Generalized gauge-invariant formulations of the strong-field approximation

The gauge problem in the so-called strong-field approximation (SFA) describing atomic or molecular systems exposed to intense laser fields is investigated. Introducing a generalized gauge and partitioning of the Hamiltonian it is demonstrated that the S-matrix expansion obtained in the SFA depends on both gauge and partitioning in such a way that two gauges always yield the same S-matrix expansion, if the partitioning is properly chosen.Comment: 11 page

Exact Keldysh theory of strong-field ionization: residue method vs saddle-point approximation

In recent articles [Mishima et al., Phys. Rev. A, 66, 033401(2002); Chao, Phys. Rev. A, 72, 053414 (2005)] it was proposed to use the residue theorem for the exact calculation of the transition amplitude describing strong-field ionization of atomic systems within Keldysh theory. This should avoid the necessity to apply the method of steepest descent (saddle-point approximation). Comparing the results of both approaches for atomic hydrogen a difference by a factor of 2 was found for the 1s, and an even more drastic deviation for the 2s state. Thus it was concluded that the use of the saddle-point approximation is problematic. In this work the deviations are explained and it is shown that the previous conclusion is based on an unjustified neglect of an important contribution occurring in the application of the residue theorem. Furthermore, the applicability of the method of steepest descent for the ionization of Rydberg states is discussed and an improvement of the standard result is suggested that successfully removes the otherwise drastic failure for large principal quantum numbers.Comment: 7 pages, 3 figures, 1 tabl

Break-down of the single-active-electron approximation for one-photon ionization of the B 1Σu+^1\Sigma_u^+ state of H2_2 exposed to intense laser fields

Ionization, excitation, and de-excitation to the ground state is studied theoretically for the first excited singlet state B $^1\Sigma_u^+$ of H$_2$ exposed to intense laser fields with photon energies in between about 3 eV and 13 eV. A parallel orientation of a linear polarized laser and the molecular axis is considered. Within the dipole and the fixed-nuclei approximations the time-dependent Schr\"odinger equation describing the electronic motion is solved in full dimensionality and compared to simpler models. A dramatic break-down of the single-active-electron approximation is found and explained to be due to the inadequate description of the final continuum states.Comment: 9 pages, 4 figure

Poverty in the Texas borderland and lower Mississippi Delta: A comparative analysis of differences by family type

We provide a comparative analysis of county-level poverty in the two poorest regions of the United States - the Texas Borderland and the Lower Mississippi Delta - with a special focus on differences by family type. Our results reveal important regional variation in both the prevalence of poverty and the composition of the poor population across major family types. Using OLS regression models of family type-specific poverty we demonstrate three key findings: 1) There are no significant regional differences in poverty levels by family type between the Borderland and the Delta, net of important structural factors that characterize the regions; 2) with the exception of the employment rate, the structural factors associated with poverty among married couple and single female-headed families are quite different; and 3) paradoxically, areas in the Borderland with high in-migration are especially likely to suffer from high married-couple family poverty. Our results suggest the need for regionally targeted and demographically tailored anti-poverty policies.family structure, Mississippi Delta, persistent poverty, poverty, regional poverty, Texas Borderland

Differential jumping performance in newly metamorphosed Blanchard\u27s cricket frogs, Acris blanchardi (Anura: Hylidae), from fish- and invertebrate-dominated ponds

Organisms that adopt phenotypically plastic anti-predator strategies during larval stages may incur fitness costs later in development. These costs are typically difficult to define in many species. The difficulty of identifying ecological trade-offs may result from experimental comparisons that do not adequately mirror naturally occurring predator-prey relatioships. To examine this, we captured 61 newly metamorphosed Acris blanchardi from ponds dominated either by fish or invertebrate predators. These predators are known to induce alternate phenotypic anti-predator responses at the larval stage. We use jumping performance as a measure of post-metamorphic fitness, and compared the morphological traits and jumping ability of frogs from ponds with and without fish. Frogs from fishless ponds can jump significantly farther than frogs from fish-dominated ponds. Morphological measurements indicated that overall frog size determined differences in jumping ability, with hind-leg length primarily explaining jumping performance. Differences in hind-leg length positively correlated with overall body size, which differed between the two types of ponds and did not result from allometric growth between them. Differences in frog sizes presumably are related to differences in developmental anti-predator morphologies in larvae

Effects of urbanization on the occurrence of Batrachochytrium dendrobatidis: do urban environments provide refuge from the amphibian chytrid fungus?

