536 research outputs found
Lifetime Measurement of the 8s Level in Francium
We measure the lifetime of the 8s level on a magneto-optically trapped sample
of ^{210}Fr atoms with time-correlated single-photon counting. The 7P_{1/2}
state serves as the resonant intermediate level for two-photon excitation of
the 8s level completed with a 1300 nm laser. Analysis of the fluorescence decay
through the the 7P_{3/2} level gives 53.30 +- 0.44 ns for the 8s level
lifetime.Comment: 4 pages, 4 figure
Sub-Doppler spectroscopy of Rb atoms in a sub-micron vapor cell in the presence of a magnetic field
We report the first use of an extremely thin vapor cell (thickness ~ 400 nm)
to study the magnetic-field dependence of laser-induced-fluorescence excitation
spectra of alkali atoms. This thin cell allows for sub-Doppler resolution
without the complexity of atomic beam or laser cooling techniques. This
technique is used to study the laser-induced-fluorescence excitation spectra of
Rb in a 50 G magnetic field. At this field strength the electronic angular
momentum J and nuclear angular momentum I are only partially decoupled. As a
result of the mixing of wavefunctions of different hyperfine states, we observe
a nonlinear Zeeman effect for each sublevel, a substantial modification of the
transition probabilities between different magnetic sublevels, and the
appearance of transitions that are strictly forbidden in the absence of the
magnetic field. For the case of right- and left- handed circularly polarized
laser excitation, the fluorescence spectra differs qualitatively. Well
pronounced magnetic field induced circular dichroism is observed. These
observations are explained with a standard approach that describes the partial
decoupling of I and J states
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Identifying jumps in financial assets: A comparison between nonparametric jump tests
We perform a comprehensive Monte Carlo comparison between nine alternative procedures available in the literature to detect jumps in financial assets using high-frequency data. We evaluate size and power properties of the procedures under alternative sampling frequencies, persistence in volatility, jump size and intensity, and degree of contamination with microstructure noise. The overall best performance is shown by the Andersen, Bollerslev, and Dobrev (2007) and Lee and Mykland (2008) intraday procedures (ABD-LM), provided the price process is not very volatile. We propose two extensions to the existing battery of tests. The first regards the finite sample improvements based on simulated critical values for the ABD-LM procedure. The second regards a procedure that combines frequencies and tests able to reduce the number of spurious jumps. Finally, we report an empirical analysis using real high frequency data on five stocks listed in the New York Stock Exchange
Collisional and thermal ionization of sodium Rydberg atoms I. Experiment for nS and nD atoms with n=8-20
Collisional and thermal ionization of sodium nS and nD Rydberg atoms with
n=8-20 has been studied. The experiments were performed using a two-step pulsed
laser excitation in an effusive atomic beam at atom density of about 2 10^{10}
cm^{-3}. Molecular and atomic ions from associative, Penning, and thermal
ionization processes were detected. It has been found that the atomic ions were
created mainly due to photoionization of Rydberg atoms by photons of blackbody
radiation at the ambient temperature of 300K. Blackbody ionization rates and
effective lifetimes of Rydberg states of interest were determined. The
molecular ions were found to be from associative ionization in Na(nL)+Na(3S)
collisions. Rate constants of associative ionization have been measured using
an original method based on relative measurements of Na_{2}^{+} and Na^{+} ion
signals.Comment: 23 pages, 10 figure
A primordial star in the heart of the Lion
Context: The discovery and chemical analysis of extremely metal-poor stars
permit a better understanding of the star formation of the first generation of
stars and of the Universe emerging from the Big Bang. aims: We report the study
of a primordial star situated in the centre of the constellation Leo (SDSS
J102915+172027). method: The star, selected from the low resolution-spectrum of
the Sloan Digital Sky Survey, was observed at intermediate (with X-Shooter at
VLT) and at high spectral resolution (with UVES at VLT). The stellar parameters
were derived from the photometry. The standard spectroscopic analysis based on
1D ATLAS models was completed by applying 3D and non-LTE corrections. results:
An iron abundance of [Fe/H]=--4.89 makes SDSS J102915+172927 one of the lowest
[Fe/H] stars known. However, the absence of measurable C and N enhancements
indicates that it has the lowest metallicity, Z<= 7.40x10^{-7} (metal-mass
fraction), ever detected. No oxygen measurement was possible. conclusions: The
discovery of SDSS J102915+172927 highlights that low-mass star formation
occurred at metallicities lower than previously assumed. Even lower metallicity
stars may yet be discovered, with a chemical composition closer to the
composition of the primordial gas and of the first supernovae.Comment: To be published in A&
The chemical abundance analysis of normal early A- and late B-type stars
Modern spectroscopy of early-type stars often aims at studying complex
physical phenomena. Comparatively less attention is paid to identifying and
studying the "normal" A- and B-type stars and testing how the basic atomic
parameters and standard spectral analysis allow one to fit the observations. We
wish to stablish whether the chemical composition of the solar photosphere can
