Showing papers in "Zeitschrift für Naturforschung A in 1972"
TL;DR: In this paper, the authors present a theoretical treatment and experimental observations of the deformation that occurs in a nematic liquid crystal when electric or magnetic fields are applied to the liquid crystal.
Abstract: Abstract We present a theoretical treatment and give experimental observations of the deformation that occurs in a nematic liquid crystal when electric or magnetic fields are applied. We consider only normal deformations in the nematic material where fluid flow and other dynamic phenomena play no role. Three important sample geometries are considered in the magnetic field, and the experimentally observed deformations are in good agreement with theory. The normal deformation induced by electric fields is of interest from a device standpoint, and we give a solution for the deformation that is valid even for large dielectric anisotropics. This solution has been experimentally verified. We give a detailed comparison of the distortions produced by electric and magnetic fields and show that the deformations are of a similar form even though the field is nonuniform in the electric case. The change in birefringence and electrical capacitance as a function of distortion is discussed as a means of observing the deformation.
173 citations
TL;DR: In this paper, the geometry of lattice defects can be studied in detail using the weak-beam method 1 of high resolution electron microscopy, and the theoretical basis of the method is discussed, and experimental conditions which are necessary if particular methods of image interpretation are to be used are considered.
Abstract: Abstract The geometry of lattice defects can be studied in detail using the weak-beam method 1 of high resolution electron microscopy. In this paper the theoretical basis of the method is discussed, and the experimental conditions which are necessary if particular methods of image interpretation are to be used are considered.
119 citations
TL;DR: In this paper, a very aproximate treatment of the phase contrast due to defocussing of phase objects having large phase shifts is evolved and shown to give a qualitativity correct account of the observations.
Abstract: Abstract High resolution electron microscope images showing the detailed distribution of metal atoms within the unit cells of complex oxide structures have been recorded recently and as a first approximation may be interpreted as amplitude-object images if obtained with the degree of defocus corresponding to the \"optimum-defocus condition\" for the phase-contrast imaging of thin phase objects. Detailed observations of images of Ti2Nb10O29 crystals having thicknesses of the order of 100 Å reveal that the thin phase-object approximation, which assumes that only small phase-shifts are involved, is inadequate to explain some features of the image intensities including the variation of contrast with crystal thickness. A very aproximate treatment of the phase contrast due to defocussing of phase objects having large phase shifts is evolved and shown to give a qualitativity correct account of the observations. The variation of image contrast with tilt away from a principle orientation is discussed. From the symmetry of the image contrast it is deduced that the symmetry of the crystal structure as derived from X-ray diffraction studies can not be correct.
95 citations
TL;DR: In this article, a recent reformulation of quasi-equilibrium theory is extended to several additional classes of ionic decompositions, and formulas for calculating average translational and rotational energies of the fragments are presented.
Abstract: Abstract A recent reformulation of quasi-equilibrium theory is extended to several additional classes of ionic decompositions. Formulae for calculating average translational and rotational energies of the fragments are presented, and kinematic corrections to the formulas are introduced. Illustrative calculations are compared with experiment, and the interesting possibility of maxima in rate constants is noted.
77 citations
TL;DR: In this paper, the radiation damage of electron irradiated organic compounds (paraffin, tetracene) was investigated in the temperature range 300 - 4 K. The radiation damage was measured by the decrease of diffraction intensities or the contrast of extinction contours.
Abstract: Abstract The radiation damage of electron irradiated organic compounds (paraffin, tetracene) was investigated in the temperature range 300 - 4 K. The radiation damage was measured by the decrease of diffraction intensities or the contrast of extinction contours. The damage of the crystals at 4 K required considerable higher electron doses than at 300 K. This result is explained as follows: The total radiation damage proceeds by two steps: 1) By primary radiation damage - excitation of energy levels with subsequent dissociation - which is almost temperature independent, 2) by secondary radiation damage - diffusion of molecular fragments - which decreases strongly with decreasing temperature.