Batrachochytrium dendrobatidis (Bd) is a widespread pathogenic fungus that is known to cause the disease, chytridiomycosis, which can be lethal to many amphibians. We compared occurrence rates on spring peepers (Pseudacris crucifer) in urban and forested breeding sites in eastern Texas, USA. All study sites were at approximately the same latitude and altitude, and samples were collected at the same time of year to isolate differences in Bd infection rates between habitat types. We found significant differences (p\u3c0.001) in the occurrence of Bd between habitats; with dramatically lower rates of occurrence at urban sites (19.5 %), compared to forested sites (62.9 %). The exact reason for the observed differences in the occurrence of Bd is not known, however, we suspect that warmer temperatures or lower population densities and lower species richness at urban sites all could play a role in our results. Our findings suggest that urban environments may provide a refuge for some amphibians from the pathogen

Spatio-temporal variability of bovine tuberculosis eradication in Spain (2006-2011)

A. Garcia-Saenz et al.In this study we analyzed the space–time variation of the risk of bovine tuberculosis (bTB) in cattle between 2006 and 2011. The results indicated that at country level, there were no significant temporal changes between years, but, at county level bTB evolution was more heterogeneous. In some counties, between some years, the prevalence and the incidence of the disease was higher as compared to the global rate in the rest of the counties of Spain. The analysis of potential risk factors indicated that both, a large number of movements from counties with high incidence (>1%), and presence of bullfighting cattle herds increased bTB risk. Red deer abundance, number of goats and number of mixed cattle-goat farms were not significantly associated with the prevalence/incidence of bTB.This research was supported by a Grant from the Ministerio de Ciencia e Innovación of Spain (AGL-2010-21098). PhD studies of Garcia-Saenz A, are funded by a FPI Grant from Ministerio de Ciencia e Innovación of Spain (BES-2011-043628) and PhD studies of Guta S., are funded by Agencia Española de Cooperación Internacional para el Desarrollo (AECID). Acevedo P., was supported by a Beatriu de Pinós fellowship funded by ‘Comissionat per a Universitats i Recerca del Departament d’Innovació, Universitats i Empresa’ of the Generalitat de Catalunya, and the COFUND Programme – Marie Curie Actions under 7th Framework Programme of the European Community. He is currently funded from the SFRH/BPD/90320/2012 post-doctoral Grant by Portuguese Fundação para a Ciência e a Tecnologia (FCT) and European Social Fund.Peer Reviewe

Late Life Depressive Symptoms and Cognitive Function among Older Mexican Adults: The Past and the Present

Objective: To evaluate associations between depression and individual cognitive domains and how changes in depressive symptoms relate to cognition three years later in the context of Mexico, a developing country experiencing rapid aging. Method: Data comes from waves 3 (2012) and 4 (2015) of the Mexican Health and Aging Study (n=12,898, age 50+). Depression is ascertained using a modified Center for Epidemiologic Studies – Depression Scale. Cognition is assessed using verbal learning, verbal memory, visual scanning, verbal fluency, visuospatial ability, visual memory, and orientation tasks. Depressive symptoms and cognitive functioning were both measured in 2012 and 2015. Scores across cognitive domains are modeled using ordinary least squares regression, adjusting for demographic, health, and economic covariates. Results: When depression and cognition were measured concurrently in 2015, depression exhibited associations with all cognitive domains. When considering a respondent’s history of depression, individuals who had elevated depressive symptoms in 2012 and recovered by 2015 continued to exhibit poorer cognitive function in 2015 in verbal learning, verbal memory, visual scanning, and verbal fluency tasks compared to individuals who were neither depressed in 2012 nor 2015. Conclusions: Depression was associated with cognition across cognitive domains among older Mexican adults. Despite improvements in depressive symptomatology, formerly depressed respondents continued to perform worse than their counterparts without a history of depression on several cognitive tasks. In addition to current mental health status, researchers should consider an individual’s history of depression when assessing the cognitive functioning of older adults