Zum problem der Exoelektronenemission aus Alkalihalogeniden

Ohne Zusammenfassung

---

     

Zur Theorie des Exzitons in den Alkalihalogeniden

Following the many-body treatment of exciton byHaken andSchottky we represent the electronic polarization of the lattice induced by the excess electron and the hole in the Hamiltonian as well as in the wave function by introducing localized pair states and their coupling with the exciton particles. By application of a unitary transformation the pair states may be eliminated and one obtains the effective Schrödinger equation of an exciton consisting of particles dressed with polarization clouds. The coupling coefficients are determined by a minimum condition for the polarization energy. The effective interaction law between electron and hole and their effective masses are investigated explicitly. By transformation of the Schrödinger equation into a system of difference equations and solution of the corresponding secular equation one obtains an expression for the binding energy of the lowest exciton state in the alkali halides, which represents a connection of the “excitation” and “transfer” models. For the iodides of Na, K and Rb the binding energies were calculated using an approximative estimate for the various matrix elements involved. They are in good agreement with the experimentally observed energy differences between the first absorption peak and the small shoulder which indicates the position of the band edge.     

Anionische Silberzentren („B-Zentren“) in Alkalihalogeniden

It is proposed that negatively charged silver ions on anion sites are responsible for theB bands found in silver doped alkalihalides. Experimental investigations confirming this model of theB center are presented. Optical absorption and emission ofB centers were measured in seven alkalihalides in the temperature range from 450 to 4 °K. Configuration coordinate diagrams were obtained for KCl, KBr, and KI. A chemical method was used to determine the charge of theB center. Thermal or optical excitation causes theB centers to dissociate into neutral silver atoms andF centers. The Ag⁰ centers are bound to interstitial positions at low temperature. At high temperature they collect together forming colloidal centers. TetragonalB _A centers were formed during optical bleaching of theB band in mixed crystals of the type KCl+ΔNaCl. Their absorption and emission spectra, optical orientation, and polarized emission were investigated. These properties are shown to be similar to those ofF _A centers.B _A fluorescence polarized almost completely in the [100] direction was observed. A Jahn-Teller splitting of theB absorption band was resolved in RbCl at low temperature. The splitting confirms the configuration5s² for the Ag^? ion. The results are compared with those for the isoelectronic centers In⁺ and Sn⁺⁺. Thin films of alkalihalides containing small amounts of silver, copper, or thallium were condensed simultaneously with alkali vapor. New bands in the UV region were found, possibly due to Cu^? and Tl^? centers.     

Paraelastisches Verhalten von S2 −-Zentren in Alkalihalogeniden

The reorientation of S₂ ^? molecule ions on anion sites in alkali halide crystals under mechanical stress has been investigated by means of electron spin resonance and optical measurements. The reorientation rate is given by an Arrhenius relation. Tunneling in the ground state of libration is unimportant contrary to the case of the O₂ ^? center. The stress-induced dichroism permitted identification and assignment of two optical absorption bands of the S₂ ^? center.     

Entstehung von Alkalimetallen bei der Elektronenbestrahlung von Alkalihalogeniden

We have observed additional peaks in the energy loss spectra of NaBr, NaI, KF, KI, and RbF foils, which are caused by the electron impact. They are identified as volume and surface plasma losses of alkali metal particles. The energy loss values are in accord with energy loss data of alkali metal foils. Electron diffraction patterns of the irradiated alkali halide foils showed additional rings of alkali metal agglomerates, the size of which are estimated to be in the range of 100Å.     

Zur Frage derZ-Zentren bei der Exoelektronenemission aus Alkalihalogeniden

Zusammenfassung Es wurde das Maximum des Exoelektronenemissionsstroms untersucht, das an NaCl infolge Verunreinigung durch Kalzium auftritt, und die Möglichkeit, dieses Maximum denZ-Zentren zuzuordnen, diskutiert.

---

Z-

, NaCl . Z-.

---

---

Wir danken herzlichst Herrn Dr. A. Bohun für das gefällige überlassen des NaCl: Ca-Materials.     

Eine neue Methode zur Bestimmung des Diffusionskoeffizienten von Wasserstoff in Alkalihalogeniden

The diffusion coefficient of hydrogen in KCl-, KBr- and KJ-crystals is measured by a new method, and the activation energy is evaluated by the temperature-dependence between 187 and 446 °C.     

