David M. Schrader

Bio: David M. Schrader is an academic researcher from Marquette University. The author has contributed to research in topics: Positron & Positronium. The author has an hindex of 15, co-authored 46 publications receiving 1309 citations.

Positron and Positronium Chemistry

Abstract: Chapter 1. Introduction (D.M. Schrader and Y.C. Jean). 2. Theoretical Aspects of Positronic Systems (D.M. Schrader). 3. Experimental Techniques in Positron and Positronium Chemistry (Y.C. Jean and D.M. Schrader). 4. Radiation Chemistry: Intraspur Effects and Positronium Formation Mechanics (Y. Ito). 5. Positrons and Positronium in Liquids (H. Nakanishi and Y.C. Jean). 6. Positrons and Positronium Annihilation in Gases (S.C. Sharma). 7. The Slow Positron as a Probe of Surface Chemistry (Y.C. Jean). 8. Positrons and Positronium in Molecular Solids (S.J. Wang and Y.C. Jean). 9. Positron Annihilation Spectroscopy for Chemical Analysis (PASCA) (K.L. Cheng and Y.C. Jean). 10. Positronium Annihilation as a Probe for the Determination of Phase Transitions and other Structural Changes in Micellar Systems, Microemulsions, Surfactant Vesicles, and Polymers (H.J. Ache). 11. Biological Applications of Positron Annihilation (Y.Y. Wang and Y.C. Jean). Subject Index. Author Index.

Principles and Applications of Positron and Positronium Chemistry

Abstract: Introduction to positron and positronium chemistry, YC Jean et al compounds of positrons and positronium, DM Schrader experimental techniques in positron spectroscopy, PG Coleman organic and inorganic chemistry of the positron and positronium, G Duplatre and I Billard physical and radiation chemistry of the positron and positronium, SV Stephanov and VM Byakov positrons and positronium in the gas phase, DM Schrader positron porosimetry, MH Weber and KG Lynn positron annihilation studies on superconducting materials, CS Sundar positronium in Si and SiO2 thin films, R Suzuki applications to polymers, PE Mallon applications to coatings and paints, YC Jean et al positron annihilation electrospectroscopy, S Amdani et al characterization of nanoparticle and nanopore materials, J Xu age momentum correlation (AMOC), H Stoll et al

Hydrogen--antihydrogen interactions

Abstract: Accurate quantum-mechanical calculations on the system hydrogen-antihydrogen ($\mathrm{H}\overline{\mathrm{H}}$) are presented. The energy is calculated using the Ritz variational principle, and is obtained as a function of the proton-antiproton distance in the Born-Oppenheimer approximation. It is everywhere lower and therefore more accurate than that recently calculated by Junker and Bardsley. In particular, the maximum in the interatomic interaction reported by these authors is not seen in this calculation. The cross section for rearrangement annihilation is calculated and found to be uniformly larger than that reported by Junker and Bardsley.

Semiempirical polarization potential for low-energy positron-atom and positron-atomic-ion interactions. I. Theory: Hydrogen and the noble gases

Abstract: A simple model for the effects of polarization is introduced for the study of positron-atom interactions. The model is also used to study positronium-atom interactions by treatment of such systems as the interactions of positrons with polarizable anions. The model depends upon the average dipole polarizability of the atom (or ion) and one disposable parameter, an effective radius. The effective radius is determined by fitting calculated scattering lengths for H and He and the calculated positronium affinity of H to their well-established values, and by interpolating on the ionization potentials for other atoms. The model produces quite satisfactory phase shifts and cross sections for positrons scattering elastically off H, He, Ne, Ar, Kr, and Xe. The model appears to be equal or superior in predictive power to the much more elaborate polarized-orbital model recently formulated by McEachran et al. The model predicts that none of the rare-gas atoms studied will bind a positron. As a measure of how far from binding they are, negative values of the positron affinity are devised by appealing to effective-range theory and a heuristic argument involving the dependence of the calculated scattering length on the disposable parameter in the model.

Simple but accurate calculations on the elastic scattering of electrons and positrons from neon and argon.

Abstract: The method for constructing model polarization potentials for electron- and positron-atom systems, previously presented for targets containing up to two electrons, is extended to neon and argon targets. The empirical character of the method is reduced to the determination of a single effective radius for each target, so that electron scattering calculations can be carried out by reference to positron scattering data, or vice versa. In this work, electron data are used to fix the value of the effective radius of each target, which is then used in positron scattering calculations. For electron scattering, exchange is approximated by a local effective potential with two adjustable parameters. The calculated elastic total and differential cross sections are nevertheless in excellent agreement with recent experimental values and are as accurate as the results from much more elaborate calculations.

