1,333 research outputs found
Laser-induced 3D alignment and orientation of quantum-state-selected molecules
A strong inhomogeneous static electric field is used to spatially disperse a
rotationally cold supersonic beam of 2,6-difluoroiodobenzene molecules
according to their rotational quantum state. The molecules in the lowest lying
rotational states are selected and used as targets for 3-dimensional alignment
and orientation. The alignment is induced in the adiabatic regime with an
elliptically polarized, intense laser pulse and the orientation is induced by
the combined action of the laser pulse and a weak static electric field. We
show that the degree of 3-dimensional alignment and orientation is strongly
enhanced when rotationally state-selected molecules, rather than molecules in
the original molecular beam, are used as targets.Comment: 8 pages, 7 figures; v2: minor update
Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules
The yield of strong-field ionization, by a linearly polarized probe pulse, is
studied experimentally and theoretically, as a function of the relative
orientation between the laser field and the molecule. Experimentally, carbonyl
sulfide, benzonitrile and naphthalene molecules are aligned in one or three
dimensions before being singly ionized by a 30 fs laser pulse centered at 800
nm. Theoretically, we address the behaviour of these three molecules. We
consider the degree of alignment and orientation and model the angular
dependence of the total ionization yield by molecular tunneling theory
accounting for the Stark shift of the energy level of the ionizing orbital. For
naphthalene and benzonitrile the orientational dependence of the ionization
yield agrees well with the calculated results, in particular the observation
that ionization is maximized when the probe laser is polarized along the most
polarizable axis. For OCS the observation of maximum ionization yield when the
probe is perpendicular to the internuclear axis contrasts the theoretical
results.Comment: 14 pages, 4 figure
A Prismatic Analyser concept for Neutron Spectrometers
A development in modern neutron spectroscopy is to avoid the need of large
samples. We demonstrate how small samples together with the right choice of
analyser and detector components makes distance collimation an important
concept in crystal analyser spectrometers. We further show that this opens new
possibilities where neutrons with different energies are reflected by the same
analyser but counted in different detectors, thus improving both energy
resolution and total count rate compared to conventional spectrometers. The
technique can be combined with advanced focusing geometries and with
multiplexing instrument designs. We present a combination of simulations and
data with 3 energies from one analyser. The data was taken on a prototype
installed at PSI, Switzerland, and shows excellent agreement with the
predictions. Typical improvements will be 2 times finer resolution and a factor
1.9 in flux gain compared to a Rowland geometry or 3 times finer resolution and
a factor 3.2 in flux gain compared to a single flat analyser slab
Control and femtosecond time-resolved imaging of torsion in a chiral molecule
We study how the combination of long and short laser pulses, can be used to
induce torsion in an axially chiral biphenyl derivative
(3,5-difluoro-3',5'-dibromo-4'-cyanobiphenyl). A long, with respect to the
molecular rotational periods, elliptically polarized laser pulse produces 3D
alignment of the molecules, and a linearly polarized short pulse initiates
torsion about the stereogenic axis. The torsional motion is monitored in
real-time by measuring the dihedral angle using femtosecond time-resolved
Coulomb explosion imaging. Within the first 4 picoseconds, torsion occurs with
a period of 1.25 picoseconds and an amplitude of 3 degrees in excellent
agreement with theoretical calculations. At larger times the quantum states of
the molecules describing the torsional motion dephase and an almost isotropic
distribution of the dihedral angle is measured. We demonstrate an original
application of covariance analysis of two-dimensional ion images to reveal
strong correlations between specific ejected ionic fragments from Coulomb
explosion. This technique strengthens our interpretation of the experimental
data.Comment: 11 pages, 9 figure
Ionization of 1D and 3D oriented asymmetric top molecules by intense circularly polarized femtosecond laser pulses
We present a combined experimental and theoretical study on strong-field
ionization of a three-dimensionally oriented asymmetric top molecule,
benzonitrile (CHN), by circularly polarized, nonresonant femtosecond
laser pulses. Prior to the interaction with the strong field, the molecules are
quantum-state selected using a deflector, and 3-dimensionally (3D) aligned and
oriented adiabatically using an elliptically polarized laser pulse in
combination with a static electric field. A characteristic splitting in the
molecular frame photoelectron momentum distribution reveals the position of the
nodal planes of the molecular orbitals from which ionization occurs. The
experimental results are supported by a theoretical tunneling model that
includes and quantifies the splitting in the momentum distribution. The focus
of the present article is to understand strong-field ionization from
3D-oriented asymmetric top molecules, in particular the suppression of electron
