Absorption (logic)

About: Absorption (logic) is a research topic. Over the lifetime, 5733 publications have been published within this topic receiving 236302 citations.

EXAFS spectroscopic analysis of the Verwey transition in Fe 3 O 4 .

Abstract: We present an extended x-ray absorption fine structure study at the Fe $K$ edge of the local structural changes in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ across the Verwey transition at ${T}_{V}\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The local structure of the $\mathrm{Fe}{\mathrm{O}}_{6}$ octahedra is distorted below ${T}_{V}$ and remains unaltered during the transition, the local distortions being already present in the pseudocubic phase above ${T}_{V}$. The phonon modes responsible for the lattice distortion are those associated with displacements of the octahedral $\mathrm{Fe}\ensuremath{-}\mathrm{Fe}$ chains. We propose that the metal-insulator transition is then caused by a change in the regime of the local distortions from a static regime at low temperatures to a dynamical one at high temperatures, the lattice dynamics above ${T}_{V}$ being the origin for the electrical conductivity.

Fermi LAT observations of the Geminga pulsar

Abstract: We report on the \textit{Fermi}-LAT observations of the Geminga pulsar, the second brightest non-variable GeV source in the $\gamma$-ray sky and the first example of a radio-quiet $\gamma$-ray pulsar. The observations cover one year, from the launch of the $Fermi$ satellite through 2009 June 15. A data sample of over 60,000 photons enabled us to build a timing solution based solely on $\gamma$ rays. Timing analysis shows two prominent peaks, separated by $\Delta \phi$ = 0.497 $\pm$ 0.004 in phase, which narrow with increasing energy. Pulsed $\gamma$ rays are observed beyond 18 GeV, precluding emission below 2.7 stellar radii because of magnetic absorption. The phase-averaged spectrum was fitted with a power law with exponential cut-off of spectral index $\Gamma$ = (1.30 $\pm$ 0.01 $\pm$ 0.04), cut-off energy $E_{0}$ = (2.46 $\pm$ 0.04 $\pm$ 0.17) GeV and an integral photon flux above 0.1 GeV of (4.14 $\pm$ 0.02 $\pm$ 0.32) $\times$ 10$^{-6}$ cm$^{-2}$ s$^{-1}$. The first uncertainties are statistical and the second are systematic. The phase-resolved spectroscopy shows a clear evolution of the spectral parameters, with the spectral index reaching a minimum value just before the leading peak and the cut-off energy having maxima around the peaks. Phase-resolved spectroscopy reveals that pulsar emission is present at all rotational phases. The spectral shape, broad pulse profile, and maximum photon energy favor the outer magnetospheric emission scenarios.

γ rays and neutrinos from a powerful cosmic accelerator

Abstract: Possibly the powerful radio quasar 3C273 will reveal its nature as an efficient proton accelerator up to energies of order ${10}^{11}$ GeV in the near future. It is shown in this paper that the shock-accelerated protons expected to be present in the quasar's plasma jet induce an unsaturated synchrotron cascade with electromagnetic radiation emerging in the x-ray and \ensuremath{\gamma}-ray ranges. While (including the synchrotron emission from the accelerated primary electrons) the broadband nonthermal emission from 3C273 can be explained over the observed 18 orders of magnitude, a flattening of the spectrum at the highest observed energies (a few GeV) is predicted that could be falsified by the Energetic Gamma Ray Experiment Telescope on board the Compton Gamma Ray Observatory. Above \ensuremath{\approxeq}100 GeV the cascade spectrum dramatically steepens again due to the absorption of the \ensuremath{\gamma}-ray photons by the host galaxy's strong infrared photon field from extended dust clouds, in accordance with the nondetection of 3C273 by Cher\'enkov telescopes. However, neutrinos from the hadronic interactions initiating the cascade are not damped and reach terrestrial experiments without any modification of their injected flux. In contrast with the neutrino flux from pp interactions, which are energetically unimportant in jets, p\ensuremath{\gamma} interactions generate a flat neutrino flux. Therefore it is emphasized that one must not simply normalize the expected neutrino flux by the observed \ensuremath{\gamma}-ray flux. Hence it is shown that the expected neutrino flux in the energy range relevant for underwater or under-ice detectors is much lower than assumed by many authors. On the other hand, with an increasing number of cosmic \ensuremath{\gamma}-ray sources at known positions, their neutrino detection should be feasible when it is realized that the angular resolution is the crucial design property for neutrino detectors.

