646 research outputs found
Dynamics of Enceladus and Dione inside the 2:1 Mean-Motion Resonance under Tidal Dissipation
In a previous work (Callegari and Yokoyama 2007, Celest. Mech. Dyn. Astr.
vol. 98), the main features of the motion of the pair Enceladus-Dione were
analyzed in the frozen regime, i.e., without considering the tidal evolution.
Here, the results of a great deal of numerical simulations of a pair of
satellites similar to Enceladus and Dione crossing the 2:1 mean-motion
resonance are shown. The resonance crossing is modeled with a linear tidal
theory, considering a two-degrees-of-freedom model written in the framework of
the general three-body planar problem. The main regimes of motion of the system
during the passage through resonance are studied in detail. We discuss our
results comparing them with classical scenarios of tidal evolution of the
system. We show new scenarios of evolution of the Enceladus-Dione system
through resonance not shown in previous approaches of the problem.Comment: 36 pages, 12 figures. Accepted in Celestial Mechanics and Dynamical
Astronom
Thermodynamic and Kinetic Stabilities of Al(III) Complexes with N2O3 Pentadentate Ligands
: Al(III) complexes have been recently investigated for their potential use in imaging with positron emission tomography (PET) by formation of ternary complexes with the radioisotope fluorine-18 (18F). Although the derivatives of 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) are the most applied chelators for [Al18F]2+ labelling and (pre)clinical PET imaging, non-macrocyclic, semi-rigid pentadentate chelators having two N- and three O-donor atoms such as RESCA1 and AMPDA-HB have been proposed with the aim to allow room temperature labelling of temperature-sensitive biomolecules. The paucity of stability data on Al(III) complexes used for PET imaging instigated a complete thermodynamic and kinetic solution study on Al(III) complexes with aminomethylpiperidine (AMP) derivatives AMPTA and AMPDA-HB and the comparison with a RESCA1-like chelator CD3A-Bn (trans-1,2-diaminocyclohexane-N-benzyl-N,N',N'-triacetic acid). The stability constant of [Al(AMPDA-HB)] is about four orders of magnitude higher than that of [Al(AMPTA)] and [Al(CD3A-Bn)], highlighting the greater affinity of phenolates with respect to acetate O-donors. On the other hand, the kinetic inertness of the complexes, determined by following the Cu2+-mediated transmetallation reactions in the 7.5-10.5 pH range, resulted in a spontaneous and hydroxide-assisted dissociation slightly faster for [Al(AMPTA)] than for the other two complexes (t1/2 = 4.5 h for [Al(AMPTA)], 12.4 h for [Al(AMPDA-HB)], and 24.1 h for [Al(CD3A-Bn)] at pH 7.4 and 25 °C). Finally, the [AlF]2+ ternary complexes were prepared and their stability in reconstituted human serum was determined by 19F NMR experiments
EUV ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization and electron energy-loss spectra
The ionization dynamics of pure He nanodroplets irradiated by EUV radiation
is studied using Velocity-Map Imaging PhotoElectron-PhotoIon COincidence
(VMI-PEPICO) spectroscopy. We present photoelectron energy spectra and angular
distributions measured in coincidence with the most abundant ions He+, He2+,
and He3+. Surprisingly, below the autoionization threshold of He droplets we
find indications for multiple excitation and subsequent ionization of the
droplets by a Penning-like process. At high photon energies we evidence
inelastic collisions of photoelectrons with the surrounding He atoms in the
droplets
Penning ionization of doped helium nanodroplets following EUV excitation
Helium nanodroplets are widely used as a cold, weakly interacting matrix for
spectroscopy of embedded species. In this work we excite or ionize doped He
droplets using synchrotron radiation and study the effect onto the dopant atoms
depending on their location inside the droplets (rare gases) or outside at the
droplet surface (alkali metals). Using photoelectron-photoion coincidence
imaging spectroscopy at variable photon energies (20-25 eV), we compare the
rates of charge-transfer to Penning ionization of the dopants in the two cases.
The surprising finding is that alkali metals, in contrast to the rare gases,
are efficiently Penning ionized upon excitation of the (n=2)-bands of the host
droplets. This indicates rapid migration of the excitation to the droplet
surface, followed by relaxation, and eventually energy transfer to the alkali
dopants
Stability analysis of static solutions in a Josephson junction
We present all the possible solutions of a Josephson junction with bias
current and magnetic field with both inline and overlap geometry, and examine
their stability. We follow the bifurcation of new solutions as we increase the
junction length. The analytical results, in terms of elliptic functions in the
case of inline geometry, are in agreement with the numerical calculations and
explain the strong hysteretic phenomena typically seen in the calculation of
the maximum tunneling current. This suggests a different experimental approach
based on the use, instead of the external magnetic field the modulus of the
elliptic function or the related quantity the total magnetic flux to avoid
hysteretic behavior and unfold the overlapping curves.Comment: 36 pages with 17 figure
Repulsion-Sustained Supercurrent and Flux Quantization in Rings of Symmetric Hubbard Clusters
We test the response to a threading magnetic field of rings of 5-site
-symmetric repulsive Hubbard clusters connected by weak intercell
links; each 5-site unit has the topology of a CuO cluster and a repulsive
interaction is included on every site. In a numerical study of the three-unit
ring with 8 particles, we take advantage of a novel exact-diagonalization
technique which can be generally applied to many-fermion problems. For O-O
hopping we find Superconducting Flux Quantization (SFQ), but for purely Cu-Cu
links bound pair propagation is hindered by symmetry. The results agree with
W=0 pairing theory.Comment: 4 pages, 2 figure
Collective Autoionization in Multiply-Excited Systems: A novel ionization process observed in Helium Nanodroplets
Free electron lasers (FELs) offer the unprecedented capability to study
reaction dynamics and image the structure of complex systems. When multiple
photons are absorbed in complex systems, a plasma-like state is formed where
many atoms are ionized on a femtosecond timescale. If multiphoton absorption is
resonantly-enhanced, the system becomes electronically-excited prior to plasma
formation, with subsequent decay paths which have been scarcely investigated to
date. Here, we show using helium nanodroplets as an example that these systems
can decay by a new type of process, named collective autoionization. In
addition, we show that this process is surprisingly efficient, leading to ion
abundances much greater than that of direct single-photon ionization. This
novel collective ionization process is expected to be important in many other
complex systems, e.g. macromolecules and nanoparticles, exposed to high
intensity radiation fields
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