352 research outputs found
Crystal structure and characterization of magnesium carbonate chloride heptahydrate
MgCO·MgCl·7HO is the only known neutral magnesium carbonate conÂtaining chloride ions at ambient conditions. According to the literature, only small and twinned crystals of this double salt could be synthesised in a concentrated solution of MgCl. For the crystal structure solution, single-crystal diffraction was carried out at a synchrotron radiation source. The monoclinic crystal structure (space group Cc) exhibits double chains of MgO octaÂhedra linked by corners, connected by carbonate units and water molÂecules. The chloride ions are positioned between these double chains parallel to the (100) plane. Dry MgCO·MgCl·7HO decomÂposes in the air to chlorartinite, Mg(OH)Cl(CO)·nHO (n = 2 or 3). This work includes an extensive charÂacterization of the title comÂpound by powder X-ray diffraction, thermal analysis, SEM and vibrational spectroscopy
Transient absorption and reshaping of ultrafast XUV light by laser-dressed helium
We present a theoretical study of transient absorption and reshaping of
extreme ultraviolet (XUV) pulses by helium atoms dressed with a moderately
strong infrared (IR) laser field. We formulate the atomic response using both
the frequency-dependent absorption cross section and a time-frequency approach
based on the time-dependent dipole induced by the light fields. The latter
approach can be used in cases when an ultrafast dressing pulse induces
transient effects, and/or when the atom exchanges energy with multiple
frequency components of the XUV field. We first characterize the dressed atom
response by calculating the frequency-dependent absorption cross section for
XUV energies between 20 and 24 eV for several dressing wavelengths between 400
and 2000 nm and intensities up to 10^12 W/cm^2. We find that for dressing
wavelengths near 1600 nm, there is an Autler-Townes splitting of the 1s ---> 2p
transition that can potentially lead to transparency for absorption of XUV
light tuned to this transition. We study the effect of this XUV transparency in
a macroscopic helium gas by incorporating the time-frequency approach into a
solution of the coupled Maxwell-Schr\"odinger equations. We find rich temporal
reshaping dynamics when a 61 fs XUV pulse resonant with the 1s ---> 2p
transition propagates through a helium gas dressed by an 11 fs, 1600 nm laser
pulse.Comment: 13 pages, 8 figures, 1 table, RevTeX4, revise
Signatures of the Madden–Julian oscillation in middle-atmosphere zonal mean temperature: triggering the interhemispheric coupling pattern
The influence of the Madden–Julian oscillation (MJO) on the middle atmosphere (MA) and particularly on MA temperature is of interest for both the understanding of MJO-induced teleconnections and research on the variability of the MA. We analyze statistically the connection of the MJO and the MA zonal mean temperature based on observations by the Microwave Limb Sounder (MLS) satellite instrument. We consider all eight MJO phases, different seasons and the state of the quasi-biennial oscillation (QBO). We show that MA temperature anomalies are significantly related to the MJO and its temporal development. The MJO signal in the zonal mean MA temperature is characterized by a particular spatial pattern in the MA, which we link to the interhemispheric coupling (IHC) mechanism, as a major outcome of this study. The signal with the largest magnitude is found in the polar MA during boreal winter with temperature deviations on the order of ±10 K when the QBO at 50 hPa is in its easterly phase. Other atmospheric conditions and locations also exhibit temperature signals, which are, however, weaker or noisier. We also analyze the change in the temperature signal while the MJO progresses from one phase to the next. We find a gradual altitude shift in parts of the IHC pattern, which can be seen more or less clearly depending on the atmospheric conditions.
