463 research outputs found
Near-UV photodissociation dynamics of CH2I2
The near-UV photodissociation dynamics of CH2I2 has been investigated using a combination of velocitymap
(slice) ion imaging and ab initio calculations characterizing the excited states. Ground state I(2
P3/2)
and spin-orbit excited I*(2
P1/2) atoms were probed using 2+1 resonance-enhanced multiphoton ionization
(REMPI) or with single-photon VUV ionization. Two-color ion images were recorded at pump
wavelengths of 355 nm, 266 nm and 248 nm, and one-color ion images at the REMPI wavelengths of
~304 nm and ~280 nm. Analysis of the ion images shows that, regardless of iodine spin-orbit state, ~20%
of the available energy is partitioned into translation ET at all excitation wavelengths indicating that the
CH2I co-fragment is formed highly internally excited. The translational energy distributions comprise a
slow, âstatisticalâ component that peaks near zero and faster components that peak away from zero. The
slow component makes an increasingly large contribution to the distribution as the excitation wavelength
is decreased. The CâI bond dissociation energy of D0 = 2.155Âą0.008 eV is obtained from the trend in the
ET release of the faster components with increasing excitation energy. The I and I* ion images are
anisotropic, indicating prompt dissociation, and are characterized by β parameters that become
increasingly positive with increasing ET. The decrease in β at lower translational energies can be
attributed to deviation from axial recoil. MRCI calculations including spin-orbit coupling have been
performed to identify the overlapping features in the absorption spectrum and characterize onedimensional
cuts through the electronically excited potential energy surfaces. The excited states are of
significantly mixed singlet and triplet character. At longer wavelengths, excitation directly accesses
repulsive states primarily of B1 symmetry, consistent with the observed !βâ, while shorter wavelengths
accesses bound states, also of B1 symmetry that are crossed by repulsive states
Jupiter â friend or foe? I: the asteroids
The asteroids are a major source of potential impactors on the Earth today. It has long been assumed that the giant planet Jupiter acts as a shield, significantly lowering the impact rate on the Earth from both cometary and asteroidal bodies. Such shielding, it is claimed, enabled the development and evolution of life in a collisional environment, which is not overly hostile. The reduced frequency of impacts, and of related mass extinctions, would have allowed life the time to thrive, where it would otherwise have been suppressed. However, in the past, little work has been carried out to examine the validity of this idea. In the first of several papers, we examine the degree to which the impact risk resulting from a population representative of the asteroids is enhanced or reduced by the presence of a giant planet, in an attempt to understand fully the impact regime under which life on Earth developed. Our results show that the situation is far less clear cut that has previously been assumed, that is, the presence of a giant planet can act to enhance the impact rate of asteroids on the Earth significantly
A conference management system with verified document confidentiality
We present a case study in verified security for realistic systems: the implementation of a conference management system, whose functional kernel is faithfully represented in the Isabelle theorem prover, where we specify and verify confidentiality properties. The various theoretical and practical challenges posed by this development led to a novel security model and verification method
generally applicable to systems describable as inputâoutput automata
UV Photodissociation Dynamics of CHI2Cl and its Role as a Photolytic Precursor for a Chlorinated Criegee Intermediate
Photolysis of geminal diiodoalkanes in the presence of molecular oxygen has become an established
route to the laboratory production of several Criegee intermediates, and such compounds also have
marine sources. Here, we explore the role that the trihaloalkane, chlorodiiodomethane (CHI2Cl), may
play as a photolytic precursor for the chlorinated Criegee intermediate ClCHOO. CHI2Cl has been
synthesized and its UV absorption spectrum measured; relative to that of CH2I2 the spectrum is
shifted to longer wavelength and the photolysis lifetime is calculated to be less than two minutes.
