X-Ray standing wave analysis of bismuth implanted in Si(110)

Perfect crystal silicon samples implanted with 60 KeV Bi atoms along the [110] surface normal direction were analyzed with X-ray standing waves. Two reflection orders, (220) and (440) were used with synchrontron radiation to study systematically the impurity distribution function at 5 different doses ranging from 0.6 to 10×10¹⁴ Bi atoms/cm². The analysis reveals the substitutional Bi position connected with a lattice expansion and the formation of precipitates at higher Bi doses as well as estimates for the Bi vibrational amplitude.     

Calibration of a Bonner sphere extension (BSE) for high-energy neutron spectrometry

In a recent work, we constructed modular multisphere system which expands upon the design of an existing, commercially available Bonner sphere system by adding concentric shells of copper, tungsten, or lead. Our modular multisphere system is referred to as the Bonner Sphere Extension (BSE). The BSE was tested in a high energy neutron beam (thermal to 800 MeV) at Los Alamos Neutron Science Center and provided improvement in the measurement of the neutron spectrum in the energy regions above 20 MeV when compared to the standard BSS (Burgett, 2008 and Howell et al., 2009).However, when the initial test of the system was carried-out at LANSCE, the BSE had not yet been calibrated. Therefore the objective of the present study was to perform calibration measurements. These calibration measurements were carried out using monoenergetic neutron ISO 8529-1 reference beams at the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany. The following monoenergetic reference beams were used for these experiments: 14.8 MeV, 1.2 MeV, 565 keV, and 144 keV. Response functions for the BSE were calculated using the Monte Carlo N-Particle Code, eXtended (MCNPX). The percent difference between the measured and calculated responses was calculated for each sphere and energy. The difference between measured and calculated responses for individual spheres ranged between 7.9 % and 16.7 % and the arithmetic mean for all spheres was (10.9 ± 1.8) %. These sphere specific correction factors will be applied for all future measurements carried-out with the BSE.     

Ultrafast dynamics of delocalized and localized electrons in carbon nanotubes

We report on the dynamics of the dielectric function of single-wall carbon nanotubes in the 10-30 THz frequency range after ultrafast laser excitation. The absence of a distinct free-carrier response is attributed to the photogeneration of strongly bound excitons in the tubes with large energy gaps. We find a feature of enhanced transmission caused by the blocking of optical transitions in small-gap tubes. The rapid decay of a featureless background with pronounced dichroism is associated with the increased absorption of spatially localized charge carriers before thermalization is completed.     

Large amplitude oscillations (switching) of bi-stable vortex structures in zero field

A ferromagnetic block (2a×2b×2c) with three unequal axes supports a vortex structure that intersects each of the two opposite surfaces of largest area. Swirls appear at points (xp, yp, c) and (?xp, yp, ?c) in state-A. In state-B the swirls are at points (?xp, ?yp, c) and (xp, ?yp, ?c). With micromagnetic simulations we demonstrate the possibility of a nucleation-free switching between those states, with possible applicability in devices. The oscillations are accompanied by large external fields from the magnetic charge at the core of the intersections of the vortices with the surface. These fields of the order of a quarter of the saturation inductance move over a distance 2xp～c in fractions of a ns.     

Self-affine convex polygons

A polygon in ${{\mathbb R}^2}$ is called self-affine if it can be dissected into k ≥ 2 affine images of itself. Self-affine convex polygons have at most five vertices. Triangles are trivially self-affine. It is shown that every convex quadrangle is self-affine, whereas only some, but not all convex pentagons are self-affine.     

Changes in the intracellular Ca2+ content in human red blood cells in the presence of glycerol

Changes of the intracellular Ca2+ content in human red blood cells (RBCs) in glycerol-containing solutions and after freeze-thawing the cells with glycerol and subsequent deglycerolization were investigated with the Ca2+-sensitive fluorescent dye fluo-4 using fluorescence microscopy. In the glycerol-containing solutions the Ca2+ content increased when compared with a physiological medium (Hepes buffered saline solution (HBSS)). This effect was most likely a result of an inhibition of the Ca2+ pump. After inhibiting the Ca2+ pump using o-vanadate, the Ca2+ uptake was not significantly different in the cells in glycerol-containing and physiological medium. Freeze-thawing and deglycerolization of RBCs resulted in a more pronounced increase in the Ca2+ content. Also in this case, the Ca2+ pump seemed to play a major role.     

Synthesis of 6-methyl-8<Emphasis Type="Italic">H</Emphasis>-dibenzo[<Emphasis Type="Italic">a</Emphasis>,<Emphasis Type="Italic">g</Emphasis>]quinolizin-8-imines <Emphasis Type="Italic">via Reissert</Emphasis> compounds

Alkylation of Reissert compounds derived from 3-methylisoquinolines with several 2-cyanobenzylbromides followed by hydrolytic cleavage provided the corresponding 1-benzyl-3-methylisoquinolines. Treatment of the latter with methylmagnesiumiodide caused cyclization to the title compounds rather than formation of 2-acetylbenzylisoquinolines.     

Analysis of ultrafast relaxation in photoexcited DNA base pairs of adenine and thymine

The photoinduced dynamics in base pairs of adenine and thymine were analyzed by femtosecond pump-probe spectroscopy. On the short-time scale up to a few picoseconds, the characteristic time constants for the dimers are quite similar to the corresponding values of the monomers. This leads to the conclusion that ultrafast intramolecular relaxation proceeds via ππ^* and nπ^* states of one component within the dimer. On the long-time scale, we obtained a novel time constant of roughly 40 ps for the thymine dimer and the adenine–thymine base pair. This time constant was never observed in the monomers and is tentatively assigned to an intermolecular relaxation process, possibly via a hydrogen transfer state.     

Exciton diffusion and relaxation in methyl-substituted polyparaphenylene polymer films

Exciton diffusion in ladder-type methyl-substituted polyparaphenylene film and solution was investigated by means of femtosecond pump-probe spectroscopy using a combined approach, analyzing exciton-exciton annihilation, and transient absorption depolarization properties. We show that the different views on the exciton dynamics offered by anisotropy decay and annihilation are required in order to obtain a correct picture of the energy transfer dynamics. Comparison of the exciton diffusion coefficient and exciton diffusion radius obtained for polymer film with the two techniques reveals that there is substantial short-range order in the film. Also in isolated chains there is considerable amount of order, as revealed from only partial anisotropy decay, which shows that only a small fraction of the excitons move to differently oriented polymer segments. It is further concluded that interchain energy transfer is faster than intrachain transfer, mainly as a result of shorter interchain distances between chromophoric units.