59 citations
TL;DR: In this article, the energy of electrons crossing the glow boundary of a cylindrical hollow cathode discharge has been directly controlled in a small, test region by the use of a wall probe installed in the side of the cathode cylinder.
Abstract: Abstract The energy of electrons crossing the glow boundary of a cylindrical hollow cathode discharge has been directly controlled in a small, test region by the use of a wall probe installed in the side of the cathode cylinder. Varying the potential of this probe disclosed that some electrons ejected from the probe surface and hence from all cathode walls penetrate into the dark space opposite to the surface of their origin. These electrons can reach the opposite wall if the pressure and the cathode diameter are too low, in which case they will be removed. This removal causes the hollow cathode discharge to extinguish at higher pressures than is the case for linear discharges. Electrons that cross the cathode cavity entirely can pass through a small hole in the cathode wall into a high vacuum chamber and be analysed energetically. The same experiment also discloses the energy distribution of electrons leaving the cathode surface. A comparison with the theory recently published 3 shows that pendelelectrons can be responsible for the currrent amplification observed in the hollow cathode discharge.
58 citations
TL;DR: In this paper, it was shown that the room-temperature nematic MBBA can be deformed by means of curvature electricity if it is contained in a thin sandwich cell with the electric field parallel and the unique axis normal to the bounding walls.
Abstract: Abstract Measurements are reported which show that the room-temperature nematic MBBA can be deformed by means of curvature electricity if it is contained in a thin sandwich cell with the electric field parallel and the unique axis normal to the bounding walls. Meyer's coefficient e33 was determined to be 4 · 10-5 dyne1/2 at room temperature; and its sign was found to be positive.
53 citations
TL;DR: In this paper, it was shown that in an uniaxial crystal the frequencies of optical phonons * which are identical to those of the short wavelength polaritons depend on the angle Θ between the optic axis and the phonon wave vector.
Abstract: Abstract From the general polariton dispersion theory it can be shown that in an uniaxial crystal the frequencies of optical phonons * which are identical to those of the short wavelength polaritons depend on the angle Θ between the optic axis and the phonon wave vector. For Θ=0 and Θ=Π/2 the phonons are exactly transversal or longitudinal so that they can be assigned to be of totally symmetric or twofold degenerate type. Careful measurements of the directional dispersion of all phonon modes of LiNbO3 form a firm basis for a new complete assignment. 6 of the total number of 13 dispersion branches previously given in the literature had to be reassigned.
47 citations
TL;DR: In this article, the diffraction phenomena of X-rays in the case ΘB≅π/2 are studied on the basis of dynamical theory, and the angular width of diffraction for Θ B = π/2, 2√|χh|, is about 103 times as broad as the one for ϘB ⪡π/ 2, 2|Xh|/sin Θb.
Abstract: Abstract The diffraction phenomena of X-rays in the case ΘB≅π/2 are studied on the basis of dynamical theory. The angular width of diffraction for ΘB= π/2, 2√|χh| , is about 103 times as broad as the one for ΘB ⪡π/2, 2|Xh|/sin ΘB . Similar characteristic phenomena are expected for electrons. The π/2 Bragg angle diffraction would be utilized as X-ray resonator.
44 citations
TL;DR: In this paper, the authors measured the kinetic energy of the product ions of the benzonitrile, benzene and thiophene molecules produced in well-defined states of excitation by charge exchange with Xe+, Kr+, Ar+, CO+, and N2+.
