Lumineszenz und Photochemie von Schwefelzentren in KCl und KBr

Interstitial hydrogen atoms are produced in alkali halides doped withSH ^–-molecules by UV-irradiation at low temperature. Tempering of the crystals leads to the formation of other sulphur centres which are investigated by optical luminescence and by electron spin resonance. The kinetics of these centres is discussed and models for several molecules are developped.     

Quantum diffusion of muonium in insulators

In this paper we review our recent experiments conducted at TRIUMF on muonium diffusion in alkali halides. First, the technique of longitudinal-field muonium spin relaxation (T ₁) due to nuclear hyperfine interaction, an indispensabletour de force for the present work. is described. It is demonstrated in KCl that the technique provides spectacular sensitivity for muonium diffusion as well as determining the average nuclear hyperfine coupling constant. The muonium hop rate shows a minimum (T ^*≃80 K) and steep increase with decreasing temperature. The result is compared with the current theory of quantum diffusion in non-metallic crystals. A few more sets of new data may be presented for other alkali halides. In addition, we show that muonium forms a delocalized state in NaCl as evidenced by a large change of the average nuclear hyperfine parameter. Related topics of local tunneling system may be briefly reviewed.     

Effects of pressure on magnetic properties of F-centers in rubidium halides

Room temperature magnetic resonance measurements have been made on additively colored RbCl and RbBr and on X-irradiated RbH at pressures up to 8 kbar. In RbCl and RbH pressure shifts of the F-center isotropic coupling constant a₁ were obtained from changes in the resolved EPR hyperfine structure. Pressure shifts of first and second shell hyperfine coupling constants of F-centers in RbBr were found from high pressure ENDOR spectra. Effects of the pressure induced polymorphic phase transformations in RbCl and RbBr are discussed. No evidence was found for a structure transformation in RbH. Room pressure ENDOR measurements on additively colored RbI were made at 77°K. The first and second shell hyperfine coupling constants are reported. All results accord well with the behavior of F-centers in other alkali halides.     

Theory of H sites in undoped crystalline semiconductors

The equilibrium sites of atomic hydrogen and muonium in both doped and undoped elemental semiconductors, as investigated by means of several theoretical methods and experimental techniques, are outlined. A particular emphasis is given to the undoped c-Si results. Two controversial points regarding the H and muonium equilibrium sites have been carefully discussed. The bond centered model has been favored with respect to the vacancy associated one. The T site quite reasonably results to be a metastable site and the most favored candidate for the normal muonium center.     

Endor study of atomic hydrogen in KI-crystals

We report the results of an ENDOR investigation of H⁰_i centres in KI. The superhyperfine (shf) and quadrupole interactions of the unpaired centre electron with two shells of K nuclei and two higher shells of I nuclei could be determined precisely. The results are compared with those obtained previously in the other potassium halides.     

Elektronen-Kern-Doppelresonanz-Untersuchung vonU 2-Zentren in Kaliumchlorid

U ₂-centers in alkali halides are neutral hydrogen atoms in interstitial lattice sites, as has been shown by EPR measurements. The hyperfine interactions with the proton and with the four nearest halogen nuclei are resolved in the EPR spectrum. In order to resolve hyperfine interactions with further nuclei of the surrounding lattice ENDOR measurements have been performed onU ₂-centers in KCl at 77 °K. The analysis of the ENDOR spectra gave precise values for the hyperfine and quadrupole interaction constants of the nearest neighbour chlorine and potassium nuclei. The isotropic hyperfine constant of the chlorine neighbours is 24 times larger than that of the potassium neighbours although both nuclei are on equivalent first shell lattice positions. The hyperfine interactions of second shell potassium nuclei [(1/2, 1/2, 3/2)-position] show an unexpectedly large isotropic hyperfine constant. One expects a pure magnetic dipole-dipole interaction for the outer shell nuclei because of the concentrated hydrogen wave function. Two further chlorine shells could be approximately analysed. A theoretical estimate of the hyperfine and quadrupole interaction constants was made by orthogonalizing the 1s hydrogen wave function to the cores of the surrounding ions. If one takes into account the mutual overlap of neighbouring potassium and chlorine ions, one gets the right order of magnitude of the measured constants and a value of 10.4∶1 for the ratio of the isotropic hyperfine constants of the first shell chlorine and potassium nuclei. The relatively large isotropic constant of the second shell potassium nuclei can also be explained on this basis.     

