1,829 research outputs found
Optical spectroscopy of complex open 4-shell ions Sn-Sn
We analyze the complex level structure of ions with many-valence-electron
open [Kr] 4 sub-shells (=7-4) with ab initio
calculations based on configuration-interaction many-body perturbation theory
(CI+MBPT). Charge-state-resolved optical and extreme ultraviolet (EUV) spectra
of Sn-Sn ions were obtained using an electron beam ion trap.
Semi-empirical spectral fits carried out with the orthogonal parameters
technique and Cowan code calculations lead to 90 identifications of
magnetic-dipole transitions and the determination of 79 energy
ground-configuration levels, questioning some earlier EUV-line assignments. Our
results, the most complete data set available to date for these ground
configurations, confirm the ab initio predictive power of CI+MBPT calculations
for the these complex electronic systems.Comment: 18 pages, 5 figure
Study of the 25Mg(d,p)26Mg reaction to constrain the 25Al(p,Îł )26Si resonant reaction rates in nova burning conditions
The rate of the Al(p, )Si reaction is one of the few key remaining nuclear uncertainties required for predicting the production of the cosmic -ray emitter Al in explosive burning in novae. This reaction rate is dominated by three key resonances (, and ) in Si. Only the resonance strength has been directly constrained by experiment. A high resolution measurement of the Mg(d, p) reaction was used to determine spectroscopic factors for analog states in the mirror nucleus, Mg. A first spectroscopic factor value is reported for the state at 6.256 MeV, and a strict upper limit is set on the value for the state at 5.691 MeV, that is incompatible with an earlier (He, He) study. These results are used to estimate proton partial widths, and resonance strengths of analog states in Si contributing to the Al(p, )Si reaction rate in nova burning conditions
The environmental security debate and its significance for climate change
Policymakers, military strategists and academics all increasingly hail climate change as a security issue. This article revisits the (comparatively) long-standing âenvironmental security debateâ and asks what lessons that earlier debate holds for the push towards making climate change a security issue. Two important claims are made. First, the emerging climate security debate is in many ways a re-run of the earlier dispute. It features many of the same proponents and many of the same disagreements. These disagreements concern, amongst other things, the nature of the threat, the referent object of security and the appropriate policy responses. Second, given its many different interpretations, from an environmentalist perspective, securitisation of the climate is not necessarily a positive development
Implementation of the Hierarchical Reference Theory for simple one-component fluids
Combining renormalization group theoretical ideas with the integral equation
approach to fluid structure and thermodynamics, the Hierarchical Reference
Theory is known to be successful even in the vicinity of the critical point and
for sub-critical temperatures. We here present a software package independent
of earlier programs for the application of this theory to simple fluids
composed of particles interacting via spherically symmetrical pair potentials,
restricting ourselves to hard sphere reference systems. Using the hard-core
Yukawa potential with z=1.8/sigma for illustration, we discuss our
implementation and the results it yields, paying special attention to the core
condition and emphasizing the decoupling assumption's role.Comment: RevTeX, 16 pages, 2 figures. Minor changes, published versio
Pressure induced structural and dynamical changes in liquid Si. An ab-initio study
The static and dynamic properties of liquid Si at high-pressure have been
studied using the orbital free ab-initio molecular dynamics method. Four
thermodynamic states at pressures 4, 8, 14 and 23 GPa are considered. The
calculated static structure shows qualitative agreement with the available
experimental data. We analize the remarkable structural changes occurring
between 8 and 14 GPa along with its effect on several dynamic properties.Comment: 10 pages, 11 figures. Accepted for publication in Journal of Physics:
Condensed Matte
Diel variations of H2O2 in Greenland: A discussion of the cause and effect relationship
Atmospheric hydrogen peroxide (H2O2) measurements at Summit, Greenland, in MayâJune, 1993 exhibited a diel variation, with afternoon highs typically 1â2 parts per billion by volume (ppbv) and nighttime lows about 0.5 ppbv lower. This variation closely followed that for temperature; specific humidity exhibited the same general trend. During a 17-day snowfall-free period, surface snow was accumulating H2O2, apparently from nighttime cocondensation of H2O and H2O2. Previous photochemical modeling (Neftel et al., 1995) suggests that daytime H2O2 should be about 1 ppbv, significantly lower than our measured values. Previous equilibrium partitioning measurements between ice and gas phase (Conklin et al., 1993) suggest that air in equilibrium with H2O2 concentrations measured in surface snow (15â18 ÎŒM) should have an H2O2 concentration 2â3 times what we measured 0.2â3.5 m above the snow surface. A simple eddy diffusion model, with vertical eddy diffusion coefficients calculated from balloon soundings, suggested that atmospheric H2O2 concentrations should be affected by any H2O2 degassed from surface snow. However, field measurements showed the absence of either high concentrations of H2O2 or a measurable concentration gradient between inlets 0.2 and 3 m above the snow. A surface resistance to degassing, that is, slow release of H2O2 from the ice matrix, is a plausible explanation for the differences between observations and modeled atmospheric profiles. Degassing of H2O2 at a rate below our detection limit would still influence measured atmospheric concentrations and help explain the difference between measurements and photochemical modeling. The cumulative evidence suggests that surface snow adjusts slowly to drops in atmospheric H2O2 concentration, over timescales of at least weeks. The H2O2 losses previously observed in pits sampled over more than 1 year are thought to have occurred later in the summer or fall, after the MayâJuly field season
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