Abstract: Abstract Ions of the benzonitrile, benzene and thiophene molecules were produced in well-defined states of excitation by charge exchange with Xe+, Kr+, Ar+, CO+, and N2+. Their subsequent unimolecular decomposition was followed, as a function of time, by allowing the dissociations to take place within a strong homogeneous draw-out field and measuring the kinetic energy of the product ions. The decompositions were found to be purely exponential within experimental error, and the corresponding decay rate constants k (for 2 · 105 sec-1 ≦ k ≦ 5 · 108 sec-1) proved to be monotonically increasing functions of the excitation energy. These are the first unambiguous measurements of this function k(E) for any molecule. In the case of benzene, the reactions C6H6+ → C6H5+ + H and C6H8+ → C4H4+ + C2H2 were found to be definitely not in competition with one another. Furthermore the dependence of the excitation energy on the impact energy was measured. A variation of the impact energy between 10 and 200 eVc.m. only changes the internal energy E by about 0.1 eV. This value appears to be in qualitative agreement with calculations on near-resonant charge transfer by Gurnee and Magee.
39 citations
TL;DR: In this paper, it is shown that these elements satisfy the most important relations which are known to be valid for the classical implication, and that these results seem to justify the interpretation of this lattice as a propositional calculus sometime called quantum logic.
Abstract: Abstract In the lattice of subspaces of the Hilbert space elements can be defined which may be con-sidered as generalized implications. It is shown, that these elements satisfy the most important relations which are known to be valid for the classical implication. These results seem to justify the interpretation of this lattice as a propositional calculus sometime called quantum logic.
Abstract: Abstract Expressions are derived for the optical potential and related mean free path as a function of energy in the range 20 eV to 100 keV for electrons and estimates are made of the contributions from the electronic excitations, thermal diffuse scattering and disorder scattering in Al, Cu and Au.
TL;DR: In this paper, a theoretical and experimental study of the phenomenon of diffusion slip in a binary gas mixture is presented, where a very general variational expression given earlier by Loyalka is rederived via the use of a method developed recently.
Abstract: Abstract A theoretical and experimental study of the phenomenon of diffusion slip in a binary gas mixture is presented. To provide some physical insight, a very general variational expression given earlier by Loyalka is rederived via the use of a method developed recently. The case of Maxwellian diffuse specular reflection is considered in some detail and the inadequacies of previous theoretical results based on the early arguments of Maxwell, kinetic models and simple intermolecular force laws are discussed. Although in general, the variational results (or the equivalent results given here) together with the assumptions of Lennard Jones potential and diffusive reflection give a satisfactory agreement with the available experimental data, it is found that for isobaric (isotopic) mixtures, in the choice of the intermolecular and gas-surface interaction parameters special care should be taken in that the results are quite sensitive to small variations in the values of these parameters.
TL;DR: In this paper, an integral representation of Dirac's density matrix is evaluated in the approximation that V(r) at the point r is replaced by a linear potential with a gradient equal to the gradient of V at r.
Abstract: Abstract For a system of independent electrons moving in a common one-body potential V (r) an integral representation of Dirac's density matrix is evaluated in the approximation that V(r) at the point r is replaced by a linear potential with a gradient equal to the gradient of V at r. The particle density ᵨ, ∇ᵨ and the kinetic-energy density εk are derived from the density matrix. After eliminating the potential and its gradient a parametric representation for εk in terms of ᵨ and y = |∇ᵨ |½ ᵨ-⅔ is obtained. Explicit analytical expressions are given in the limits y → 0 and y → ∞ and compared with the inhomogeneity corrections of Kirzhnits and v. Weizsäcker.
TL;DR: In this article, the longitudinal proton relaxation time T1 in methyl naphthalene crystals, differing in the arrangement and number of the substituted CH3 groups, was measured as a function of the temperature above 77 °K and the magnetic field between 0.9 and 20 kOe.
Abstract: Abstract The longitudinal proton relaxation time T1 in methyl naphthalene crystals, differing in the arrangement and number of the substituted CH3 groups, was measured as a function of the temperature above 77 °K and the magnetic field between 0.9 and 20 kOe. The results can be described by hindered rotation of the methyl groups with the jumping times and activation energies strongly dependent on the group arrangement. In the β-position the rotational barrier of 0.8 kcal/mol is predominantly determined by the infermolecular interaction, whereas in the case of the a-position and for adjacent CH3’s the hindering potential of 2.4 kcal/mol arises largely from the intramolecular term.
