5,425 research outputs found
Produção de sementes de hortaliças para a agricultura familiar.
Esta publicação tem por objetivo apresentar aspectos técnicos da produção de sementes das diversas hortaliças, disponibilizando esse conhecimento para a incorporação ao processo produtivo dos agricultores familiares envolvidos na produção de sementes de hortaliças
Single and Double Photoionization and Photodissociation of Toluene by Soft X-rays in Circumstellar Environment
The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl
derivatives occurs mainly in the dust shells of asymptotic giant branch (AGB)
stars. The bands at 3.3 and 3.4 m, observed in infrared emission spectra
of several objects, are attributed C-H vibrational modes in aromatic and
aliphatic structures, respectively. In general, the feature at 3.3 m is
more intense than the 3.4 m. Photoionization and photodissociation
processes of toluene, the precursor of methylated PAHs, were studied using
synchrotron radiation at soft X-ray energies around the carbon K edge with
time-of-flight mass spectrometry. Partial ion yields of a large number of ionic
fragments were extracted from single and 2D-spectra, where electron-ion
coincidences have revealed the doubly charged parent-molecule and several
doubly charged fragments containing seven carbon atoms with considerable
abundance. \textit{Ab initio} calculations based on density functional theory
were performed to elucidate the chemical structure of these stable dicationic
species. The survival of the dications subjected to hard inner shell ionization
suggests that they could be observed in the interstellar medium, especially in
regions where PAHs are detected. The ionization and destruction of toluene
induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB
star. In this context, a minimum photodissociation radius and the half-life of
toluene subjected to the incidence of the soft X-ray flux emitted from a
companion white dwarf star were determined.Comment: 11 pages, 4 figures, accept for publication in Ap
Non-hexagonal-ring defects and structures induced by strain in graphene and in functionalized graphene
We perform {\textit ab initio} calculations for the strain-induced formation
of non-hexagonal-ring defects in graphene, graphane (planar CH), and graphenol
(planar COH). We find that the simplest of such topological defects, the
Stone-Wales defect, acts as a seed for strain-induced dissociation and
multiplication of topological defects. Through the application of inhomogeneous
deformations to graphene, graphane and graphenol with initially small
concentrations of pentagonal and heptagonal rings, we obtain several novel
stable structures that possess, at the same time, large concentrations of
non-hexagonal rings (from fourfold to elevenfold) and small formation energies
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