The Systematics of Muonium Hyperfine Constants

The hyperfine constants for muonium in elemental and binary inorganic solids suggest formation of three different families of defect centre, with distinct electronic structures. The overall range of values, spanning nearly five orders of magnitude, and their correlation with host properties such as band gap and electron affinity, reveal a deep-to-shallow instability which has profound implications for the electrical properties of hydrogen impurity in electronic materials, both semiconducting and dielectric.     

Solid state physics at ISOLDE

Radioactive atoms have been used in solid state physics and in materials science for decades. Besides their classical applications as tracers for diffusion studies, nuclear techniques such as Mössbauer spectroscopy, perturbedγγ angular correlation,β-NMR, and emission channeling make use of nuclear properties (via hyperfine interactions or emittedα orβ particles) to gain microscopic information on structural and dynamical properties of solids. During the last decade, the availability of many different radioactive isotopes as clean ion beams at ISOL facilities like ISOLDE/CERN has triggered a new era involving methods sensitive to the optical and electronic properties of solids, especially in the field of semiconductor physics. This overview will browse through ongoing solid state physics experiments with radioactive ion beams at ISOLDE. A wide variety of problems is under study, involving bulk properties, surfaces and interfaces in many different systems like semiconductors, superconductors, magnetic systems, metals and ceramics.

     

应变对析氢反应催化剂TiC2性能影响的研究

应变工程是一种有效地用来调整原子薄膜材料的电子、磁性和光学性能的策略.利用第一性原理计算,我们表明应变也可以有效地调节Ti C₂的析氢反应(HER)的催化活性,这是电解水电化学制氢所必需的.我们主要考虑0-8%范围的拉伸应变,研究发现,在25%的氢覆盖率下双轴拉伸比单轴拉伸能更有效的提高HER活性,但b方向拉伸后的Ti C₂结构具有更高的氢最大覆盖率,且b方向的拉伸应变对不同氢覆盖率的Ti C₂单层片的催化性能都有很大的提高.电子结构计算表明,拉伸应变可以激活相对惰性的内部价电子,从而引起体系的失稳和催化活性的提高.在本工作中获得的见解可能有助于利用应变作为一种有效手段来提高二维材料的催化活性,并探索更有效地调整其电子结构和催化活性的新方法.     

Atomic hydrogen and muonium in alkali halides

Atomic hydrogen can be trapped at interstitial and substitutional cation and anion sites in alkali halides. The geometrical structure of these defects was established by electron nuclear double resonance (ENDOR). From the analysis of the ENDOR spectra also detailed information was obtained on the electronic structure. In this article the major experimental and theoretical results for atomic hydrogen in several alkali halides are briefly reviewed. Special emphasis is given to the isotope effects upon replacing hydrogen by deuterium. The nature of the dynamical hyperfine and superhyperfine interactions is discussed. Its magnitude to be expected for muonium is estimated. Recent results on muonium centres are discussed on the basis of the knowledge about the hydrogen centres.     

Partitioning Entropy with Action Mechanics: Predicting Chemical Reaction Rates and Gaseous Equilibria of Reactions of Hydrogen from Molecular Properties

Our intention is to provide easy methods for estimating entropy and chemical potentials for gas phase reactions. Clausius’ virial theorem set a basis for relating kinetic energy in a body of independent material particles to its potential energy, pointing to their complementary role with respect to the second law of maximum entropy. Based on this partitioning of thermal energy as sensible heat and also as a latent heat or field potential energy, in action mechanics we express the entropy of ideal gases as a capacity factor for enthalpy plus the configurational work to sustain the relative translational, rotational, and vibrational action. This yields algorithms for estimating chemical reaction rates and positions of equilibrium. All properties of state including entropy, work potential as Helmholtz and Gibbs energies, and activated transition state reaction rates can be estimated, using easily accessible molecular properties, such as atomic weights, bond lengths, moments of inertia, and vibrational frequencies. We conclude that the large molecular size of many enzymes may catalyze reaction rates because of their large radial inertia as colloidal particles, maximising action states by impulsive collisions. Understanding how Clausius’ virial theorem justifies partitioning between thermal and statistical properties of entropy, yielding a more complete view of the second law’s evolutionary nature and the principle of maximum entropy. The ease of performing these operations is illustrated with three important chemical gas phase reactions: the reversible dissociation of hydrogen molecules, lysis of water to hydrogen and oxygen, and the reversible formation of ammonia from nitrogen and hydrogen. Employing the ergal also introduced by Clausius to define the reversible internal work overcoming molecular interactions plus the configurational work of change in Gibbs energy, often neglected; this may provide a practical guide for managing industrial processes and risk in climate change at the global scale. The concepts developed should also have value as novel methods for the instruction of senior students.     