TL;DR: In this article, the authors showed that the spin lattice relaxation time T1 of protons in naphthalene and anthracene crystals is decreased by the presence of metastable triplet excitons in these crystals.
Abstract: Abstract The spin lattice relaxation time T1 of protons in naphthalene and anthracene crystals is decreased by the presence of metastable triplet excitons in these crystals. The exciton induced relaxation time T1, Exc is measured at 300 °K and 77 °K and at Larmor frequencies between 0.4 and 44 MHz. From the frequency dependence of T1, Exc the exciton-proton correlation time τc at the two temperatures is determined to 5.5 · 10 -12 sec and 4.1·10 -12 sec (±10%) in anthracene and 2.1·10-11 sec and 1·10 -11 sec in naphthalene, assuming scalar proton-electron interaction and statistical incoherent hopping motion of the excitons. The predictions of a theoretical model for exciton-migration are in satisfying agreement with the experimental results as far as experimental data for the parameters of that model are available.
TL;DR: In this article, the relaxation times T1, T2, T1Q and T2Q were calculated in the weak collision case in the presence of anisotropy of the chemical shifts.
Abstract: Abstract The relaxation times T1 , T2 , T1Q and T2Q are calculated in the weak collision case in the presence of anisotropy of the chemical shifts.
TL;DR: In this paper, the Bethe theory of electron diffraction is used to calculate the specular intensity as a function of incident angle, and a generalized Hill's determinant method is used for numerical calculations instead of the more common but slower matrix-eigenvalue technique.
Abstract: Abstract The Bethe theory of electron diffraction is used to calculate reflection electron diffraction intensities for medium and high energy electrons. A generalized Hill's determinant method is used for the numerical calculations instead of the more common but slower matrix-eigenvalue technique. Results of a \"systematics\" calculation of the specular intensity as a function of incident angle are compared with some experimental values for the Si (111) surface. The application of the Bethe theory to crystals where the surface structure differs from the bulk is also considered.
TL;DR: In this article, positive ions were extracted from the negative glow (NG) of a cylindrical hollow cathode discharge (h.c.d.) through a plane sampling probe (diameter 1 mm, orifice dia. 50 [xm, Molyb-denum) and analyzed in a 60° mass spectrometer.
Abstract: Abstract Positive ions were extracted from the negative glow (NG) of a cylindrical hollow cathode discharge (h.c.d.) through a plane sampling probe (diameter 1 mm, orifice dia. 50 [xm, Molyb-denum) and analyzed in a 60° mass spectrometer. The discharge was operated in pure Argon with an admixture of about 0.1% of H20. The principal ions were Ar+, Ar++,r2+, ArH+ , H20+ and H3O+ . The individual ion currents were measured as functions of the probe potential U\\>, at dif-ferent discharge currents (2 - 7 mA) and of the gas pressure (0,05 - 0,5 torr). The reaction scheme for the production and conversion of these ions in the NG could be established. In general, the ions were extracted from the axis of the h.c.d. When the sampling probe was mounted excentrically (3 mm out of the axis), the ratio of secondary to primary ions was higher in the axis. At probe potentials above +15 v referred to anode no ions from the NG reached the probe, but electrons were accelerated so that ionization of the neutral gas could arise in and behind the orifice. The Ar++ current as a function of Ub together with other results suggested the presence of very fast electrons (energy > 100 v) in the sampling orifice. They could be identified as electrons from the cathode fall region which were scattered into the orifice. Studies of the ion currents from Ar-H2 mixture proved the sampling probe suitable as an electron impact ion source.
TL;DR: In this paper, the principles of new electron optical imaging systems which make possible the threedimensional image construction of a small object are described, and it is shown that there is a good chance to correct "ring zone segment"-systems to reach atomic resolution with or without image-reconstruction-calculations.