Some questions of the theory of luminescent centres in crystalline phosphors

The calculation of the properties of admixture centres in the adiabatic approximation consists of two stages: 1) calculation of the adiabatic potentials, and 2) calculation of the different properties of centres on the basis of the known adiabatic potentials and wave functions. This paper is a survey of the theoretical studies carried out on these two problems in the Institute of Physics and Astronomy of the Academy of Sciences of the Estonian SSR. The calculations of adiabatic potentials relate mainly to luminescent centres in the phosphors of alkali halides, the many theoretical problems of which are discussed. The calculations of the spectra are bound up with the use and elaboration of the method of moments, which has permitted very general results to be obtained (the Mössbauer and Spolský effects are also discussed).The paper will be published in full in Czech. J. Phys. A, probably in the May 1963 issue.     

Electronic structure and dynamics of muonium in semiconductors

In this paper, a selection of recent results on muonium in semiconductors is presented. These are primarily taken from Si and GaAs and encompass the electronic structure of the diamagnetic centers, charge state cycling, spin‐exchange scattering and interconversion between muonium states. These experiments illustrate the power of μSR for investigating the behavior of muonium and, by analogy, the technologically relevant isolated hydrogen centers in semiconductors. This revised version was published online in August 2006 with corrections to the Cover Date.     

Triple resonance investigation of F centres in BaFCl

Abstract

In BaFCl two types of F centres can be produced, where the electron occupies a Cl^? vacancy [F(Cl^?)-centres] or an F^? vacancy [F(F^?)-centres]. When producing F(F^?)-centres the simultaneous production of F(Cl^?)-centres cannot be avoided. Both ESR spectra overlap strongly. The ENDOR spectra contain very many lines of both centres and have a very complicated angular dependence. It is shown that the angular dependence of the ENDOR spectra of both F centres can be measured separately by applying a double ENDOR technique (called also Triple Resonance). The ratio of the signal heights of the ENDOR lines compared to the double ENDOR lines is characteristic for the neighbour shell to which the nuclei involved belong. The use of this effect for the analysis of the spectra and its explanation are discussed.     

Electronic structure of thallous centres and Tl+ - Vk recombination in KCl crystal

Parameters of impurity Tl ion required for the calculation of different thallous centres in alkali halides are obtained for the semiempirical method of the intermediate neglect of differential overlap (INDO). The electronic structure of Tl⁺, Tl²⁺ centres in KCl is calculated. The potential energy curves for the recombination of nearest Tl⁺, V_$\text{k}$ centres against the breathing vibrational mode of the V_$\text{k}$ centre are calculated. This recombination (hole trapping) is found to be nonradiative tunneling with small activation energy rather than radiative one. A number of experimental data is also discussed in the light of the present calculations.     

Electronic structure, dynamics, and metastability of muonium in semiconductors

Muonium centers are light hydrogen-like centers formed when positive muons are stopped in crystalline semiconductors. Detailed information on the hyperfine structure, dynamics and metastability of muonium are obtained using a combination of muon spin rotation or relaxation, muon level-crossing resonance and related methods. The expected close similarity to hydrogen, especially with regard to electronic structure, is important since the equivalent information on isolated hydrogen is either less detailed or completely absent. There are also interesting differences between muonium and hydrogen. In particular muonium dynamics are expected to exhibit enhanced quantum mechanical effects since the muon has only 1/9th the proton mass. In this paper we review the current status of experiments.     

The use of ENDOR to identify the atomic structure of defects in diamond

Although nearly 100 paramagnetic defects have been catalogued in diamond by spin Hamiltonian parameters measured by electron paramagnetic resonance (EPR), very few of these have been unambiguously associated with an atomic model. It has been necessary to use electron nuclear double resonance (ENDOR) to obtain enough information to make proper assignment of such models. The reason for the limitation of EPR, and the way in which ENDOR overcomes these limitations are discussed. The interpretation of hyperfine structure in terms of unpaired electrons in molecular orbitals, and of quadrupole interactions in terms of all electrons, paired and unpaired, as a source of information about molecular structure in diamond, is evaluated by reference to some well documented examples. The measurements so far made by ENDOR on defects in diamond are reviewed, and the salient contribution for the assignment of a model for each defect is explained. The details revealed by ENDOR considerably increase knowledge about defects, particularly those involving substitutional nitrogen atoms. This in turn helps in understanding the complex electron and atom, migration processes which go on under appropriate conditions of temperature and pressure, or optical excitation. The possibilities are discussed for using ENDOR to increase the number of well characterized centres.     