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.     

Nuclear motion corrections to the binding energy in hydrogen

Most of the calculations of nuclear recoil corrections to the atomic binding in hydrogen have been done using the covariant Bethe-Salpeter equation. In this paper an alternative to the Bethe-Salpeter approach in the form of a modified Dirac equation is presented. It contains the usual Hamiltonian for an electron in the field of a static proton, but it also includes the proton's kinetic energy and an interaction term due to transverse photons. The part of the interaction which produces the hyperfine splitting is extracted and treated perturbatively, whereas the remainder of the potential is retained, rearranged, and approximated in such a way as to make the resulting equation soluble. In a simple way, we are able to obtain reduced mass corrections to the fine structure and the hyperfine structure of hydrogen. An extension of the work, which enables us to calculate additional recoil terms not included in our lowest order effective potential, is briefly described.     

Electron microscopy of pharmaceutical systems

During the last decades, the focus of research in pharmaceutical technology has steadily shifted towards the development and optimisation of nano-scale drug delivery systems. As a result, electron microscopic methods are increasingly employed for the characterisation of pharmaceutical systems such as nanoparticles and microparticles, nanoemulsions, microemulsions, solid lipid nanoparticles, different types of vesicles, nanofibres and many more. Knowledge of the basic properties of these systems is essential for an adequate microscopic analysis. Classical transmission and scanning electron microscopic techniques frequently have to be adapted for an accurate analysis of formulation morphology, especially in case of hydrated colloidal systems. Specific techniques such as environmental scanning microscopy or cryo preparation are required for their investigation. Analytical electron microscopic techniques such as electron energy-loss spectroscopy or energy-dispersive X-ray spectroscopy are additional assets to determine the elemental composition of the systems, but are not yet standard tools in pharmaceutical research. This review provides an overview of pharmaceutical systems of interest in current research and strategies for their successful electron microscopic analysis. Advantages and limitations of the different methodological approaches are discussed and recent findings of interest are presented.     

Recent Progress in the Study on Muonium Centers in Semiconductors

The electronic structures and the locations of muonum centers (Mu) were investigated in single crystalline ZnO. Mu centers with extremely small hyperfine parameters have been observed below 40 K. It is inferred from their small ionization energy and hyperfine parameters that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO. This revised version was published online in August 2006 with corrections to the Cover Date.     

BIMEVOX as dense membrane in catalytic reactor (ME = Co,Cu, Ta)

A catalytic dense membrane reactor allows to physically separate the oxygen feed from the reactant (hydrocarbon) feed with a catalytic membrane chosen among oxide ion conducting materials. The membrane plays a double role, it provides the oxygen needed for selective oxidation and acts as a catalyst. The catalytic properties of BIMEVOX (ME = Co, Cu, Ta) membranes were examined in the mild oxidation of propene and of propane. During the complex transient state observed when the surface is rough, the nature and distribution of products are different from those obtained with mirror-polished membranes in a former work. In particular, syngas is formed with propene as well as with propane, and it precedes the production of hydrogen and coke. The complex behaviour differs according to ME and seems to be related to the different nature of electron semi-conduction induced by each dopant.     

