Angstrom Level Research Articles

Dynamic selectivity filters in ion channels.

This work was supported by a Wellcome Trust (UK) International Prize Travelling Fellowship and the NIH (NS-11756).

Free volume studies in miscible polymer blend systems

AbstractThis paper summarises the currently available literature concerned with measurement of free volume in miscible, amorphous polymer blends using positron annihilation lifetime spectroscopy (PALS) which probes excluded volume at the angstrom level. Previously reported data is compared with new data from a range of different blend systems. Miscible blends tend to show a negative deviation of free volume size (and to a lesser degree free volume fraction) on mixing due to the intimacy of packing of the blend component macromolecules. A largely immiscible system is also reported and shows a different behaviour (positive deviation of free volume size) and this is ascribed to additional free volume at the interface.

Nonlinear Optical Properties of Hierarchical Systems

AbstractNonlinear optical processes provide many of the functions needed for Photonics Technology. Molecular materials are hierarchical systems in which the structure can be controlled from the angstrom level to macroscopic scale to optimize the nonlinear optical properties. In this article basic concepts of nonlinear optics and photonics are reviewed. Multifunctional molecular hierarchical systems are discussed which involves heterostructure design at molecular and bulk levels. For optimization of nonlinear response at the molecular level, guidance from theoretical modeling and study of sequentially built and systematically derivatized structures has proven to be useful. The use of Langmuir-Blodgett film approach to control the order and conformation with monolayer resolution is discussed. Examples of our approach to optimize the response at the bulk level involve the use of orientation control by electric field poling and use of composite heterostructures. An important advancement in heterostructure design is the use of sol-gel processing to make organic-inorganic composites which have successfully exhibited electro-optic modulation and femtosecond Kerr gate operations. Finally, novel photorefractive molecular materials are discussed.

Effect of temperature on gas permeation of polymer blends. II. Copolyester/copolyester–polyurethane

AbstractThe transport behavior of H2, N2, CO2 was studied for blends of an aliphatic copolyester with a copolyester‐polyurethane (PU) at various temperatures from 20 to 80°C. It was found that there was an abrupt change in slope near the melting point of the copolyester in Arrhenius plot of the permeability vs. temperature. This was considered to be due to the phase changes in the blends. The relationship between the logarithm of gas permeability and blend composition were only slightly dependent on temperature owing to the low degree of crystallinity of the copolyester; however, the activation energy of gas permeation for the blends had different values below and above the melting temperature of copolyester. The Tg measurements by DSC suggested that the copolyester/PU blends are miscible. However, the gas permeation measurements revealed the blends are not molecularly homegeneous. The miscibility of the blends did not extend to the angstrom level.

A new calculational method to assess the therapeutic potential of auger electron emission

This paper discusses a new computer code to estimate the efficacy of Auger electron sources in cancer therapy. Auger electron emission accompanies the decay of many radionuclides already commonly used in nuclear medicine, for example; 99mTc and 201TI. The range of these electrons is in general sub-cellular, therefore, the toxicity of the source depends on the site of decay relative to the genetic material of the cell. Electron track structure methods have been used which enable the study of energy deposition from Auger sources down to the Angstrom level. A figure for the minimum energy required per single strand break is obtained by fitting our energy deposition calculations for 125I decays in a model of the DNA to experimental data on break lengths from 125I labeled plasmid fragments. This method is used to investigate the efficiency of double strand break production by other Auger sources which have potential value for therapy. The high RBE of Auger sources depends critically on the distance between the source and target material. The application of Auger emitters for therapy may necessitate a carrier molecule that can append the source to the DNA. Many DNA localizing agents are known in the field of chemotherapy, some of which could be carrier molecules for Auger sources; the halogenated thymidine precursors are under scrutiny in this field. The activation of Auger cascades in situ by high energy, collimated X ray and neutron beams is also assessed.

Characterization of nanometer-scale epitaxial structures by grazing-incidence x-ray diffraction and specular reflectivity

X-ray rocking curve analysis is a powerful and nondestructive technique for the characterization of heteroepitaxial structures. Conventionally, measurements are performed in symmetrical scattering geometry using a double-crystal x-ray diffractometer but the technique can be extended to the study of very thin layers (<200 Å) by the use of glancing-incidence scattering geometry and a triple-crystal diffractometer. These structures can also be studied by the technique of total external x-ray reflectivity. This is sensitive to the electron-density profile of the heterostructure as a function of depth. By combining the above techniques we have found it possible to obtain structural information on layers as thin as 20 Å. Such measurements permit accurate measurement of individual layer thicknesses and interface roughnesses on the angstrom level. The lattice parameter strain can be obtained by modeling of the intensity distribution of the crystal truncation rod.

Grazing Incidence X-Ray Scattering Studies of Single Quantum Wells

AbstractX-ray scattering techniques at grazing incidence have been used to characterize single quantum well hetero–structures. Double–and triple-axis diffractometry has been used to determine lattice mismatch and layer thickness of a 250Å thick layer of AlInAs grown by MBE on an InP substrate and capped by a 45Å GaAs layer. Reflectivity measurements in the triple – crystal mode permit accurate measurement of individual layer thicknesses, relative electron density and interface roughnesses on the Angstrom level.

Does Doping Method Influence the Electrical Properties of Polyacetylene?

Abstract The influence of doping method on the electrical properties of polyacetylene are considered. Cis-polyacetylene was doped chemically and electrochemically with the (IrCl6)2-species. The conduction mechanism was found to be independent of doping method. Electrochemical doping, however, results in a more homogeneous distribution of dopant at the 15–200 angstrom level and a higher conducting material. Electrochemical doping also results in the incorporation of a non-hydrolyzed dopant species.

A REVIEW OF INFRA-RED PHOTOGRAPHY

In medicine as in other fields, photography finds its usefulness as a valuable adjunct to didactic teaching. It is an accurate and impartial portrayal of the object viewed. The ether spectrum is arbitrarily divided. That portion of the spectrum which is visible lies between about 4,000 and 8,000 angstroms. Beyond the 4,000 angstrom level are the shorter rays: the ultraviolet, roentgen, gamma and cosmic rays, in the order given. The rays longer than the red (at 8,000 angstroms) are the infra-red rays (heat rays) and the hertzian rays, a portion of which has been utilized in radio broadcasting short wave and radio communication. There is no sharp definition between the rays but rather a gradual merging. All photographic plates or films are more sensitive than the human eye in the ultraviolet area. They register from about 3,300 angstroms. The shortest wavelength that the eye perceives is 4,000 angstroms. The