Diffusion of deuterium and hydrogen in doped and undoped MgO crystals

Abstract: Infrared absorption of O${\mathrm{D}}^{\ensuremath{-}}$ and O${\mathrm{H}}^{\ensuremath{-}}$ ions is used to study diffusion rates of ${\mathrm{D}}^{+}$ and ${\mathrm{H}}^{+}$ in doped and undoped MgO crystals heated at high temperatures in ${\mathrm{D}}_{2}$O and ${\mathrm{H}}_{2}$O vapors. Crystals doped with H, Li, Na, Al, V, Cr, Fe, Co, Ni, Cu, Ga, Ag, and Yb are studied. With the exception of the H- and Li-doped crystals, the diffusion characteristics of the doped and undoped MgO crystals are essentially the same. The minimum temperature observed for ${\mathrm{D}}^{+}$ diffusion is 1750 K, and the diffusion coefficient is \ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{2}$/sec at 1873 K. The ratio of the diffusion coefficients of ${\mathrm{H}}^{+}$ and ${\mathrm{D}}^{+}$ at 1873 K is $\frac{D({\mathrm{H}}^{+})}{D({\mathrm{D}}^{+})}=1.1\ifmmode\pm\else\textpm\fi{}0.1$. Complete removal of soluble ${\mathrm{H}}^{+}$ can be accomplished by application of an electric field at 1300 K. In H-doped crystals an exchange of ${\mathrm{H}}^{+}$ for ${\mathrm{D}}^{+}$ results in large concentrations of O${\mathrm{D}}^{\ensuremath{-}}$ ions. In Li-doped crystals, deuterons diffuse much more readily; the threshold temperature for ${\mathrm{D}}^{+}$ diffusion is only 800 K. The higher diffusivity is attributed to a catalytic behavior of ${\mathrm{Li}}_{2}$O precipitates.

Optical properties of copper clusters embedded in alumina: An experimental and theoretical study of size dependence

Abstract: Optical properties of copper clusters of $3\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter, produced by laser vaporization and embedded in alumina, are investigated and compared to a semi-quantal model [based on time-dependent local-density approximation (TDLDA) and density functional theory (DFT)], including the absorption and screening properties of the ionic core background and the surrounding matrix. To begin with, the experiments show that the alumina trapped clusters are oxidized if codeposited at room temperature whereas they do remain metallic by holding the substrate temperature at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. On the other hand, reducing under ${\mathrm{H}}_{2}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ atmosphere of samples made at room temperature is an alternative to elaborate embedded metallic copper clusters. Consequently, the alumina matrix evaporated on a substrate at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is optically characterized and the oxidization level of the copper clusters is carefully investigated through x-ray photoelectron spectroscopy (XPS). Concerning the optical properties of the copper clusters, a strong damping and a broadening of the surface plasmon resonance (SPR) with decreasing size is observed, in good agreement with theoretical calculations. This damping, much more important than in gold clusters, is correlated to a strong coupling of the collective excitation with interband transitions. These results allow us to validate the semiquantal model for the three systems Au, Ag, and Cu, in which the size effects are ruled by the electronic spillout phenomenon, the surface of ineffective ionic core polarization and the local porosity at the metal/matrix interface. Lastly, the inhomogeneous effects such as size dispersion are shown to be of less importance compared to intrinsic size effects in the optical properties.