Hyperfine interactions of impurities in defect sites in ion-implanted metals

The lattice location of ion-implanted radioactive isotopes in metals and their interactions with defects in their own radiation damage cascade are of importance for studies of the hyperfine interactions of such probe atoms by nuclear methods. Recent results from Mössbauer and PAC experiments in particular are reviewed. Emphasis is put on lattice site identifications, which can be inferred from measured lattice-dynamical and hyperfine interaction parameters of probe atoms. Some general conclusions on the hyperfine interactions in interstitial- and vacancy-type complexes are drawn.     

Heavy-atom skeleton quantization and proton tunneling in "intermediate-barrier" hydrogen bonds

Quantum effects on proton transfer through barriers of several kcal/mol in hydrogen bonds are investigated theoretically in malonaldehyde. Such "intermediate-barrier" proton transfer processes play a key role in the catalytic activity of some enzymes. Tunneling is shown to be significant in this reaction even at room temperature. More importantly, the quantum nature of the heavy molecular frame atoms is found to substantially enhance proton tunneling. These findings have far-reaching implications for common modeling strategies of proton transfer in complex systems such as biomolecules.     

Synthesis,Morphological Control,and Properties of Silver Nanoparticles in Potential Applications

Nanostructured materials, especially nanoparticles (NPs), of noble metal NPs such as silver (Ag) have been the focus of research in recent decades because of their distinct physical, chemical, and biological properties. These materials have attracted considerable attention because of their potential applications, such as catalysis, biosensing, drug delivery, and nanodevice fabrication. Previous studies on Ag NPs have clearly demonstrated that their electromagnetic, optical, and catalytic properties are strongly influenced by their shape, size, and size distribution, which can be varied by using different synthetic methods, reducing agents, and stabilizers. The valuable optical properties of Ag NPs have allowed for new approaches in sensing and imaging applications, offering a wide range of detection modes, such as colorimetric, scattering, and surface‐enhanced Raman scattering techniques, at extremely low detection limits. Here, an overview of the various chemical, physical, and biological properties of Ag NP fabrication approaches to obtain the various shapes and sizes is presented.     

2s Hyperfine splitting in light hydrogen-like atoms: Theory and experiment

Since the combination D ₂₁ = 8f _HFS(2s)-f _HFS(1s) of hyperfine intervals in hydrogen and light two-body hydrogen-like atomic systems weakly depends on the nuclear structure, comparison between theory and experiment can be sensitive to high order QED corrections. New theoretical and experimental results are presented. Calculations have been performed for the hydrogen and deuterium atoms and for the helium-3 ion. Experiments on the 2s hyperfine splitting (responsible for the dominant contribution to the error in D ₂₁) have been conducted for hydrogen and deuterium. The theory and experiment are in good agreement, and their accuracy is comparable to that attained in verifying the QED theory of the hyperfine splitting in leptonic atoms (muonium and positronium).     

Dynamics of defects in semiconductors

Hyperfine interaction techniques involving radioactive probe atoms like the perturbed angular correlation technique (PAC) and the Mössbauer effect have, due to their inherent sensitivity, successfully been applied to the study of defects in semiconductors. By probing the characteristic charge distribution around the probe atom interacting with a defect, they contributed to the microscopic understanding of the nature, structure and stability of complexes formed between radioactive dopant atoms and defects present in elemental and compound semiconductors. Moreover, dynamic effects can be studied by hyperfine interaction probe techniques. In this case, dynamics always means the fluctuation of a charge distribution resulting in a time dependent hyperfine interaction within the time scale defined by the lifetime of the isomeric nuclear state used for the measurement. Such fluctuations can either be caused by structural changes like local rearrangements of a defect complex or by electronic transitions in the semiconductor resulting in a change of the charge state of a defect complex. Examples using PAC to monitor such processes will be discussed for the semiconductor silicon.     

