96 research outputs found
Symmetric eikonal model for projectile-electron excitation and loss in relativistic ion-atom collisions
At impact energies GeV/u the projectile-electron
excitation and loss occurring in collisions between highly charged ions and
neutral atoms is already strongly influenced by the presence of atomic
electrons. In order to treat these processes in collisions with heavy atoms we
generalize the symmetric eikonal model, used earlier for considerations of
electron transitions in ion-atom collisions within the scope of a three-body
Coulomb problem. We show that at asymptotically high collision energies this
model leads to an exact transition amplitude and is very well suited to
describe the projectile-electron excitation and loss at energies above a few
GeV/u. In particular, by considering a number of examples we demonstrate
advantages of this model over the first Born approximation at impact energies
--30 GeV/u, which are of special interest for atomic physics
experiments at the future GSI facilities.Comment: 14 pages, 5 figure
Electron correlation and interference effects in strong-field processes
Several correlation and interference effects in strong-field physics are
investigated. We show that the interference of continuum wave packets can be
the dominant mechanism of high-harmonic generation (HHG) in the
over-the-barrier regime. Next, we combine HHG with resonant x-ray excitation to
force the recolliding continuum electron to recombine with a core hole rather
than the valence hole from that it was previously tunnel ionized. The scheme
opens up perspectives for nonlinear xuv physics, attosecond x-ray pulses, and
spectroscopy of core orbitals. Then, a method is proposed to generate
attochirp-free harmonic pulses by engineering the appropriate electron wave
packet. Finally, resonant photoionization mechanisms involving two atoms are
discussed which can dominate over the direct single-atom ionization channel at
interatomic distances in the nanometer range.Comment: to be published in Springer Proceedings "Multiphoton Processes and
Attosecond Physics
Patterns and determinants of halophilic Archaea (class Halobacteria) diversity in Tunisian endorheic salt lakes and sebkhet systems
We examined the diversity and community structure of members of the halophilic Archaea (class Halobacteria) in samples from central and southern Tunisian endorheic salt lakes and sebkhet (also known as sebkha) systems using targeted 16S rRNA gene diversity survey and quantitative PCR (qPCR) approaches. Twenty-three different samples from four distinct locations exhibiting a wide range of salinities (2% to 37%) and physical characteristics (water, salt crust, sediment, and biofilm) were examined. A total of 4,759 operational taxonomic units at the 0.03 (species-level) cutoff (OTU0.03s) belonging to 45 currently recognized genera were identified, with 8 to 43 genera (average, 30) identified per sample. In spite of the large number of genera detected per sample, only a limited number (i.e., 2 to 16) usually constituted the majority (>/=80%) of encountered sequences. Halobacteria diversity showed a strong negative correlation to salinity (Pearson correlation coefficient = -0.92), and community structure analysis identified salinity, rather than the location or physical characteristics of the sample, as the most important factor shaping the Halobacteria community structure. The relative abundance of genera capable of biosynthesis of the compatible solute(s) trehalose or 2-sulfotrehalose decreased with increasing salinities (Pearson correlation coefficient = -0.80). Indeed, qPCR analysis demonstrated that the Halobacteria otsB (trehalose-6-phosphatase)/16S rRNA gene ratio decreases with increasing salinities (Pearson correlation coefficient = -0.87). The results highlight patterns and determinants of Halobacteria diversity at a previously unexplored ecosystem and indicate that genera lacking trehalose biosynthetic capabilities are more adapted to growth in and colonization of hypersaline (>25% salt) ecosystems than trehalose producers.Peer reviewedMicrobiology and Molecular Genetic
Detection of Intercellular Adhesion (ica) Genes Involved in Biofilm and Slime Formation in Clinical Isolates of Staphylococcus Aureus Harboring mecA Gene
BACKGROUND AND OBJECTIVE: Prevalence of methicillin resistant Staphylococcus aureus (MRSA) strains is one of the most important health care problems and life-threatening in worldwide. The methicillin resistant S. aureus strains producing biofilm and slime have potential to colonize and transmit. The present study was conducted to detect intercellular adhesion (ica) genes involved in biofilm and slime formation in clinical isolates of methicillin resistant S. aureus harboring mecA gene.
METHODS: In this cross-sectional study, a total of 85 bacterial isolates suspected to S. aureus were prepared from clinical samples. The antibiotic susceptibility testing of bacteria to the penicillin, gentamicin, oxacillin, ciprofloxacin, ofloxacin and vancomycin was carried out based on disk diffusion agar method. Biofilm and slime formation of bacteria were examined by tissue culture polystyrene plate (TCP) and Congo red agar (CRA). The presence and frequency of ¬icaA, icaD and mecA genes were detected by multiplex PCR.
FINDINGS: 45 out of 85 (52.94%) S. aureus isolates were resistant to the methicillin. All of methicillin resistant S. aureus were able to produce biofilm and slime. Consumedly surface hydrophobicity was seen in 55.55% and 100% of strains producing strong biofilm and slime, respectively. The ¬icaA, icaD and mecA genes were present in all biofilm and slime producing isolates.
CONCLUSION: Our results showed that the all methicillin resistant S. aureus isolates with some abilities, including polysaccharide intercellular adhesion, bacterial attachment, biofilm and slime production were positive for icaA and icaD genes
Bound-bound pair production in relativistic collisions
Electron-positron pair production is considered in the relativistic collision
of a nucleus and an anti-nucleus, in which both leptons are created in bound
states of the corresponding nucleus-lepton system. Compared to free and
bound-free pair production this process is shown to display a qualitatively
different dependency both on the impact energy and charged of the colliding
particles. Interestingly, at high impact energies the cross section for this
process is found to be larger than that for the analogous atomic process of
non-radiative electron capture although the latter does not involve the
creation of new particles.Comment: 4 pages, 3 figure
Strongly Enhanced Backward Emission of Electrons in Transfer and Ionization
We studied three-dimensional angular distributions and longitudinal momentum spectra of electrons ejected in transfer plus ionization (TI), i.e., the ejection of one and the capture of a second target electron, for ion-helium collisions. We observe a pronounced structure strongly focused opposite to the projectile beam direction, which we associate with a new correlated TI mechanism proposed recently. This process contributes significantly to the total cross sections over a broad range of perturbations η, even at η as large as 0.5, where uncorrelated TI mechanisms were thought to be dominant
Simultaneous loss and excitation of projectile electrons in relativistic collisions of U(1s) ions with atoms
We study relativistic collisions between helium-like uranium ions initially
in the ground state and atoms in which, in a single collision event, one of the
electrons of the ion is emitted and the other is transferred into an excited
state of the residual hydrogen-like ion. We consider this two-electron process
at not very high impact energies, where the action of the atom on the electrons
of the ion can be well approximated as occurring solely due to the interaction
with the nucleus of the atom and, hence, the process can be regarded as a
four-body problem. Using the independent electron model we show that a very
substantial improvement in the calculated cross sections is obtained if,
instead of the first order approximation, the relativistic symmetric eikonal
and continuum-distorted-wave-eikonal-initial-state models are employed to
describe the single-electron probabilities for the excitation and loss,
respectively.Comment: 12 pages, 2 figures, submitted to J.Phys.
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