10,510 research outputs found
Optical properties of Ge-oxygen defect center embedded in silica films
The photo-luminescence features of Ge-oxygen defect centers in a 100nm thick
Ge-doped silica film on a pure silica substrate were investigated by looking at
the emission spectra and time decay detected under synchrotron radiation
excitation in the 10-300 K temperature range. This center exhibits two
luminescence bands centered at 4.3eV and 3.2eV associated with its
de-excitation from singlet (S1) and triplet (T1) states, respectively, that are
linked by an intersystem crossing process. The comparison with results obtained
from a bulk Ge-doped silica sample evidences that the efficiency of the
intersystem crossing rate depends on the properties of the matrix embedding the
Ge-oxygen defect centers, being more effective in the film than in the bulk
counterpart.Comment: 10 pages, 3 figures, in press on J. Non cryst. solids (2007
Replica bounds for diluted non-Poissonian spin systems
In this paper we extend replica bounds and free energy subadditivity
arguments to diluted spin-glass models on graphs with arbitrary, non-Poissonian
degree distribution. The new difficulties specific of this case are overcome
introducing an interpolation procedure that stresses the relation between
interpolation methods and the cavity method. As a byproduct we obtain
self-averaging identities that generalize the Ghirlanda-Guerra ones to the
multi-overlap case.Comment: Latex file, 15 pages, 2 eps figures; Weak point revised and
corrected; Misprints correcte
Na-O anticorrelation and HB. IX. Kinematics of the program clusters. A link between systemic rotation and HB morphology?
We use accurate radial velocities for 1981 member stars in 20 Galactic
globular clusters, collected within our large survey aimed at the analysis of
the Na-O anti-correlation, to study the internal kinematics of the clusters. We
performed the first systematic exploration of the possible connections between
cluster kinematics and the multiple populations phenomenon in GCs. We did not
find any significant correlation between Na abundance and either velocity
dispersion or systemic rotation. We searched for systemic rotation in the eight
clusters of our sample that lack such analysis from previous works in the
literature (NGC2808, NGC5904, NGC6171, NGC6254, NGC6397, NGC6388, NGC6441, and
NGC6838). These clusters are found to span a large range of rotational
amplitudes, from ~0.0 km/s (NGC6397) to ~13.0 km/s (NGC6441). We found a
significant correlation between the ratio of rotational velocity to central
velocity dispersion (V_{rot}/sigma_0) and the Horizontal Branch Morphology
parameter (B-R)/(B+R+V). V_{rot}/sigma_0 is found to correlate also with
metallicity, possibly hinting to a significant role of dissipation in the
process of formation of globular clusters. V_{rot} is found to correlate well
with (B-R)/(B+R+V), M_V, sigma_0 and [Fe/H]. All these correlations strongly
suggest that systemic rotation may be intimately linked with the processes that
led to the formation of globular clusters and the stellar populations they
host.Comment: Accepted for publication on Astronomy & Astrophysics. Pdflatex, 16
pages, 16 pdf figures. The position angles of the rotation axes have been
corrected, since the values reported in the previous version were erroneous.
The results of the analysis are unchanged. The manuscript has also been
processed by a language edito
Photodetachment o F- by a few-cycle circularly polarized laser field
We report on calculations of the above threshold detachment of
F- by a few-cycle circularly polarized laser field, discussing the effects of
both the carrier-envelope relative phase and the number of the cycle
contained in a pulse on the angular distribution of ejected photoelectron.
The results are analyzed in terms of a two-step semiclassical model: after
the electrons are detached through tunnelling their motion is determined by
the electric field pulse according to the classical dynamics laws.
Anisotropies in the angular distributions of the electrons ejected on the
plane perpendicular to the laser propagation direction are found that depend
on the number of cycle of the laser pulse
Polarization and angular distribution of the radiation emitted in laser-assisted recombination
The effect of an intense external linear polarized radiation field on the
angular distributions and polarization states of the photons emitted during the
radiative recombination is investigated. It is predicted, on symmetry grounds,
and corroborated by numerical calculations of approximate recombination rates,
that emission of elliptically polarized photons occurs when the momentum of the
electron beam is not aligned to the direction of the oscillating field.
Moreover, strong modifications to the angular distributions of the emitted
photons are induced by the external radiation field.Comment: 5 pages, 3 figure
Two-color ionization of hydrogen by short intense pulses
Photoelectron energy spectra resulting by the interaction of hydrogen with
two short pulses having carrier frequencies, respectively, in the range of the
infrared and XUV regions have been calculated. The effects of the pulse
duration and timing of the X-ray pulse on the photoelectron energy spectra are
discussed. Analysis of the spectra obtained for very long pulses show that
certain features may be explained in terms of quantum interferences in the time
domain. It is found that, depending on the duration of the X-ray pulse, ripples
in the energy spectra separated by the infrared photon energy may appear.