Muonium and hydrogen in semiconductors

As impurities in semiconductors muonium and hydrogen should have many similarities. In principle properties such as structures which depend primarily on their identical adiabatic potential energies should be much the same while dynamic characteristic such as diffusion would be quite different. The experimental results are in fact quite different for the two but for other reasons. This paper indicates something of the current interest in hydrogen impurities, which is mostly focussed on passivation and hydrogen pairs with other impurities or intrinsic defects, and then discusses muonium in semiconductors, where the abundant information has to do with the isolated impurity. Consequently a complete picture of hydrogen/muonium at the moment requires the study of both.     

Static and Intermittent Hyperfine Coupling for the Muoniated Radical in Tellurium

The hyperfine constant for muonium defect centres in elemental tellurium, measured spectroscopically at low temperature, corresponds to 7% of the free-atom value. The centres are tentatively identified as the diatomic species TeMu, analogous to the hydroxyl radical, OH. Their spectrum disappears as the centres ionize around 80 K but the muon spin response indicates that the same centres form and reionise, rapidly and repeatedly, above 200 K. The capture and loss of charge carriers provide a model for the electrical activity of hydrogen impurity in this low-gap semiconductor.     

Applications of μSR to the study of fullerenes

A review is given of the current status of SR results on muonium states in the pure fullerenes C₆₀ and C₇₀ as well as on the alkali doped series of C₆₀. In particular the results of studies of the hyperfine interaction of the muonated radicals are discussed in order to determine their electronic distribution and the rotational ordering and dynamics of the fullerenes. Unique information on the alkali doped C₆₀ species is obtained from studies of the endohedral muonium species.     

Interionic potentials in alkali halides and their use in simulations of the molten salts

After an outline of work on rare-gas systems, which serves as a target for parallel work on alkali halides, and an initial brief survey of those parts of this parallel work for which results have been obtained, interionic potential models for alkali halides are considered in some detail. The rigid ion potentials of Fumi and Tosi are discussed and then a major part of the section is devoted to deriving a new set of polarizable ion potentials, which incorporate the ideas behind the lattice dynamical shell model. Extensions which include many-body terms in the potentials are considered briefly and finally the information which can be obtained from alkali halide diatomic molecules is discussed.

In the third section methods of computer simulation for ionic liquids are outlined, concentrating on the molecular dynamics method, and some of the properties which can be obtained by analysing the ion trajectories are listed. Results from simulations, including some new work on LiF, NaCl and RbI, are reviewed.     

A new theory of compressible ions — structures of the alkali halides

Ions in ionic crystals are considered to exist in compressible space-filling polyhedral cells analogous to the Wigner-Seitz cell in metals. Repulsion arises from the compression energy of the ions written as a surface integral over the ionic cells. Two adjustable parameters are introduced per ion with the provision that the same parameters can be used in any crystal of any structure in which the ion occurs. The 18 parameters for the 5 alkali and 4 halogen ions have been determined from PV data on the 20 alkali halides. The important successes of the theory are: (i) All the twenty alkali halides are correctly predicted to occur in their observed structures (ii) The thermal transition in CsCl is explained (iii) The pressure transitions in the alkali halides are predicted well (iv) The calculated values of the variation of transition pressures with temperature agree well with experiment. These results are much better than those obtained by earlier theories.     

Comments on the stability of charge states and locations for hydrogen and muonium in semiconductors

This paper brings together some current concepts concerning hydrogen states in semiconductors (illustrated with reference to silicon but quite generally applicable) and highlight certain findings of recent quantum electronic structure calculations for defect centres involving hydrogen and its isotopes. As regards muonium, the situation which held in the early μSR literature [1] is entirely reversed: the location and structure of Mu^* are now well established [2], whereas the precise location and the nature of the metastability of Mu′ have become open questions! In fact, neither state is stable in the presence of other electrically active impurities, which undoubtedly accounts for the difficulty in detecting paramagnetic protium. The implications for the interpretation of existing data, and for various possible future experiments, are examined.