Fano resonances in nanoscale structures

Abstract: Modern nanotechnology allows one to scale down various important devices (sensors, chips, fibers, etc.) and thus opens up new horizons for their applications. The efficiency of most of them is based on fundamental physical phenomena, such as transport of wave excitations and resonances. Short propagation distances make phase-coherent processes of waves important. Often the scattering of waves involves propagation along different paths and, as a consequence, results in interference phenomena, where constructive interference corresponds to resonant enhancement and destructive interference to resonant suppression of the transmission. Recently, a variety of experimental and theoretical work has revealed such patterns in different physical settings. The purpose of this review is to relate resonant scattering to Fano resonances, known from atomic physics. One of the main features of the Fano resonance is its asymmetric line profile. The asymmetry originates from a close coexistence of resonant transmission and resonant reflection and can be reduced to the interaction of a discrete (localized) state with a continuum of propagation modes. The basic concepts of Fano resonances are introduced, their geometrical and/or dynamical origin are explained, and theoretical and experimental studies of light propagation in photonic devices, charge transport through quantum dots, plasmon scattering in Josephson-junction networks, and matter-wave scattering in ultracold atom systems, among others are reviewed.

Microcomputer program for analysis of positron annihilation lifetime spectra

Abstract: A user friendly program (LT) for analysis of the lifetime spectra has been designed for personal computers. The program is compared with other existing programs such as POSITRONFIT, CONTIN and MELT. LT enables one to analyse both discrete and continuous spectra as well as mixed- partially linear and partially continuous spectra. The searched parameters are found by using a fully automatic procedure and/or by a nonlinear optimising procedure, starting from a set of chosen parameters. The program is tested for series of simulated data as well as some chosen experimental ones, especially for the two polymers PTFE and hard polyethylene. The analysis of these polymer spectra is carried out on the basis of the log-normal distribution model and a new model of the lifetime spectrum developed by Dauwe et al., recently adapted to LT.

Zeolitic imidazolate framework (ZIF-8) based polymer nanocomposite membranes for gas separation

Abstract: As synthesised ZIF-8 nanoparticles (size ∼ 60 nm and specific surface area ∼ 1300–1600 m2 g−1) were directly incorporated into a model polymer matrix (Matrimid® 5218) by solution mixing. This produces flexible transparent membranes with excellent dispersion of nanoparticles (up to loadings of 30 wt%) with good adhesion within the polymer matrix, as confirmed by scanning electron microscopy, dynamic mechanical thermal analysis and gas sorption studies. Pure gas (H2, CO2, O2, N2 and CH4) permeation tests showed enhanced permeability of the mixed matrix membrane with negligible losses in selectivity. Positron annihilation lifetime spectroscopy (PALS) indicated that an increase in the free volume of the polymer with ZIF-8 loading together with the free diffusion of gas through the cages of ZIF-8 contributed to an increase in gas permeability of the composite membrane. The gas transport properties of the composite membranes were well predicted by a Maxwell model whilst the processing strategy reported can be extended to fabricate other polymer nanocomposite membranes intended for a wide range of emerging energy applications.

Positron studies of condensed matter

Abstract: This review is concerned with the utilization of the positron-electron annihilation phenomenon in studies of the physics of condensed matter. The theory of the annihilation process is outlined, the importance of the two-photon mode of decay is established, and it is described how the observable results depend on the initial positron and electron states of the system. A brief account of the principal experimental techniques is included. Discussion of the complex nature of positron behavior in molecular substances suggests the particular value of investigations directed at the two photon pick-off mode of decay of the orthopositronium atoms that are formed in these materials. The possibility of positronium formation in ionic and metallic materials is also considered. The many important electron aspects of positron annihilation in metals are dealt with. The independent particle approach to the analysis of two photon angular distributions is discussed and illustrated. A survey of electronic structure investigations includes studies of polycrystalline and single crystal specimens. The relevance of angular correlation measurements in investigations of the Fermi surfaces of metals and alloys is discussed. More recently developed applications to the study of defected and disordered systems are dealt with. A preliminary account of the interpretation of multicomponentmore » lifetime spectra in terms of varying numbers of distinguishable positron states provides the basis for a discussion of studies of positron trapping by defects, voids. and surfaces in ionic and metallic solids, liquids and powders. (auth)« less

Defect identification in semiconductors with positron annihilation: Experiment and theory

Abstract: Positron annihilation spectroscopy is particularly suitable for studying vacancy-type defects in semiconductors. Combining state-of-the-art experimental and theoretical methods allows for detailed identification of the defects and their chemical surroundings. Also charge states and defect levels in the band gap are accessible. In this review the main experimental and theoretical analysis techniques are described. The usage of these methods is illustrated through examples in technologically important elemental and compound semiconductors. Future challenges include the analysis of noncrystalline materials and of transient defect-related phenomena.