Abstract: Abstract Threedimensionally Imaging Electron Microscopes The principles of new electron optical imaging systems will be described which make possible the threedimensional image construction of a small object. Data of threedimensional Fourier space are collected by the registration of several images using primary beams with different tilting angles. The simplest device of such a type - a magnetic fly's eye system - will lead to spherical aberrrations larger than about 20 mm. It will be shown, that there is a good chance to correct “ring zone segment”-systems to reach atomic resolution with or without image-reconstruction-calculations. Not only microscopes with conventional bright field and dark field imaging but also transmission scanning microscopes can be constructed usind these principles.
TL;DR: In this article, a high-resolution Raman spectrum of the oxygen molecule has been analyzed taking into account the spin-splitting of its rotational levels, the contribution of 16O18O-isotope and N2-impurity.
Abstract: Abstract A high-resolution Raman spectrum of the oxygen molecule has been analysed taking into account the spin-splitting of its rotational levels, the contribution of 16O18O-isotope and N2-impurity. The intensity contour has been simulated by a computer program.
TL;DR: In this article, time-dependent gain measurements of a pulsed iodine laser amplifier have been used to determine decay rates of excited iodine atoms 52P1/2, and rate constants are reported for the iodine deactivation by CF3I and C2F6.
Abstract: Abstract Time-dependent gain measurements of a pulsed iodine laser amplifier have been used to deter-mine decay rates of excited iodine atoms 52P1/2 . Rate constants are reported for the iodine deactivation by CF3I and C2F6 . Attempts to correlate the results with predictions from a kinetic model indicate that excited iodine atoms are formed not only in the primary photolysis but also by additional chemical processes.
TL;DR: In this paper, a method is described to obtain information on the true difference potential of interatomic interaction from the quasistatic wings of spectral lines broadened by neutral atoms.
Abstract: Abstract A new method is described to obtain information on the true difference potential of interatomic interaction from the quasistatic wings of spectral lines broadened by neutral atoms. Based on the one-particle-approximation of the quasistatic theory of line broadening, it does not rely on any particular model function. Furthermore, from measurements of the temperature dependence of the line wings it may yield additional information on the potentials for the initial or final state of the line separately. The method is appplied to measurements of the red wing of the mercury absorption line 61S0 - 63P1 λ 2537 Å, perturbed by argon.
TL;DR: In this paper, the phase diagrams of the Li2SO4 - Rb2SO 4 and Li2 SO4 - CsSO 4 were determined by means of differential thermal analyses and electrical conductivity measurements.
Abstract: Abstract The phase diagrams of the systems Li2SO4 - Rb2SO4 and Li2SO4 - Cs2SO4 have been determined by means of differential thermal analyses and electrical conductivity measurements. We have also modified our previously published phase diagram of the system Li2SO4 - Na2SO4 , especially in the region near the equimolar mixture.
TL;DR: In this article, it was shown that desorption of CO from a Pd surface reaches its maximum rate within the time constant of the system ( ~ 0.5 msec) and this effect can be used to investigate the kinetics of adsorption /desorption processes by means of relaxation methods.
Abstract: Abstract Absorption of a pulse of laser light in the surface region of a solid causes a fast temperature increase, followed, after the end of the laser pulse, by a rapid decrease to the base temperature. Experiments with a Nd laser (power 105 - 106 W/cm2, half width of the pulse 150 μsec) showed that desorption of CO from a Pd surface reaches its maximum rate within the time constant of the system ( ~ 0.5 msec). This effect can be used for example to investigate the kinetics of adsorption /desorption processes by means of relaxation methods.
TL;DR: In this article, the width and shift operator occurring in a previously developed unified theory of Stark broadening by plasma electrons is approximated by different expressions which include time ordering to various orders (up to order four).
Abstract: Abstract The \"width and shift\" operator occurring in a previously developed unified theory of Stark broadening by plasma electrons is approximated by different expressions which include time ordering to various orders (up to order four). For the special case of the Lyman-a line in the dipole approximation results also including time ordering to all orders are calculated and compared to the approximate results. Numerical evaluations for this case are presented using the wing expression of the unified theory. A particularly good agreement with the exact profile is obtained for an approximation which maintains time ordering to fourth order. As for the accuracy of un-time-ordered profiles, our results are less optimistic than those of Godfrey et al. The role of an accurate thermal average concerning the effect of time ordering is emphasized.