be regarded as a reference for early A- and late B-type stars. We have obtained
optical high-resolution, high signal-to-noise ratio spectra of three slowly
rotating early-type stars (HD 145788, 21 Peg and pi Cet) that show no obvious
sign of chemical peculiarity, and performed a very accurate LTE abundance
analysis of up to 38 ions of 26 elements (for 21 Peg), using a vast amount of
spectral lines visible in the spectral region covered by our spectra. We
provide an exhaustive description of the abundance characteristics of the three
analysed stars with a critical review of the line parameters used to derive the
abundances. We compiled a table of atomic data for more than 1100 measured
lines that may be used in the future as a reference. The abundances we obtained
for He, C, Al, S, V, Cr, Mn, Fe, Ni, Sr, Y, and Zr are compatible with the
solar ones derived with recent 3D radiative-hydrodynamical simulations of the
solar photosphere. The abundances of the remaining studied elements show some
degree of discrepancy compared to the solar photosphere. Those of N, Na, Mg,
Si, Ca, Ti, and Nd may well be ascribed to non-LTE effects; for P, Cl, Sc and
Co, non-LTE effects are totally unknown; O, Ne, Ar, and Ba show discrepancies
that cannot be ascribed to non-LTE effects. The discrepancies obtained for O
(in two stars) and Ne agree with very recent non-LTE abundance analysis of
early B-type stars in the solar neighbourhood.Comment: Accepted for publication on Astronomy and Astrophysic
The Intuitive Bioclimatism and Embedded Sustainability of Cypriot Vernacular Farmhouses, as Principles for their Strategic Restoration and Reuse
This research aims to investigate the bioclimatic design, sustainability and environmental behaviour of Cypriot vernacular farmhouses, as part of small-scale family-owned farmsteads. These farmhouses, located in village outskirts, are still in operation, greatly supporting the primary sector of Cypriot economy. Current uses have conduced arbitrary modifications in their morphology, layout, construction and usage, which in the past had contributed positively to thermal comfort and household autonomy. In the present, these modifications caused farmhouses to become climatically inept, devoid of Bioclimatism and Sustainability. This research employs multiple case studies with a fully-integrated mixed-methods design. Findings are drawn via triangulation of qualitative and quantitative data from ethnographic participant observation, post-occupancy evaluation survey, interviews, in-situ documentation and environmental monitoring. Initial findings show that there is a tangible and intangible relationship between the dwelling, its inhabitants and the environment, strengthened in time due to tradition, accumulated knowledge and experience, and born out of necessity, scarcity and practicality. It is an interdependent, dynamic and adaptive reciprocity, induced by the complex interplay of socio-cultural, economic, technological, aesthetic and environmental factors. The end result was for bioclimatic design to be applied intuitively and sustainability to be embedded in the form, configuration, construction and operational modes of farmhouses. As Cyprus attempts to conform to its EU obligations regarding the sustainable development of rural environments and make the transition towards nearly-zero energy buildings, the Cypriot vernacular farmhouse can offer valuable lessons in building design and performance, whilst provide clear guidance for its strategic restoration and reuse
Documenting the Recovery of Vascular Services in European Centres Following the Initial COVID-19 Pandemic Peak: Results from a Multicentre Collaborative Study
Objective: To document the recovery of vascular services in Europe following the first COVID-19 pandemic peak. Methods: An online structured vascular service survey with repeated data entry between 23 March and 9 August 2020 was carried out. Unit level data were collected using repeated questionnaires addressing modifications to vascular services during the first peak (March – May 2020, “period 1”), and then again between May and June (“period 2”) and June and July 2020 (“period 3”). The duration of each period was similar. From 2 June, as reductions in cases began to be reported, centres were first asked if they were in a region still affected by rising cases, or if they had passed the peak of the first wave. These centres were asked additional questions about adaptations made to their standard pathways to permit elective surgery to resume. Results: The impact of the pandemic continued to be felt well after countries’ first peak was thought to have passed in 2020. Aneurysm screening had not returned to normal in 21.7% of centres. Carotid surgery was still offered on a case by case basis in 33.8% of centres, and only 52.9% of centres had returned to their normal aneurysm threshold for surgery. Half of centres (49.4%) believed their management of lower limb ischaemia continued to be negatively affected by the pandemic. Reduced operating theatre capacity continued in 45.5% of centres. Twenty per cent of responding centres documented a backlog of at least 20 aortic repairs. At least one negative swab and 14 days of isolation were the most common strategies used for permitting safe elective surgery to recommence. Conclusion: Centres reported a broad return of services approaching pre-pandemic “normal” by July 2020. Many introduced protocols to manage peri-operative COVID-19 risk. Backlogs in cases were reported for all major vascular surgeries
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