87 research outputs found
Classical Effects of Laser Pulse Duration on Strong-field Double Ionization
We use classical electron ensembles and the aligned-electron approximation to
examine the effect of laser pulse duration on the dynamics of strong-field
double ionization. We cover the range of intensities
for the laser wavelength 780 nm. The classical scenario suggests that the
highest rate of recollision occurs early in the pulse and promotes double
ionization production in few-cycle pulses. In addition, the purely classical
ensemble calculation predicts an exponentially decreasing recollision rate with
each subsequent half cycle. We confirm the exponential behavior by trajectory
back-analysis
Non-sequential double ionization below laser-intensity threshold: Anticorrelation of electrons without excitation of parent ion
Two-electron correlated spectra of non-sequential double ionization below
laser-intensity threshold are known to exhibit back-to-back scattering of the
electrons, viz., the anticorrelation of the electrons. Currently, the widely
accepted interpretation of the anticorrelation is recollision-induced
excitation of the ion plus subsequent field ionization of the second electron.
We argue that another mechanism, namely simultaneous electron emission, when
the time of return of the rescattered electron is equal to the time of
liberation of the bounded electron (the ion has no time for excitation), can
also explain the anticorrelation of the electrons in the deep below
laser-intensity threshold regime. Our conclusion is based on the results of the
numerical solution of the time-dependent Schr\"{o}dinger equation for a model
system of two one-dimensional electrons as well as an adiabatic analytic model
that allows for a closed-form solution.Comment: 6 pages and 3 figure
Correlated multi-electron dynamics in ultrafast laser pulse - atom interactions
We present the results of the detailed experimental study of multiple
ionization of Ne and Ar by 25 and 7 fs laser pulses. For Ne the highly
correlated "instantaneous" emission of up to four electrons is triggered by a
recollisional electron impact, whereas in multiple ionization of Ar different
mechanisms, involving field ionization steps and recollision-induced
excitations, play a major role. Using few-cycle pulses we are able to suppress
those processes that occur on time scales longer than one laser cycle.Comment: 9 pages, 4 figure
Many-electron tunneling in atoms
A theoretical derivation is given for the formula describing N-electron
ionization of atom by a dc field and laser radiation in tunneling regime.
Numerical examples are presented for noble gases atoms.Comment: 11 pages, 1 EPS figure, submitted to JETP (Jan 99
An integrated ultra-high vacuum apparatus for growth and in situ characterization of complex materials
Here we present an integrated ultra-high vacuum apparatus \u2013 named MBE-Cluster \u2013 dedicated to the growth
and in situ structural, spectroscopic and magnetic characterization of complex materials. Molecular Beam
Epitaxy (MBE) growth of metal oxides, e.g. manganites, and deposition of patterned metallic layers can be
fabricated and in situ characterized by reflection high-energy electron diffraction (RHEED), low-energy
electron diffraction (LEED) - Auger Electron Spectroscopy, X-ray photoemission spectroscopy (PES) and
azimuthal longitudinal magneto-optic Kerr effect (MOKE). The temperature can be controlled in the range
from 5 to 580 K, with the possibility of application of magnetic fields H up to \ub17 kOe and electric fields E for
voltages up to \ub1500 V. The MBE-Cluster operates for in-house research as well as user facility in combination
with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator (HHG) facility for timeresolved
spectroscopy
A theoretical approach for the interpretation of pulsating PMS intermediate-mass stars
The investigation of the pulsation properties of pre-main-sequence
intermediate-mass stars is a promising tool to evaluate the intrinsic
properties of these stars and to constrain current evolutionary models. Many
new candidates of this class have been discovered during the last decade and
very accurate data are expected from space observations obtained for example
with the CoRoT satellite. In this context we aim at developing a theoretical
approach for the interpretation of observed frequencies, both from the already
available ground-based observations and from the future more accurate and
extensive CoRoT results. To this purpose we have started a project devoted to
the computations of fine and extensive grids of asteroseismic models of
intermediate mass pre-main-sequence stars. The obtained frequencies are used to
derive an analytical relation between the large frequency separation and the
stellar luminosity and effective temperature and to develop a tool to compare
theory and observations in the echelle diagram. The predictive capabilities of
the proposed method are verified through the application to two test stars. As
a second step, we apply the procedure to two true observations from multisite
campaigns and we are able to constrain their stellar parameters, in particular
the mass, in spite of the small number of frequencies. We expect that with a
significantly higher number of frequencies both the stellar mass and age could
be constrained and, at the same time, the physics of the models could be
tested.Comment: Accepted for publication on A&
Asteroseismic fundamental properties of solar-type stars observed by the NASA Kepler Mission
We use asteroseismic data obtained by the NASA Kepler Mission to estimate the
fundamental properties of more than 500 main-sequence and sub-giant stars. Data
obtained during the first 10 months of Kepler science operations were used for
this work, when these solar-type targets were observed for one month each in a
survey mode. Stellar properties have been estimated using two global
asteroseismic parameters and complementary photometric and spectroscopic data.
