1,160 research outputs found
Relativistic and retardation effects in the two--photon ionization of hydrogen--like ions
The non-resonant two-photon ionization of hydrogen-like ions is studied in
second-order perturbation theory, based on the Dirac equation. To carry out the
summation over the complete Coulomb spectrum, a Green function approach has
been applied to the computation of the ionization cross sections. Exact
second-order relativistic cross sections are compared with data as obtained
from a relativistic long-wavelength approximation as well as from the scaling
of non-relativistic results. For high-Z ions, the relativistic wavefunction
contraction may lower the two-photon ionization cross sections by a factor of
two or more, while retardation effects appear less pronounced but still give
rise to non-negligible contributions.Comment: 6 pages, 2 figure
The 'Fast' and 'Slow' Light Induced Defects in Diluted and Undiluted Hydrogenated Amorphous Silicon Solar Cells and Materials
International audienceStudies have been carried out on a-Si:H p-in solar cells and corresponding i-layer films fabricated with and without hydrogen dilution for kinetics with high intensity and 1 sun illuminations. The results show a striking similarity between the kinetics in the fill factors (FF) of the p-in solar cells and the mobility lifetime (µτ) products of the corresponding i layer films. New results are presented on thermal annealing after 10 sun degradation which further substantiate the presence of fast and slow defects in the light induced changes of a-Si:H materials, as do the degradation kinetics of both cells and films under 1 sun illumination to their degraded steady states (DSS). Initial (fast) and subsequent (slow) regimes approaching DSS are present at temperatures between 25°C and 100°C, with the two regimes having distinctly different dependences on temperature. The DSS in the films and cells improve monotonically with temperature whereas the initial regimes show a clear reversal in their temperature dependence between 40°C and 50°C. The inability to express these results of 1 sun kinetics with rate equations containing only single time constants for creation and annealing provides further evidence that more than one defect is responsible for light induced degradation in a-Si:H materials and solar cells
The Selective Electrochemical Conversion of Preactivated CO_2 to Methane
This work reports the selective electrochemical conversion of CO_2 to methane, the reverse reaction of fossil fuel combustion. This reaction is facilitated by preactivation of the CO_2 molecule with an N-heterocyclic carbene (NHC) to form a zwitterionic species in the first step. In the presence of Ni(cyclam)^(2+) and CF_3CH_2OH, this species is shown to undergo further electrochemical reduction of the bound-CO_2 fragment at glassy carbon cathodes in dichloromethane electrolyte solution. Labeling studies confirm the origin of the carbon and protons in the methane product are the preactivated CO_2 and trifluoroethanol respectively
Enhancement of Josephson phase diffusion by microwaves
We report an experimental and theoretical study of the phase diffusion in
small Josephson junctions under microwave irradiation. A peculiar enhancement
of the phase diffusion by microwaves is observed. The enhancement manifests
itself by a pronounced current peak in the current-voltage characteristics. The
voltage position of the peak increases with the power of
microwave radiation as , while its current amplitude
weakly decreases with . As the microwave frequency increases, the peak
feature evolves into Shapiro steps with finite slope. Our theoretical analysis
taking into account the enhancement of incoherent superconducting current by
multi-photon absorption is in good agreement with experimental data.Comment: 5 pages, 4 figure
LAPW frozen-phonon calculation, shell model lattice dynamics and specific-heat measurement of SnO
An ab-initio Linear Augmented Plane-Wave (LAPW) calculation of the
zone-centered phonon frequencies of SnO has been performed. E symmetry has
been ascribed to the mode observed at 113 cm in Raman measurements,
discarding a previous B assignement. The other phonon modes measured by
Raman spectroscopy are also well reproduced. A new shell-model has also been
developed, that gives good agreement of the zone-centered frequencies compared
to the measured data and the LAPW results. Specific heat measurements have been
performed between 5 K and 110 K. Computation of the specific heat and the
M\"{o}ssbauer recoilless fraction with the improved shell-model shows a good
agreement with the experimental data as a function of temperature.Comment: 11 pages, 1 figure. to appear in Phys. Rev. B (November 1999
Pemoline and Tetramisole \u27Positives\u27 in English Racehorses Following Levamisole Administration
Pemoline is a central nervous system stimulant that has been used to treat attention-deficit hyperactivity disorder and narcolepsy in humans; its identification in horses could be considered evidence of attempts to influence performance. Two recent pemoline \u27positives\u27 in English racehorses led us to review the chemical relationships between tetramisole, levamisole, aminorex and pemoline. Pemoline is a simple oxidation product of aminorex, which has been shown in the United States and elsewhere to be an equine metabolite of levamisole. Based on the clear structural relationships between aminorex and pemoline, we conclude that levamisole can metabolise to pemoline in horses and that pemoline identifications in horses post levamisole administration are likely to be associated with levamisole administration. Levamisole should not be administered to horses about to compete because of its ability to metabolise to two central nervous system stimulants, aminorex and pemoline
Charge-imbalance effects in intrinsic Josephson systems
We report on two types of experiments with intrinsic Josephson systems made
from layered superconductors which show clear evidence of nonequilibrium
effects: 1. In 2-point measurements of IV-curves in the presence of high-
frequency radiation a shift of the voltage of Shapiro steps from the canonical
value hf/(2e) has been observed. 2. In the IV-curves of double-mesa structures
an influence of the current through one mesa on the voltage measured on the
other mesa is detected. Both effects can be explained by charge-imbalance on
the superconducting layers produced by the quasi-particle current, and can be
described successfully by a recently developed theory of nonequilibrium effects
in intrinsic Josephson systems.Comment: 8pages, 9figures, submitted to Phys. Rev.
Dynamic screening of a localized hole during photoemission from a metal cluster
Recent advances in attosecond spectroscopy techniques have fueled the
interest in the theoretical description of electronic processes taking place in
the subfemtosecond time scale. Here we study the coupled dynamic screening of a
localized hole and a photoelectron emitted from a metal cluster using a
semi-classical model. Electron density dynamics in the cluster is calculated
with Time-Dependent Density Functional Theory and the motion of the
photoemitted electron is described classically. We show that the dynamic
screening of the hole by the cluster electrons affects the motion of the
photoemitted electron. At the very beginning of its trajectory, the
photoemitted electron interacts with the cluster electrons that pile up to
screen the hole. Within our model, this gives rise to a significant reduction
of the energy lost by the photoelectron. Thus, this is a velocity dependent
effect that should be accounted for when calculating the average losses
suffered by photoemitted electrons in metals.Comment: 15 pages, 5 figure
- …