181 research outputs found
Properties of metastable alkaline-earth-metal atoms calculated using an accurate effective core potential
The first three electronically excited states in the alkaline-earth-metal
atoms magnesium, calcium, and strontium comprise the (nsnp) triplet P^o_J
(J=0,1,2) fine-structure manifold. All three states are metastable and are of
interest for optical atomic clocks as well as for cold-collision physics. An
efficient technique--based on a physically motivated potential that models the
presence of the ionic core--is employed to solve the Schroedinger equation for
the two-electron valence shell. In this way, radiative lifetimes, laser-induced
clock shifts, and long-range interaction parameters are calculated for
metastable Mg, Ca, and Sr.Comment: 13 pages, 9 table
Calculations of collisions between cold alkaline earth atoms in a weak laser field
We calculate the light-induced collisional loss of laser-cooled and trapped
magnesium atoms for detunings up to 50 atomic linewidths to the red of the
^1S_0-^1P_1 cooling transition. We evaluate loss rate coefficients due to both
radiative and nonradiative state-changing mechanisms for temperatures at and
below the Doppler cooling temperature. We solve the Schrodinger equation with a
complex potential to represent spontaneous decay, but also give analytic models
for various limits. Vibrational structure due to molecular photoassociation is
present in the trap loss spectrum. Relatively broad structure due to absorption
to the Mg_2 ^1Sigma_u state occurs for detunings larger than about 10 atomic
linewidths. Much sharper structure, especially evident at low temperature,
occurs even at smaller detunings due to of Mg_2 ^1Pi_g absorption, which is
weakly allowed due to relativistic retardation corrections to the forbidden
dipole transition strength. We also perform model studies for the other
alkaline earth species Ca, Sr, and Ba and for Yb, and find similar qualitative
behavior as for Mg.Comment: 20 pages, RevTex, 13 eps figures embedde
Increasing transnational sea‐ice exchange in a changing Arctic Ocean
The changing Arctic sea‐ice cover is likely to impact the trans‐border exchange of sea ice between the exclusive economic zones (EEZs) of the Arctic nations, affecting the risk of ice‐rafted contamination. We apply the Lagrangian Ice Tracking System (LITS) to identify sea‐ice formation events and track sea ice to its melt locations. Most ice (52%) melts within 100 km of where it is formed; ca. 21% escapes from its EEZ. Thus, most contaminants will be released within an ice parcel's originating EEZ, while material carried by over 1 00,000 km2 of ice—an area larger than France and Germany combined—will be released to other nations' waters. Between the periods 1988–1999 and 2000–2014, sea‐ice formation increased by ∼17% (roughly 6 million km2 vs. 5 million km2 annually). Melting peaks earlier; freeze‐up begins later; and the central Arctic Ocean is more prominent in both formation and melt in the later period. The total area of ice transported between EEZs increased, while transit times decreased: for example, Russian ice reached melt locations in other nations' EEZs an average of 46% faster while North American ice reached destinations in Eurasian waters an average of 37% faster. Increased trans‐border exchange is mainly a result of increased speed (∼14% per decade), allowing first‐year ice to escape the summer melt front, even as the front extends further north. Increased trans‐border exchange over shorter times is bringing the EEZs of the Arctic nations closer together, which should be taken into account in policy development—including establishment of marine‐protected areas
Combined CI+MBPT calculations of energy levels and transition amplitudes in Be, Mg, Ca, and Sr
Configuration interaction (CI) calculations in atoms with two valence
electrons, carried out in the V(N-2) Hartree-Fock potential of the core, are
corrected for core-valence interactions using many-body perturbation theory
(MBPT). Two variants of the mixed CI+MBPT theory are described and applied to
obtain energy levels and transition amplitudes for Be, Mg, Ca, and Sr
Nematode and Arthropod Genomes Provide New Insights into the Evolution of Class 2 B1 GPCRs
Nematodes and arthropods are the most speciose animal groups and possess Class 2 B1 G-protein coupled receptors
(GPCRs). Existing models of invertebrate Class 2 B1 GPCR evolution are mainly centered on Caenorhabditis elegans and
Drosophila melanogaster and a few other nematode and arthropod representatives. The present study reevaluates the
evolution of metazoan Class 2 B1 GPCRs and orthologues by exploring the receptors in several nematode and arthropod
genomes and comparing them to the human receptors. Three novel receptor phylogenetic clusters were identified and
designated cluster A, cluster B and PDF-R-related cluster. Clusters A and B were identified in several nematode and
arthropod genomes but were absent from D. melanogaster and Culicidae genomes, whereas the majority of the members of
the PDF-R-related cluster were from nematodes. Cluster A receptors were nematode and arthropod-specific but shared a
conserved gene environment with human receptor loci. Cluster B members were orthologous to human GCGR, PTHR and
Secretin members with which they probably shared a common origin. PDF-R and PDF-R related clusters were present in
representatives of both nematodes and arthropods. The results of comparative analysis of GPCR evolution and diversity in
protostomes confirm previous notions that C. elegans and D. melanogaster genomes are not good representatives of
nematode and arthropod phyla. We hypothesize that at least four ancestral Class 2 B1 genes emerged early in the metazoan
radiation, which after the protostome-deuterostome split underwent distinct selective pressures that resulted in duplication
and deletion events that originated the current Class 2 B1 GPCRs in nematode and arthropod genomes.This work was supported by the Portuguese Foundation for Science and Technology (FCT) project PTDC/BIA-BCM/114395/2009, by the European
Regional Development Fund through COMPETE and FCT under the project ‘‘PEst-C/MAR/LA0015/2011.’’ RCF is in receipt of an FCT grant (SFRH/BPD/89811/2012)
and JCRC is supported by auxiliary research contract FCT Pluriannual funds attributed to CCMAR. The funders had no role in study design, data collection and
analysis, decision to publish, or preparation of the manuscript
Aging of the mammalian gastrointestinal tract: a complex organ system
Gastrointestinal disorders are a major cause of morbidity in the elderly population. The gastrointestinal tract is the most complex organ system; its diverse cells perform a range of functions essential to life, not only secretion, digestion, absorption and excretion, but also, very importantly, defence. The gastrointestinal tract acts not only as a barrier to harmful materials and pathogens but also contains the vast number of beneficial bacterial populations that make up the microbiota. Communication between the cells of the gastrointestinal tract and the central nervous and endocrine systems modifies behaviour; the organisms of the microbiota also contribute to this brain–gut–enteric microbiota axis. Age-related physiological changes in the gut are not only common, but also variable, and likely to be influenced by external factors as well as intrinsic aging of the cells involved. The cellular and molecular changes exhibited by the aging gut cells also vary. Aging intestinal smooth muscle cells exhibit a number of changes in the signalling pathways that regulate contraction. There is some evidence for age-associated degeneration of neurons and glia of the enteric nervous system, although enteric neuronal losses are likely not to be nearly as extensive as previously believed. Aging enteric neurons have been shown to exhibit a senescence-associated phenotype. Epithelial stem cells exhibit increased mitochondrial mutation in aging that affects their progeny in the mucosal epithelium. Changes to the microbiota and intestinal immune system during aging are likely to contribute to wider aging of the organism and are increasingly important areas of analysis. How changes of the different cell types of the gut during aging affect the numerous cellular interactions that are essential for normal gut functions will be important areas for future aging research
- …