Die optische Absorption der U2-Zentren in Alkalihalogeniden des NaCl-Typs

The UV absorption of the U₂ center in nine alkalihalides at 20 °K is presented. In the bromides and iodides the U₂ band consists of three components. This splitting is interpreted as a combined effect of spin-orbit coupling and dynamic lattice distortion. The energy of the strongest U₂ band is calculated by using the effective charge of the H atom in the U₂ ground state as a parameter. This comes out to be ?0.17 e in chlorides, ?0.20 e in bromides, and ?0.25 e in iodides. In mixed crystals of KBr+Δ KJ U₂ centers of the general kind HBr _n J_4?n (n=0, ..., 4) are investigated. They form three classes of U₂ bands, which are distinguished by thep-orbital in the excited state. This can bep _BrBr,p _BrJ,p _JJ depending on what halide ions the defect electron is located in the excited state. Dichroic bleaching of each band with polarized light allows to orient the complexes. This demonstrates that by optical stimulation the H atom can diffuse through the interstitial lattice. By warming up to 70 °K HBr₄ changes to HBr₃J which becomes unstable above 80 °K.     

Die elastische Wechselwirkung zwischen O 2 − -Zentren in Alkalihalogeniden-Zentren in Alkalihalogeniden

The interaction between two elastic dipoles has been studied directly by means of electron spin resonance. The investigated system consists of pairs of O ₂ ^? centers on nearest neighbor anion sites in KCl, KBr and KI. Since the O ₂ ^? centers reorient at temperatures as low as 1°K a pair assumes the configuration of lowest energy at these temperatures. The investigation of the EPR spectrum of the pairs and its response to externally applied uniaxial stresses has yielded the orientations of the axes of the pair partners in the configuration of lowest energy as well as some information on configurations of higher energy. The experiments are in agreement with the predictions based on continuum elasticity theory. Electric quadrupole interaction and covalency effects are less important. In particular the magnetic interaction is small compared to the elastic interaction.     

Radionuclides in deposition in the Ignalina NPP region in 2005

Results of radionuclide activity concentration measurements in deposition and ground-level air conducted at the station of the Institute of Physics situated 3.5 km from the Ignalina Nuclear Power Plant (Ignalina NPP) in 2005 are analyzed. Atmospheric depositional fluxes of ⁷Be, ⁶⁰Co and ¹³⁷Cs are estimated. Radionuclide particle deposition rates are calculated. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model of the global dispersion and deposition is used to explain variations in the ¹³⁷Cs activity concentration in the ground-level air in the Ignalina NPP region in 2005. An increase in the ¹³⁷Cs activity concentration of up to 17.4 μBq m⁻³ on 30 October-5 November is studied. Modelling results show that the Chernobyl NPP Unit 4 Sarcophagus and the radiocaesium resuspension from the Chernobyl NPP accident polluted regions are sources of ¹³⁷Cs to the environment of the Ignalina NPP. Results on solubility in water of aerosols — carriers of ¹³⁷Cs — are discussed.     

Thermal convection in Rivlin-Ericksen elastico-viscous fluid in porous medium in hydromagnetics

The thermal instability of a layer of Rivlin-Ericksen elastico-viscous fluid in porous medium acted on by a uniform magnetic field is considered. For stationary convection, Rivlin-Ericksen elastico-viscous fluid behaves like a Newtonian fluid. The magnetic field is found to have stabilizing effect whereas medium permeability has destabilizing effect. The magnetic field introduces oscillatory modes in the system, A sufficient condition for the non-existence of overstability is also obtained.     

Universal scaling in BCS superconductivity in three dimensions in non-s waves

The solutions of a renormalized BCS equation are studied in three space dimensions in s, p and d waves for finite-range separable potentials in the weak to medium coupling region. In the weak-coupling limit, the present BCS model yields a small coherence length and a large critical temperature, , appropriate for some high- materials. The BCS gap, , and specific heat as a function of zero-temperature condensation energy are found to exhibit potential-independent universal scalings. The entropy, specific heat, spin susceptibility and penetration depth as a function of temperature exhibit universal scaling below in p and d waves. Received: 18 July 1997 / Revised: 8 September 1997 / Accepted: 29 September 1997     

Activities in fibre-optical communications in Germany

This paper offers a review of the contributions of German research groups in the field of fibre-optical communications until now. Since the latest publications are also taken into account which in part are not yet printed, a survey is given at the same time of current activities in this field in Germany.