279 research outputs found
Two-photon double ionization of neon using an intense attosecond pulse train
We present the first demonstration of two-photon double ionization of neon
using an intense extreme ultraviolet (XUV) attosecond pulse train (APT) in a
photon energy regime where both direct and sequential mechanisms are allowed.
For an APT generated through high-order harmonic generation (HHG) in argon we
achieve a total pulse energy close to 1 J, a central energy of 35 eV and a
total bandwidth of eV. The APT is focused by broadband optics in a
neon gas target to an intensity of Wcm. By tuning
the photon energy across the threshold for the sequential process the double
ionization signal can be turned on and off, indicating that the two-photon
double ionization predominantly occurs through a sequential process. The
demonstrated performance opens up possibilities for future XUV-XUV pump-probe
experiments with attosecond temporal resolution in a photon energy range where
it is possible to unravel the dynamics behind direct vs. sequential double
ionization and the associated electron correlation effects
Photoionization in the time and frequency domain
Ultrafast processes in matter, such as the electron emission following light
absorption, can now be studied using ultrashort light pulses of attosecond
duration (s) in the extreme ultraviolet spectral range. The lack of
spectral resolution due to the use of short light pulses may raise serious
issues in the interpretation of the experimental results and the comparison
with detailed theoretical calculations. Here, we determine photoionization time
delays in neon atoms over a 40 eV energy range with an interferometric
technique combining high temporal and spectral resolution. We spectrally
disentangle direct ionization from ionization with shake up, where a second
electron is left in an excited state, thus obtaining excellent agreement with
theoretical calculations and thereby solving a puzzle raised by seven-year-old
measurements. Our experimental approach does not have conceptual limits,
allowing us to foresee, with the help of upcoming laser technology, ultra-high
resolution time-frequency studies from the visible to the x-ray range.Comment: 5 pages, 4 figure
Attosecond electron-spin dynamics in Xe 4d photoionization
The photoionization of xenon atoms in the 70-100 eV range reveals several
fascinating physical phenomena such as a giant resonance induced by the dynamic
rearrangement of the electron cloud after photon absorption, an anomalous
branching ratio between intermediate Xe states separated by the spin-orbit
interaction and multiple Auger decay processes. These phenomena have been
studied in the past, using in particular synchrotron radiation, but without
access to real-time dynamics. Here, we study the dynamics of Xe 4d
photoionization on its natural time scale combining attosecond interferometry
and coincidence spectroscopy. A time-frequency analysis of the involved
transitions allows us to identify two interfering ionization mechanisms: the
broad giant dipole resonance with a fast decay time less than 50 as and a
narrow resonance at threshold induced by spin-flip transitions, with much
longer decay times of several hundred as. Our results provide new insight into
the complex electron-spin dynamics of photo-induced phenomena
Comparative analysis of viral shedding in pediatric and adult subjects with central nervous system-associated enterovirus infections from 2013 to 2015 in Switzerland.
Several enterovirus (EV) genotypes can result in aseptic meningitis, but their routes of access to the central nervous system remain to be elucidated and may differ between the pediatric and adult populations.
To assess the pattern of viral shedding in pediatric and adult subjects with acute EV meningitis and to generate EV surveillance data for Switzerland.
All pediatric and adult subjects admitted to the University Hospitals of Geneva with a diagnosis of EV meningitis between 2013 and 2015 were enrolled. A quantitative EV real-time reverse transcriptase (rRT)-PCR was performed on the cerebrospinal fluid (CSF), blood, stool, urine and respiratory specimens to assess viral shedding and provide a comparative analysis of pediatric and adult populations. EV genotyping was systematically performed.
EV positivity rates differed significantly between pediatric and adult subjects; 62.5% of pediatric cases (no adult case) were EV-positive in stool and blood for subjects for whom these samples were all collected. Similarly, the EV viral load in blood was significantly higher in pediatric subjects. Blood C-reactive protein levels were lower and the number of leucocytes/mm3 in the CSF were higher in non-viremic than in viremic pediatric subjects, respectively. A greater diversity of EV genotypes was observed in pediatric cases, with a predominance of echovirus 30 in children ≥3 years old and adults.
In contrast to adults, EV-disseminated infections are predominant in pediatric subjects and show different patterns of EV viral shedding. This observation may be useful for clinicians and contribute to modify current practices of patient care
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