas pressures of 1 mm Hg (diffusion) and 20 mm Hg (recombination).

Nonlinear scattered light with twice the frequency of the incident beam is in phase with the mechanism by which it is produced for some particular angle of emission, and hence has a resonant character in that direction. The angle is a function of the electron density in the plasma. The intensity and the angular width of the resonant beam are discussed. The experimental application to present laboratory plasmas may be practicable, when strong lasers are developed in the infrared region.

Numerical calculations of pressure increase in the plasma of a cylindrical arc column are presented. We assume applicability of the corona-equation. Results are given for magnetic fields up to 50 kΓ, external pressures of 10 –1 to 2·10 5 dyn/cm 2 and temperatures up to 80 000°K. Finally, the results are compared with calculations using the SAHA-equation.

A two-step model of the heating processes in a plasma jet is given. In this model, a hot interior and colder sheath is postulated, and the conservation theorems for mass, momentum and energy are solved so that the plasma velocity and temperature can be calculated if the power supply, pressure fall over the heating length, and the cross-sectional area of the luminous plasma stream are known. The accelerating action of the electromagnetic forces is considered as well as the J o u l e heating caused by the arc current. This model was tested by using a newly developed method to measure the average velocity and by using spectroscopy to measure the temperature of the plasma jet. The applicability of the “two step” model is shown by these experiments.

The numerical solution of the energy balance of a cylindrically symmetric hydrogen arc immersed in an axial, strong, homogeneous magnetic field led to the current-voltage-characteristic and the radial temperature distribution. Three types of arc models were used, each with different assumptions on radiation losses. The results show that, by the reduction of the thermal conductivity perpendicular to the magnetic field for a fully ionized plasma, the necessary electrical power is diminished for temperatures along the axis higher than about 2·10 4 °K. For example, an arc with 1 cm radius in a hydrogen atmosphere of 5 · 10 4 dyne/cm 2 , in an external magnetic field of 20 kGauss, a temperature along the axis of 10 5 °K requires about 3,5 kW per cm arc length. Of this, radiation losses account for about 0,5 kW per cm. However, without a superimposed magnetic field the electrical power is about 200 kW per cm.

In der vorliegenden Arbeit wird die lagenweise Desorption von Wasseradsorptionsschichten von in Luft gespaltenen A lkalihalogeniden bei der Erhitzung im Hochvakuum untersucht.