The statistical link between the MJO and the MA temperature highlights illustratively the far-reaching connections across different atmospheric layers and geographical regions in the atmosphere. Additionally, it highlights close linkages of known dynamical features of the atmosphere, particularly the MJO, the IHC, the QBO and sudden stratospheric warmings (SSWs). Because of the wide coverage of atmospheric regions and included dynamical features, the results might help to further constrain the underlying dynamical mechanisms and could be used as a benchmark for the representation of atmospheric couplings on the intraseasonal timescale in atmospheric models.</p
Phase Measurement of Resonant Two-Photon Ionization in Helium
We study resonant two-color two-photon ionization of Helium via the 1s3p 1P1
state. The first color is the 15th harmonic of a tunable titanium sapphire
laser, while the second color is the fundamental laser radiation. Our method
uses phase-locked high-order harmonics to determine the {\it phase} of the
two-photon process by interferometry. The measurement of the two-photon
ionization phase variation as a function of detuning from the resonance and
intensity of the dressing field allows us to determine the intensity dependence
of the transition energy.Comment: 4 pages, 5 figures, under consideratio
Resonance fluorescence in ultrafast and intense x-ray free-electron-laser pulses
The spectrum of resonance fluorescence is calculated for a two-level system excited by an intense, ultrashort x-ray pulse made available for instance by free-electron lasers such as the Linac Coherent Light Source. We allow for inner-shell hole decay widths and destruction of the system by further photoionization. This two-level description is employed to model neon cations strongly driven by x rays tuned to the 1s 2p-1 --> 1s-1 2p transition at 848 eV; the x rays induce Rabi oscillations which are so fast that they compete with Ne 1s-hole decay. We predict resonance fluorescence spectra for two different scenarios: first, chaotic pulses based on the self-amplified spontaneous emission principle, like those presently generated at x-ray free-electron-laser facilities and, second, Gaussian pulses which will become available in the foreseeable future with self-seeding techniques. As an example of the exciting opportunities derived from the use of seeding methods, we predict, in spite of above obstacles, the possibility to distinguish at x-ray frequencies a clear signature of Rabi flopping in the spectrum of resonance fluorescence
Participatory modeling updates expectations for individuals and groups, catalyzing behavior change and collective action in water-energy-food nexus governance
Participatory modeling is a potentially high-impact approach for catalyzing fundamental sustainability transformations. We test if participation in a group system dynamics modeling exercise increases participants' agency through a novel method to evaluate potential behavioral change using expectations measures. A water-energy-food nexus a functionally interdependent but under-conceptualised system with low consensus and high scientific uncertainty -- was mapped and its evolution simulated by 46 participants in three interventions in a region undergoing hydropower infrastructure development in North-eastern Cambodia. Participants' system-related expectations were measured before and after the interventions. Our results suggest that participants became significantly more optimistic about their individual agency to increase agricultural and fishing income, and interestingly, less likely to participate in local government development planning procedures. Findings also reveal how some uncertainties for multiple variables were reduced within and across the groups. Such converging expectations suggest that participatory modelling could contribute to making collective solutions and institutionalised agreements more likely. This research contributes to innovation in sustainability because it unpacks some underlying mechanics of how participatory processes can lead to new adaptive capacities, shared perspectives and collective actions
Delirium as a predictor of sepsis in post-coronary artery bypass grafting patients: a retrospective cohort study
Article deposited according to agreement with BMC, December 6, 2010.YesFunding provided by the Open Access Authors Fund
Non-Hermitian Rayleigh-Schroedinger Perturbation Theory
We devise a non-Hermitian Rayleigh-Schroedinger perturbation theory for the
single- and the multireference case to tackle both the many-body problem and
the decay problem encountered, for example, in the study of electronic
resonances in molecules. A complex absorbing potential (CAP) is employed to
facilitate a treatment of resonance states that is similar to the
well-established bound-state techniques. For the perturbative approach, the
full CAP-Schroedinger Hamiltonian, in suitable representation, is partitioned
according to the Epstein-Nesbet scheme. The equations we derive in the
framework of the single-reference perturbation theory turn out to be identical
to those obtained by a time-dependent treatment in Wigner-Weisskopf theory. The
multireference perturbation theory is studied for a model problem and is shown
to be an efficient and accurate method. Algorithmic aspects of the integration
of the perturbation theories into existing ab initio programs are discussed,
and the simplicity of their implementation is elucidated.Comment: 10 pages, 1 figure, RevTeX4, submitted to Physical Review
Hydrogen bonding in infinite hydrogen fluoride and hydrogen chloride chains
Hydrogen bonding in infinite HF and HCl bent (zigzag) chains is studied using
the ab initio coupled-cluster singles and doubles (CCSD) correlation method.
The correlation contribution to the binding energy is decomposed in terms of
nonadditive many-body interactions between the monomers in the chains, the
so-called energy increments. Van der Waals constants for the two-body
dispersion interaction between distant monomers in the infinite chains are
extracted from this decomposition. They allow a partitioning of the correlation
contribution to the binding energy into short- and long-range terms. This
finding affords a significant reduction in the computational effort of ab
initio calculations for solids as only the short-range part requires a
sophisticated treatment whereas the long-range part can be summed immediately
to infinite distances.Comment: 9 pages, 4 figures, 3 tables, RevTeX4, corrected typo
- …