Moreover, the temporal shape of the low frequency radiation field may be
inferred by the breadth of the photoelectron energy spectra.Comment: 12 pages, 8 figure
Resolving the radio nebula around beta Lyrae
In this paper we present high spatial resolution radio images of the puzzling
binary system beta Lyrae obtained with MERLIN at 5 GHz. We find a nebula
surrounding the binary with a brightness temperature of 11000+-700K
approximately 40AU across. This definitively confirms the thermal origin of the
radio emission, which is consistent with emission from the wind of the B6-8II
component (mass loss of order of 10^-7 Msun per year), ionized by the radiation
field of the hotter companion. This nebula, surrounding the binary, is the
proof that beta Layrae evolved in a non-conservative way, i. e. not all the
mass lost by the primary is accretted by the secondary, and present
measurements indicate that almost 0.015Msun had been lost from the system since
the onset of the Roche lobe overflow phase. Moreover, the nebula is aligned
with the jet-like structures inferred from recent optical measurements,
indicating a possible connection among them.Comment: 5 pages, 2 figures. Accepted for publication in A&
Comorbid depressive disorders in ADHD. the role of ADHD severity, subtypes and familial psychiatric disorders
ObjectiveaaTo evaluate the presence of Major Depressive Disorder (MDD) and Dysthymic Disorder (DD) in a sample of Italian children with Attention Deficit Hyperactivity Disorder (ADHD) and to explore specific features of comorbid depressive disorders in ADHD. MethodsaaThree hundred and sixty-six consecutive, drug-naïve Caucasian Italian outpatients with ADHD were recruited and comorbid disorders were evaluated using DSM-IV-TR criteria. To evaluate ADHD severity, parents of all children filled out the ADHD Rating Scale. Thirty-seven children with comorbid MDD or DD were compared with 118 children with comorbid conduct disorder and 122 without comorbidity for age, sex, IQ level, family psychiatric history, and ADHD subtypes and severity. Resultsaa42 of the ADHD children displayed comorbid depressive disorders: 16 exhibited MDD, 21 DD, and 5 both MDD and DD. The frequency of hyperactive-impulsive subtypes was significantly lower in ADHD children with depressive disorders, than in those without any comorbidity. ADHD children with depressive disorders showed a higher number of familial psychiatric disorders and higher score in the Inattentive scale of the ADHD Rating Scale, than children without any comorbidity. No differences were found for age, sex and IQ level between the three groups. Conclusions: Consistent with previous studies in other countries, depressive disorders affect a significant proportion of ADHD children in Italy. Patient assessment and subsequent treatment should take into consideration the possible presence of this comorbidity, which could specifically increase the severity of ADHD attention problems
Complexity transitions in global algorithms for sparse linear systems over finite fields
We study the computational complexity of a very basic problem, namely that of
finding solutions to a very large set of random linear equations in a finite
Galois Field modulo q. Using tools from statistical mechanics we are able to
identify phase transitions in the structure of the solution space and to
connect them to changes in performance of a global algorithm, namely Gaussian
elimination. Crossing phase boundaries produces a dramatic increase in memory
and CPU requirements necessary to the algorithms. In turn, this causes the
saturation of the upper bounds for the running time. We illustrate the results
on the specific problem of integer factorization, which is of central interest
for deciphering messages encrypted with the RSA cryptosystem.Comment: 23 pages, 8 figure
Active chlorine and nitric oxide formation from chemical rocket plume afterburning
Chlorine and oxides of nitrogen (NO(x)) released into the atmosphere contribute to acid rain (ground level or low-altitude sources) and ozone depletion from the stratosphere (high-altitude sources). Rocket engines have the potential for forming or activating these pollutants in the rocket plume. For instance, H2/O2 rockets can produce thermal NO(x) in their plumes. Emphasis, in the past, has been placed on determining the impact of chlorine release on the stratosphere. To date, very little, if any, information is available to understand what contribution NO(x) emissions from ground-based engine testing and actual rocket launches have on the atmosphere. The goal of this work is to estimate the afterburning emissions from chemical rocket plumes and determine their local stratospheric impact. Our study focuses on the space shuttle rocket motors, which include both the solid rocket boosters (SRB's) and the liquid propellant main engines (SSME's). Rocket plume afterburning is modeled employing a one-dimensional model incorporating two chemical kinetic systems: chemical and thermal equilibria with overlayed nitric oxide chemical kinetics (semi equilibrium) and full finite-rate chemical kinetics. Additionally, the local atmospheric impact immediately following a launch is modeled as the emissions diffuse and chemically react in the stratosphere
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