The photodissociation dynamics have been investigated using DC slice imaging, probing ground state
I and spin-orbit excited I* atoms with 2+1 REMPI and single-photon VUV ionization. Total
translational energy distributions are bimodal for I atoms and unimodal for I*, with around 72% of
the available energy partitioned in to the internal degrees of freedom of the CHICl radical product,
independent of photolysis wavelength. A bond dissociation energy of D0 = 1.73Âą0.11 eV is inferred
from the wavelength dependence of the translational energy release, which is slightly weaker than
typical CâI bonds. Analysis of the photofragment angular distributions indicate dissociation is
prompt and occurs primarily via transitions to states of Aâł symmetry. Complementary high-level
MRCI calculations, including spin-orbit coupling, have been performed to characterize the excited
states and confirm that states of Aâł symmetry with highly mixed singlet and triplet character are
predominantly responsible for the absorption spectrum. Transient absorption spectroscopy has
been used to measure the absorption spectrum of ClCHOO produced from the reaction of CHICl with
O2 over the range 345â440 nm. The absorption spectrum, tentatively assigned to the syn conformer,
is at shorter wavelengths relative to that of CH2OO and shows far weaker vibrational structure
The Horizontal Component of Photospheric Plasma Flows During the Emergence of Active Regions on the Sun
The dynamics of horizontal plasma flows during the first hours of the
emergence of active region magnetic flux in the solar photosphere have been
analyzed using SOHO/MDI data. Four active regions emerging near the solar limb
have been considered. It has been found that extended regions of Doppler
velocities with different signs are formed in the first hours of the magnetic
flux emergence in the horizontal velocity field. The flows observed are
directly connected with the emerging magnetic flux; they form at the beginning
of the emergence of active regions and are present for a few hours. The Doppler
velocities of flows observed increase gradually and reach their peak values
4-12 hours after the start of the magnetic flux emergence. The peak values of
the mean (inside the +/-500 m/s isolines) and maximum Doppler velocities are
800-970 m/s and 1410-1700 m/s, respectively. The Doppler velocities observed
substantially exceed the separation velocities of the photospheric magnetic
flux outer boundaries. The asymmetry was detected between velocity structures
of leading and following polarities. Doppler velocity structures located in a
region of leading magnetic polarity are more powerful and exist longer than
those in regions of following polarity. The Doppler velocity asymmetry between
the velocity structures of opposite sign reaches its peak values soon after the
emergence begins and then gradually drops within 7-12 hours. The peak values of
asymmetry for the mean and maximal Doppler velocities reach 240-460 m/s and
710-940 m/s, respectively. An interpretation of the observable flow of
photospheric plasma is given.Comment: 20 pages, 10 figures, 3 tables. The results of article were presented
at the ESPM-13 (12-16 September 2011, Rhodes, Greece, Abstract Book p. 102,
P.4.12,
http://astro.academyofathens.gr/espm13/documents/ESPM13_abstract_programme_book.pdf
Differences between the impact regimes of the terrestrial planets: implications for primordial D:H ratios
It is often assumed that the terrestrial worlds have experienced identical impact regimes over the course of their formation and evolution, and, as a result, would have started life with identical volatile budgets. In this work, through illustrative dynamical simulations of the impact flux on Venus, the Earth, and Mars, we show that these planets can actually experience greatly different rates of impact from objects injected from different reservoirs. For example, we show scenarios in which Mars experiences far more asteroidal impacts, per cometary impactor, than Venus, with the Earth being intermediate in value between the two. This difference is significant, and is apparent in simulations of both quiescent and highly stirred asteroid belts (such as could be produced by a mutual mean-motion resonance crossing between Jupiter and Saturn, as proposed in the Nice model of the Late Heavy Bombardment). We consider the effects such differences would have on the initial volatilisation of the terrestrial planets in a variety of scenarios of both endogenous and exogenous hydration, with particular focus on the key question of the initial level of deuteration in each planet's water budget. We conclude that each of the terrestrial worlds will have experienced a significantly different distribution of impactors from various reservoirs, and that the assumption that each planet has the same initial volatile budget is, at the very least, a gross over-simplification
The status of GEO 600
The GEO 600 laser interferometer with 600m armlength is part of a worldwide network of gravitational wave detectors. GEO 600 is unique in having advanced multiple pendulum suspensions with a monolithic last stage and in employing a signal recycled optical design. This paper describes the recent commissioning of the interferometer and its operation in signal recycled mode
Physics of Solar Prominences: II - Magnetic Structure and Dynamics
Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.Comment: 75 pages, 31 pictures, review pape
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
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