Homogeneous sets of effective temperatures were available for the entire
ensemble from complementary photometry; spectroscopic estimates of T_eff and
[Fe/H] were available from a homogeneous analysis of ground-based data on a
subset of 87 stars. [Abbreviated version... see paper for full abstract.]Comment: Accepted for publication in ApJS; 90 pages, 22 figures, 6 tables.
Units on rho in tables now listed correctly as rho(Sun
Knocking down gene function with an RNA aptamer expressed as part of an intron
We developed a powerful expression system to produce aptamers and other types of functional RNA in yeast to examine their effects. Utilizing the intron homing process, the aptamer-coding sequences were integrated into hundreds of rRNA genes, and the aptamers were transcribed at high levels by RNA polymerase I without any additional promoter being introduced into the cell. We used this system to express an aptamer against the heat shock factor 1 (HSF1), a conserved transcription factor responsible for mobilizing specific genomic expression programs in response to stressful conditions such as elevated temperature. We observed a temperature sensitive growth retardation phenotype and specific decrease of heat shock gene expression. As HSF1 enables and promotes malignant growth and metastasis in mammals, and this aptamer binds yeast HSF1 and its mammalian ortholog with equal affinity, the results presented here attest to the potential of this aptamer as a specific and effective inhibitor of HSF1 activity
CoRoT\,102699796, the first metal-poor Herbig Ae pulsator: a hybrid Sct- Dor variable?
We present the analysis of the time series observations of CoRoT\,102699796
obtained by the CoRoT satellite that show the presence of five independent
oscillation frequencies in the range 3.6-5 c/d. Using spectra acquired with
FLAMES@VLT, we derive the following stellar parameters: spectral type F1V,
T=7000200 K, log(g)=, [M/H]=,
sin= km/s, L/L=21. Thus, for the first
time we report the existence of a metal poor, intermediate-mass PMS pulsating
star. Ground-based and satellite data are used to derive the spectral energy
distribution of CoRoT\,102699796 extending from the optical to mid-infrared
wavelengths. The SED shows a significant IR excess at wavelengths greater than
. We conclude that CoRoT\,102699796 is a young Herbig Ae (F1Ve) star
with a transitional disk, likely associated to the HII region [FT96]213.1-2.2.
The pulsation frequencies have been interpreted in the light of the
non-radial pulsation theory, using the LOSC code in conjunction with static and
rotational evolutionary tracks. A minimization algorithm was used to find the
best-fit model with M=1.84 M, T=6900 K which imply an
isochronal age of t2.5 Myr. This result is based on the interpretation of
the detected frequencies as -modes of low-moderate -value. To our
knowledge, this is the first time that such modes are identified in a
intermediate-mass PMS pulsating star. Since CoRoT\,102699796 lies in the region
of the HR diagram where the Sct and Dor instability strips
intersect, we argue that the observed pulsation characteristics are
intermediate between these classes of variables, i.e. CoRoT\,102699796 is
likely the first PMS hybrid Dor- Sct pulsator ever studied.Comment: 13 pages, 7 figures. Accepted for publication in Monthly Notices of
the RA
Solar-like oscillations in low-luminosity red giants: first results from Kepler
We have measured solar-like oscillations in red giants using time-series
photometry from the first 34 days of science operations of the Kepler Mission.
The light curves, obtained with 30-minute sampling, reveal clear oscillations
in a large sample of G and K giants, extending in luminosity from the red clump
down to the bottom of the giant branch. We confirm a strong correlation between
the large separation of the oscillations (Delta nu) and the frequency of
maximum power (nu_max). We focus on a sample of 50 low-luminosity stars (nu_max
> 100 muHz, L <~ 30 L_sun) having high signal-to-noise ratios and showing the
unambiguous signature of solar-like oscillations. These are H-shell-burning
stars, whose oscillations should be valuable for testing models of stellar
evolution and for constraining the star-formation rate in the local disk. We
use a new technique to compare stars on a single echelle diagram by scaling
their frequencies and find well-defined ridges corresponding to radial and
non-radial oscillations, including clear evidence for modes with angular degree
l=3. Measuring the small separation between l=0 and l=2 allows us to plot the
so-called C-D diagram of delta nu_02 versus Delta nu. The small separation
delta nu_01 of l=1 from the midpoint of adjacent l=0 modes is negative,
contrary to the Sun and solar-type stars. The ridge for l=1 is notably
broadened, which we attribute to mixed modes, confirming theoretical
predictions for low-luminosity giants. Overall, the results demonstrate the
tremendous potential of Kepler data for asteroseismology of red giants.Comment: accepted by ApJ Letters, to appear in special Kepler issue. Updated
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