Particle Energization in an Expanding Magnetized Relativistic Plasma

Using a 2-1/2-dimensional particle-in-cell (PIC) code to simulate the relativistic expansion of a magnetized collisionless plasma into a vacuum, we report a new mechanism in which the magnetic energy is efficiently converted into the directed kinetic energy of a small fraction of surface particles. We study this mechanism for both electron-positron and electron-ion (mi/me=100, me is the electron rest mass) plasmas. For the electron-positron case the pairs can be accelerated to ultra-relativistic energies. For electron-ion plasmas most of the energy gain goes to the ions.Comment: 7 pages text plus 5 figures, accepted for publication by Physical Review Letter

Effect of Photoperiod and Temperature on the Development of Sorghum

Three varieties of Sorghum bicolor (L.) Moench, (i.e., ‘Early Hegari,’ ‘80-Day Milo,’ and ‘Wheatland’) were grown in controlled environment chambers and subjected to all combinations of 10-, 12-, and 14-hour photoperiods, 27 and 32 C day temperatures, and 16 and 21 C night temperatures. Days to floral initiation were determined for each variety under each treatment combination. In addition, days to anthesis and days in the floral period (from initiation to anthesis) were determined for the treatment combinations involving 21 C night temperatures. Ten-hour days hastened floral initiation and anthesis of each variety at all temperature combinations. Fourteen-hour days delayed development, but with some temperature regimes the delay was not significant, compared to the shorter days. The rate of development for the varieties under 12-hour days was highly dependent upon day and night temperatures, since floral initiation ranged from as early as that obtained with 10-hour days to later than that obtained with 14-hour days. The response to day temperature during the floral period was small, but statistically significant. The time to anthesis followed a pattern similar to that for the time to floral initiation

Measuring surface-area-to-volume ratios in soft porous materials using laser-polarized xenon interphase exchange NMR

We demonstrate a minimally invasive nuclear magnetic resonance (NMR) technique that enables determination of the surface-area-to-volume ratio (S/V) of soft porous materials from measurements of the diffusive exchange of laser-polarized 129Xe between gas in the pore space and 129Xe dissolved in the solid phase. We apply this NMR technique to porous polymer samples and find approximate agreement with destructive stereological measurements of S/V obtained with optical confocal microscopy. Potential applications of laser-polarized xenon interphase exchange NMR include measurements of in vivo lung function in humans and characterization of gas chromatography columns.Comment: 14 pages of text, 4 figure

A complementary view on the growth of directory trees

Trees are a special sub-class of networks with unique properties, such as the level distribution which has often been overlooked. We analyse a general tree growth model proposed by Klemm {\em et. al.} (2005) to explain the growth of user-generated directory structures in computers. The model has a single parameter $q$ which interpolates between preferential attachment and random growth. Our analysis results in three contributions: First, we propose a more efficient estimation method for $q$ based on the degree distribution, which is one specific representation of the model. Next, we introduce the concept of a level distribution and analytically solve the model for this representation. This allows for an alternative and independent measure of $q$. We argue that, to capture real growth processes, the $q$ estimations from the degree and the level distributions should coincide. Thus, we finally apply both representations to validate the model with synthetically generated tree structures, as well as with collected data of user directories. In the case of real directory structures, we show that $q$ measured from the level distribution are incompatible with $q$ measured from the degree distribution. In contrast to this, we find perfect agreement in the case of simulated data. Thus, we conclude that the model is an incomplete description of the growth of real directory structures as it fails to reproduce the level distribution. This insight can be generalised to point out the importance of the level distribution for modeling tree growth.Comment: 16 pages, 7 figure

Isotopic composition of stratospheric ozone

We present a kinetic calculation for the isotopic composition of stratospheric ozone. The calculated enrichments of ^(49)O_3 and ^(50)O_3 are in agreement with atmospheric measurements made at midlatitudes. Integrating the kinetic fractionation processes in the formation and photolysis of ozone, we obtain enrichments of ∼7.5–10.5 and ∼7.5–12.5% (referenced to atmospheric O_2) for δ^(49)O_3 and δ^(50)O_3, respectively, at altitudes between 20 and 35 km; the photolysis in the Hartley band of ozone is responsible for the observed altitude variation. The overall magnitude of the ozone enrichments (∼10%) is large compared with that commonly known in atmospheric chemistry and geochemistry. The heavy oxygen atom in ozone is therefore useful as a tracer of chemical species and pathways that involve ozone or its derived products. For example, the mass anomalies of oxygen in two greenhouse gases, CO_2 and N_2O, are likely the consequences of the transfer of heavy oxygen atoms from ozone

Photoluminescence, photoabsorption and photoemission studies of hydrazone thin film used as hole transporting material in OLEDs

A fotoluminescência de filmes finos de 1-(3-metilfenil)-1,2,3,4-tetrahidroquinolina-6-carboxialdeído-1,1’-difenilhidrazona foi monitorada em função da irradiação com luz UV. A intensidade da emissão decresce exponencialmente com o tempo de exposição, sugerindo degradação das amostras. Com o objetivo de investigar os mecanismos de degradação e determinar a estrutura eletrônica desse material orgânico usado com sucesso como camada transportadora de buracos na fabricação de diodos orgânicos emissores de luz (OLEDs), foram empregadas as técnicas de fotoabsorção e de fotoemissão nas bordas 1s do carbono e do nitrogênio bem como na banda de valência. A influência da luz solar foi simulada usando radiação síncrotron não-monocromática. Após exposição, todos os espectros apresentam um decréscimo nos sinais de fotoabsorção e de fotoemissão, que é menos acentuado na borda do carbono, apresentando, entretanto, um decréscimo drástico na borda do nitrogênio e na região de valência. O estudo sugere que a perda de nitrogênio é a principal causa para a quebra do sistema π, levando, dessa forma, à falha do dispositivo fabricado com esse composto.Photoluminescence (PL) emission of 1-(3-methylphenyl)-1,2,3,4-tetrahydroquinoline-6-carboxyaldehyde-1,1’-diphenylhydrazone (MTCD) thin films was monitored as a function of UV irradiation, and it was found to decrease exponentially with the exposure time. In order to gain insight into the degradation mechanisms and evaluate the electronic structure of this organic material used with good results as hole transporting layer (HTL) in the fabrication of organic light emitting diodes (OLEDs), synchrotron radiation-based photoabsorption and photoemission techniques at the carbon and nitrogen 1s edges as well as at the valence band were employed. The influence of sunlight was simulated using non-monochromatized synchrotron radiation. After exposure all the spectra show a decrease of the photoabsorption and photoemission signals, however, while it is less accentuated at the carbon edge, at the nitrogen edge and at the valence region it decreases drastically. The loss of nitrogen is suggested to be the main step in the disruption of the π system, leading to the failure of the devices fabricated with this compound as hole transporting layer