Functionalization of carbon nanotubes/graphene by polyoxometalates and their enhanced photo-electrical catalysis

Carbon nanotubes and graphene are carbon-based materials, which possess not only unique structure but also properties such as high surface area, extraordinary mechanical properties, high electronic conductivity, and chemical stability.Thus, they have been regarded as an important material, especially for exploring a variety of complex catalysts. Considerable efforts have been made to functionalize and fabricate carbon-based composites with metal nanoparticles. In this review,we summarize the recent progress of our research on the decoration of carbon nanotubes/graphene with metal nanoparticles by using polyoxometalates as key agents, and their enhanced photo-electrical catalytic activities in various catalytic reactions. The polyoxometalates play a key role in constructing the nanohybrids and contributing to their photo-electrical catalytic properties.     

Perturbed angular correlation study of the ion exchange of indium into silicalite zeolites

Two indium-containing silicalite zeolites (In/H–ZSM5) catalysts prepared by wet impregnation and ionic exchange were characterized by the Perturbed Angular Correlation (PAC) technique using ¹¹¹In as probe to determine the nature of the indium species. Some of these species take part in the catalytic reaction of the selective reduction (SCR) of NO_x with methane. PAC experiments were performed at 500oC in air before and after reduction–reoxidation treatments on the catalysts in order to determine the origin of the different hyperfine interactions and then the degree of ionic exchange. Complementary catalytic activity characterizations were also performed. PAC experiments performed on the catalyst obtained by wet impregnation showed that all In-atoms form In₂O₃ crystallites while almost 70% of In-atoms form In₂O₃ in the catalyst obtained by ionic exchange. The PAC experiments of both catalysts performed after the reduction–reoxidation treatment revealed the presence of two hyperfine interactions, different from those corresponding to indium in In₂O₃. These hyperfine interactions should be associated to disperse In species responsible of the catalytic activity located in the ionic exchange-sites of the zeolites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Energy levels of the hydrogen atom due to a generalized Dirac equation

The consequences of a generalized Dirac equation are discussed for the energy levels of the hydrogen atom. Apart from the usual generalizations of the Dirac equation by adding new interaction terms, we generalize the anticommutation rule of the Dirac matrices, which leads to spin-dependent propagation properties. Such a theory can be looked at as a model theory for testing Lorentz invariance or as an outcome of pregeometric dynamical induction schemes for space-time structure.For special examples of generalized Dirac matrices including perturbation terms with respect to the SRT Dirac matrices, we derive the energy level of the hydrogen atom and find a hyperfine splitting due to these perturbations. A comparison of this additional splitting with Lamb shift measurements gives us upper limits for possible perturbations, which turn out to be of measurable magnitude. Spin precession experiments give much more restrictive limits. So, it turns out that the hydrogen atom is not such a sensitive indicator for the Lorentz invariance as widely believed.     

Electronic structure of the muonium center as a shallow donor in ZnO

The electronic structure and the location of muonium centers (Mu) in single-crystalline ZnO were determined for the first time. Two species of Mu centers with extremely small hyperfine parameters have been observed below 40 K. Both Mu centers have an axial-symmetric hyperfine structure along with a <0001> axis, indicating that they are located at the antibonding (AB(O, parallel )) and bond-center (BC( parallel )) sites. It is inferred from their small ionization energy ( approximately 6 and 50 meV) and hyperfine parameters ( approximately 10(-4) times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO.     

Dynamics of Adsorbate Islands with Nanoscale Resolution

Surface catalytic processes produce, under certain conditions, small clusters of adsorbed atoms or groups, called islands which, after they have been formed, move as individual entities. Here we consider the catalytic reduction of NO with hydrogen on platinum. (i) Using video field ion microscopy, we observe the dynamic motion of small hydroxyl islands on the Pt(001) plane; despite changes in their morphology, the islands dimensions are confined to values corresponding to 10 to 30 Pt atoms suggesting cooperative effects to be in operation. (ii) We construct an automaton (or lattice Monte-Carlo) model on the basis of a set of elementary processes governing the microscopic dynamics. The agreement between the simulation results and the experimental observations suggests a possible mechanism for the formation and dynamics of hydroxyl islands.