Anti-Chudakov effect in high-energy electron–positron pair ionization loss in thin target

The problem of electron–positron pair ionization loss on different distances from its creation point in substance is considered. It is shown that together with the Chudakov effect of pair loss reduction the opposite effect of exceeding by the pair loss of the doubled value of single electron (positron) loss may take place. It is demonstrated that for pair ionization loss in the same substance where the pair is created such effect is rather small but can enhance by the order of magnitude if consider the pair loss in thin plate situated in the direction of pair motion on some distance from this substance. It is pointed out that such effect should be most significant at relatively low pair energies of several hundreds MeV.     

Plant Coumarins. 3. (+)-PTeryxin from <Emphasis Type="Italic">Peucedanum terebinthaceum</Emphasis>

The potentially medically valuable pyranocoumarin (+)-pteryxin was isolated for the first time from Peucedanum terebinthaceum Fischer et Turcz. The structure of (+)-pteryxin was rigorously proved using mass spectrometry, NMR, IR, and UV spectroscopy and comparison of the spectral characteristics of this compound and its basic hydrolysis products (+)-cis- and (−)-trans-khellactone and angelic and acetic acids. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 468–470, September-October, 2008.     

Compositions of nanoparticles of hexagonal symmetry

The formulas for calculation of the number of atoms in nanoparticles with symmetry group D _6h are reported. The numbers of atoms are determined by six structurally invariant numbers and the “quantum number” of the group order n. Eight classes of nanostructures with symmetry group D _6h are revealed: C _{? + 12z} , where z = 0, 1, 2, …, and C _? is C ₂, C ₆, C ₈, or C ₁₄. The sum rule for the coordination numbers of all atoms of subshells related to symmetry elements is established. Two-dimensional nanoparticles are considered.     

Relativistic-electron scattering on an atomic string of a crystal at ultrasmall angles of particle incidence on a string

The classical and quantum scattering of fast electrons on an atomic string of a crystal is considered at angles of particle incidence on the string that are much smaller than the critical angle of axial channeling. The investigation was performed within the simplest approximation of the continuous atomic-string potential in the form of a cutoff Coulomb potential. For this case, the azimuthal scattering of particles at an angle exceeding 180° in the plane orthogonal to the string axis is shown to be possible for all impact-parameter values. It is demonstrated that, in particle scattering on a string, an effect can occur that is similar to the Ramsauer-Townsend effect, which consists in a considerable reduction of the total cross section for slow-electron scattering on atoms.     

Optical and luminescent VUV spectroscopy of La2Be2O5 crystals

Electronic excitations and the processes of their radiative relaxation are studied in pure and Ce³⁺ ion-doped crystals of lanthanum beryllate excited by synchrotron radiation in the x-ray and VUV ranges by methods of optical and luminescent vacuum ultraviolet time-resolved spectroscopy. Manifestations of excitons of the valence band are absent in the reflection spectra. However, a fast (τ=1.7 ns) and a slow (microsecond range) channel corresponding to two possible types of self-trapped excitons (STE) are found in radiative relaxation of intrinsic electronic excitations at T=10 K. The slow channel corresponds to emission of STE formed through recombination, the fast channel corresponds to emission of relaxed metastable excitons from the STE state. In the energy region higher than 14 eV (E>2E _g), the effect of multiplication of electronic excitations due to generation of secondary electron-hole pairs resulting from inelastic scattering of both hot photoelectrons and hot photoholes is exhibited.     

Crystalline fullerene deuteride C60D24: Spectral investigation

X-ray diffraction patterns, infrared and x-ray photoelectron spectra, and electron energy-loss spectra are obtained for fullerene deuteride C₆₀D₂₄ prepared by treating solid C₆₀ with D₂ gas. It is established that the deuteride is a polycrystalline powder with an fcc lattice (a ₀=14.55 Å). The product of thermal decomposition of the deuteride consists mainly of fullerene molecules separated by a distance much greater than in the initial fullerite.     

Water sorption and activation energy in polyimide thin films

The water sorption behavior and the activation energy were investigated for various chemical structure polyimide thin films; BPDA‐PDA, BPDA‐ODA, PMDA‐ODA, and 6FDA‐ODA. The activation energy for the water diffusion varied in the range of 5.53 to 9.27kcal/mol, and was in the increasing order: BPDA‐PDA < BPDA‐ODA < PMDA‐ODA < 6FDA‐ODA. BPDA‐PDA and BPDA‐ODA polyimide films showed relatively well‐ordered morphological structure, which results in relatively low diffusion coefficient and high activation energy. It was found that the diffusion coefficient and the activation energy are significantly related to the in‐plane orientation, crystallinity, and packing order in the polyimide thin films. The morphological structure was predominant factors for the water diffusion coefficient and activation energy in the polyimide thin films. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2714–2720, 2000     

Results of the Experiments on the Crystallization of a Ge–Si–Sb Solid Solution under the Conditions of Microgravity on the Soyuz–Apollo Spacecraft

Physics of the Solid State - Objective factors that allow the experiment on growing crystals of a Ge–Si–Sb solid solution on the Soyuz–Apollo spacecraft to take a special place...     

Fullerite intercalated with argon at room temperature: Synthesis and physicochemical properties

Fullerite with the bulk formula Ar _x C₆₀, where 0.60 < x < 0.75, was synthesized by precipitation from a mixture of argon-saturated solutions. The fcc unit cell parameter in the sample was 1.422 nm; the orientational phase transition (OPT) temperature was 247 K. These values noticeably differ from the relevant parameters of undoped fullerite (1.416 nm and 260 K, respectively). Heating accompanied by argon evolution restores the fullerite C₆₀ structure.