TL;DR: In this article, the authors analyzed the scattering amplitude for fast electrons diffracted from a plate of crystal is analyzed in terms of multiple scattering diagrams and showed that to every individual scattering process in an arbitrary orientation there exists a process in the reciprocity configuration equal in amplitude and phase, that is, to every diagram there corresponds a reciprocal diagram.
Abstract: Abstract The scattering amplitude for fast electrons diffracted from a plate of crystal is analyzed in terms of multiple scattering diagrams. It is shown that to every individual scattering process in an arbitrary orientation there exists a process in the reciprocity configuration equal in amplitude and phase, that is, to every diagram there corresponds a reciprocal diagram. It is shown that the geometric part of the diagram may be expressed in terms of the divided differences of the kinematic shape transform.
TL;DR: In this paper, the authors measured the Miscibility gaps for the solid systems AgCl + NaCl and AgBr+NaBr using a high temperature X-ray technique and calculated the thermodynamic properties of the two systems using only experimental solubility data.
Abstract: Abstract Miscibility gaps for the solid systems AgCl + NaCl and AgBr+NaBr have been measured by a high temperature X-ray technique. For the two studied systems the solubility curves are very nearly symmetrical in respect to the compositions xNaCl = .513 and xNaBr = .506, while the upper critical temperature are 198 °C for AgCl + NaCl and 285 °C for AgBr+NaBr. The thermodynamic properties of the two solid systems have been calculated using only experimental solubility data. Values of activity and of enthalpy of mixing were estimated and compared with those reported in literature.
TL;DR: In this paper, the following three primary processes were observed at all wavelengths C2H4 + hν → C 2H2 + H2, (1) C 2 H2 + 2H → C2 H3 + H.
Abstract: Abstract Ethylene photolysis was investigated at 147, 163, 185 and 193 nm. The following three primary processes were observed at all wavelengths C2H4 + hν → C2H2 + H2, (1) C2H4 + hν → C2H2 + 2H, (2) C2H4 + hν → C2H3 + H. (3) (1) was independent of all experimental parameters tested: temperature, pressure, wavelength, scavenger, and light intensity. (2) decreases with increasing pressure and increases with increasing photon energy. (3) shows almost no pressure dependence at 185 nm and decreases in importance with increasing photon energy. A kinetic analysis shows that participation of at least three different states of ethylene must be invoked to explain the results. A tentative correlation of these states with the spectroscopic states of ethylene is given.
TL;DR: In this paper, the untersuchung der IR-und Raman-Spektren von (CD3)3SiCl and (CD 3)3SiOSi (CD-3) 3 führt zu einer Neuzuordnung der ρCH3-Schwingungen in Trimethylsilylverbindungen.
Abstract: Abstract Die Untersuchung der IR-und Raman-Spektren von (CD3)3SiCl und (CD3)3SiOSi (CD3)3 führt zu einer Neuzuordnung der ρCH3-Schwingungen in Trimethylsilylverbindungen. In Normalkoordina-tenanalysen an diesen Verbindungen sowie an HSi (OCH3)3/HSi (OCD3)3, die sich auf die gemes-senen Isotopenverschiebungen stützen, wird das Schwingungsverhalten der Methylgruppen durch die Einbeziehung von ρCH3 (CD3)-Koordinaten angemessener wiedergegeben als durch eine Betrachtung als Punktmassen. Für die vornehmlich interessierenden Valenzkraftkonstanten ergeben sich folgende verbesserte Werte: f(SiCl) 2,84; f(SiO) 4,60 (Hexamethyldisiloxan) bzw. 5,13 mdyn/Å (Tri-methoxysilan) und f(SiC) 2,84-2,89 mdyn/Å