emission in nodal planes of molecular orbitals. In the preceding article
[Dimitrovski et al., Phys. Rev. A 83, 023405 (2011)] the focus is to understand
the strong-field ionization of one-dimensionally-oriented polar molecules, in
particular asymmetries in the emission direction of the photoelectrons.Comment: 12 pages, 9 figure
Inside Out: Modern Imaging Techniques to Reveal Animal Anatomy
Animal anatomy has traditionally relied on detailed dissections to produce anatomical illustrations, but modern imaging modalities, such as MRI and CT, now represent an enormous resource that allows for fast non-invasive visualizations of animal anatomy in living animals. These modalities also allow for creation of three-dimensional representations that can be of considerable value in the dissemination of anatomical studies. In this methodological review, we present our experiences using MRI, CT and μCT to create advanced representation of animal anatomy, including bones, inner organs and blood vessels in a variety of animals, including fish, amphibians, reptiles, mammals, and spiders. The images have a similar quality to most traditional anatomical drawings and are presented together with interactive movies of the anatomical structures, where the object can be viewed from different angles. Given that clinical scanners found in the majority of larger hospitals are fully suitable for these purposes, we encourage biologists to take advantage of these imaging techniques in creation of three-dimensional graphical representations of internal structures
Quantum-state selection, alignment, and orientation of large molecules using static electric and laser fields
Supersonic beams of polar molecules are deflected using inhomogeneous
electric fields. The quantum-state selectivity of the deflection is used to
spatially separate molecules according to their quantum state. A detailed
analysis of the deflection and the obtained quantum-state selection is
presented. The rotational temperatures of the molecular beams are determined
from the spatial beam profiles and are all approximately 1 K. Unprecedented
degrees of laser-induced alignment and
orientation of iodobenzene molecules are demonstrated when the state-selected
samples are used. Such state-selected and oriented molecules provide unique
possibilities for many novel experiments in chemistry and physics.Comment: minor changes, references update
Stroke and recurrent haemorrhage associated with antithrombotic treatment after gastrointestinal bleeding in patients with atrial fibrillation:nationwide cohort study
Study question What are the risks of all cause mortality, thromboembolism, major bleeding, and recurrent gastrointestinal bleeding associated with restarting antithrombotic treatment after gastrointestinal bleeding in patients with atrial fibrillation? Methods This Danish cohort study (1996-2012) included all patients with atrial fibrillation discharged from hospital after gastrointestinal bleeding while receiving antithrombotic treatment. Restarted treatment regimens were single or combined antithrombotic drugs with oral anticoagulation and antiplatelets. Follow-up started 90 days after discharge to avoid confounding from use of previously prescribed drugs on discharge. Risks of all cause mortality, thromboembolism, major bleeding, and recurrent gastrointestinal bleeding were estimated with competing risks models and time dependent multiple Cox regression models. Study answer and limitations 4602 patients (mean age 78 years) were included. Within two years, 39.9% (95% confidence interval 38.4% to 41.3%, n=1745) of the patients had died, 12.0% (11.0% to 13.0%, n=526) had experienced thromboembolism, 17.7% (16.5% to 18.8%, n=788) major bleeding, and 12.1% (11.1% to 13.1%, n=546) recurrent gastrointestinal bleeding. 27.1% (n=924) of patients did not resume antithrombotic treatment. Compared with non-resumption of treatment, a reduced risk of all cause mortality was found in association with restart of oral anticoagulation (hazard ratio 0.39, 95% confidence interval 0.34 to 0.46), an antiplatelet agent (0.76, 0.68 to 0.86), and oral anticoagulation plus an antiplatelet agent (0.41, 0.32 to 0.52), and a reduced risk of thromboembolism was found in association with restart of oral anticoagulation (0.41, 0.31 to 0.54), an antiplatelet agent (0.76, 0.61 to 0.95), and oral anticoagulation plus an antiplatelet agent (0.54, 0.36 to 0.82). Restarting oral anticoagulation alone was the only regimen with an increased risk of major bleeding (1.37, 1.06 to 1.77) compared with non-resumption of treatment; however, the difference in risk of recurrent gastrointestinal bleeding was not significant between patients who restarted an antithrombotic treatment regimen and those who did not resume treatment. What this study adds Among patients with atrial fibrillation who experience gastrointestinal bleeding while receiving antithrombotic treatment; subsequent restart of oral anticoagulation alone was associated with better outcomes for all cause mortality and thromboembolism compared with patients who did not resume treatment. This was despite an increased longitudinal associated risk of bleeding. Funding, competing interests, data sharing This study was supported by a grant from Boehringer-Ingelheim. Competing interests are available in the full paper on bmj.com. The authors have no additional data to share
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