Bound excitons in time-dependent density-functional theory: optical and energy-loss spectra

A robust and efficient frequency dependent and nonlocal exchange correlation f(xc)(r,r(');omega) is derived by imposing time-dependent density-functional theory (TDDFT) to reproduce the many-body diagrammatic expansion of the Bethe-Salpeter polarization function. As an illustration, we compute the optical spectra of LiF, SiO2, and diamond and the finite momentum transfer energy-loss spectrum of LiF. The TDDFT results reproduce extremely well the excitonic effects embodied in the Bethe-Salpeter approach, both for strongly bound and resonant excitons. We provide a working expression for f(xc) that is fast to evaluate and easy to implement.     

Secondary instabilities and vortex formation in collisionless-fluid magnetic reconnection

It is shown that the pattern of current layers formed within a magnetic island in the nonlinear phase of magnetic field line reconnection in a collisionless two-dimensional fluid plasma is subject to the onset of a secondary instability, the effect of which increases with decreasing electron temperature. In the cold electron limit the saturation of the island growth is accompanied by a turbulent redistribution of the current layers and by the development of long lived fluid vortices while, in the opposite limit, the current layer structure remains regular.     

MARC: A code for the retrieval of atmospheric parameters from millimeter-wave limb measurements

A new data analysis software is presented that has been developed for the retrieval of atmospheric minor constituents from limb-sounding observations made in the millimeter and sub-millimeter spectral regions. The code, which is called MARC (Millimetre-wave Atmospheric-Retrieval Code), has been designed to analyze the observations of the MARSCHALS (Millimetre-wave Airborne Receivers for Spectroscopic CHaracterisation in Atmospheric Limb-Sounding) instrument which operates on the M-55 stratospheric aircraft. The main objective of the analysis of MARSCHALS observations will be to assess long-wave measurement capabilities for the study of the upper troposphere and lower stratosphere regions. The key questions will be the accuracy and spatial resolution that can be achieved by long-wave measurements in presence of clouds and horizontal gradients.MARC performs a global-fit multi-target retrieval, in which optimal estimation is used and errors of the forward model parameters are taken into account for the definition of the cost function minimized in the retrieval. With these features it is easy to use the variables of the problem as either forward model constant parameters or retrieved unknowns with minimum impact on the stability of the retrieval. MARC can perform a wide spectral-band analysis of the observations without a selection of the analyzed channels, and the retrieval process provides an error budget of the retrieved unknowns that includes both the forward model errors and the measurement errors. The error budget obtained in this way is smaller than that obtained when accounting a posteriori for the systematic errors. The new combination of the retrieval features makes possible an efficient and optimal exploitation of the information content of the observations.     

On the generation of high flux densities within absorbing plasmas due to self-focussing of a laser beam

We investigate the possibility of achieving high flux densities by self-focussing a laser beam within a plasma. We have performed computer simulations on self-focussing of short (≦1 ns) laser pulses in high density laser-produced plasmas. Special consideration has been given to the role of absorption as it affects the laser-intensity obtained within a self-focussed filament.     

Theory of (e, 2e) reaction near solid surfaces: Application to Si

The use of the (e, 2e) technique in surface state spectroscopy is discussed. A general formulation based on Green function formalism is developed, that allows to take into account the electron-solid interaction. The case of the Si(111) surface is considered in detail. It is shown that the (e, 2e) cross section strongly depends on surface conditions; hence this technique could be an efficient tool for testing reconstruction models.     

Reactivity of arylazomethylenetriphenylphosphoranes towards electrophiles: reaction with cs2.

- The title reaction proceeds via the direct attack of the electrophilic reagent onto the nitrogen atom β to the ylidic carbon of the phosphoranes, f     

Measurement of the 50Cr(γ, n)49Cr cross section with monochromatic γ-rays

The ⁵⁰Cr(γ, n)⁴⁹Cr cross section has been measured over the energy range E_γ = 20.43 to E_γ = 22.22 MeV using monochromatic γ-rays from the ³H(p, γ)⁴He reaction and detecting the resulting ⁴⁹Cr positon activity. The γ-rays were monitored by a 12.7 cm × 15.2 cm NaI(Tl) crystal. The positon activity was determined by a coincidence detector consisting of two 7.62 cm × 7.62 cm NaI(Tl) crystals set on the annihilation radiation photopeaks. The γ-ray energy resolution was less than 110 keV over the whole energy range. Structure in the cross section was not observed.     

Experimental results on a search for narrow resonances in e+e- annihilation into hadrons in the mass region 2520–2990 MeV

A search for possible new resonances produced in e⁺e^- annihilation at Adone has been performed in the mass region 2520–2990 MeV. No evidence has been found for new resonances. We obtain an upper limit on the energy integrated resonant cross section of about 6% of the J/ψ (3100) corresponding value.