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Absolute cross section for Si2+(3s21Sâ3s3p1P) electron-impact excitation
We have measured the absolute cross section for electron-impact excitation (EIE) of Si2+(3s21Sâ3s3p1P) from energies below threshold to 11 eV above. A beams modulation technique with inclined electron and ion beams was used. Radiation at 120.7 nm from the excited ions was detected using an absolutely calibrated optical system. The fractional population of the Si2+(3s3p3Po) metastable state in the incident ion beam was determined to be 0.210±0.018 (1.65Ï). The data have been corrected for contributions to the signal from radiative decay following excitation from the metastable state to 3s3p1P and 3p23P, and excitation from the ground state to levels above the 3s3p1P level. The experimental 0.56±0.08-eV energy spread allowed us to resolve complex resonance structure throughout the studied energy range. At the reported ±14% total experimental uncertainty level (1.65Ï), the measured structure and absolute scale of the cross section are in good agreement with 12-state close-coupling R-matrix calculations
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Reevaluation of experiments and new theoretical calculations for electron-impact excitation of C3+
Experimental absolute-rate coefficients for electron-impact excitation of C3+â(2s2S1/2â2p2P1/2,3/2) near threshold [D. W. Savin, L. D. Gardner, D. B. Reisenfeld, A. R. Young, and J. L. Kohl, Phys. Rev. A 51, 2162 (1995)] have been reanalyzed to include a more accurate determination of optical efficiency and revised radiometric uncertainties which reduce the total systematic uncertainty of the results. Also, new R matrix with pseudostates (RMPS) calculations for this transition near threshold are presented. Comparison of the RMPS results to those of simpler close-coupling calculations indicates the importance of accounting for target continuum effects. The reanalyzed results of Savin et al. are in excellent agreement with the RMPS calculations; comparisons are also made to other measurements of this excitation. Agreement with the RMPS results is better for fluorescence technique measurements than for electron-energy-loss measurements
Noncollinear magnetic ordering in small Chromium Clusters
We investigate noncollinear effects in antiferromagnetically coupled clusters
using the general, rotationally invariant form of local spin-density theory.
The coupling to the electronic degrees of freedom is treated with relativistic
non-local pseudopotentials and the ionic structure is optimized by Monte-Carlo
techniques. We find that small chromium clusters (N \le 13) strongly favor
noncollinear configurations of their local magnetic moments due to frustration.
This effect is associated with a significantly lower total magnetization of the
noncollinear ground states, ameliorating the disagreement between Stern-Gerlach
measurements and previous collinear calculations for Cr_{12} and Cr_{13}. Our
results further suggest that the trend to noncollinear configurations might be
a feature common to most antiferromagnetic clusters.Comment: 9 pages, RevTeX plus .eps/.ps figure
PrevalĂȘncia de atividade fĂsica em adolescentes e fatores associados
OBJECTIVE: To estimate the prevalence and identify correlates of physical activity among adolescents. METHODS: Cross-sectional study nested within a cohort of 4,325 subjects from the city of Pelotas, Southern Brazil, aged 14-15 years in 2008. Physical activity was analyzed using three different approaches: (1) prevalence of any leisure-time physical activity; (2) prevalence of any active commuting to school; and (3) prevalence of engaging in at least 300 minutes per week of both (1) and (2) combined. Independent variables included sociodemographic, behavioral, social, and biological characteristics, and number of different leisure-time physical activites practiced. Statistical analyses were carried out using Poisson regression. RESULTS: The proportion of adolescents involved in any type of leisure-time physical activity was 75.6%, while 73.4% displayed some form of active commuting to school. Prevalence of total physical activity score (>; 300 min/week) was 48.2%, being greater among boys (62.6%) than among girls (34.5%). Furthermore, prevalence increased along with the number of physical activity modalities practiced (pOBJETIVO: Estimar la prevalencia de actividad fĂsica entre adolescentes y identificar los factores asociados. MĂTODOS: Estudio transversal anidado en una cohorte con 4.325 individuos de 14-15 años en Pelota, Sur de Brasil, en 2008. La actividad fĂsica fue analizada por medio de tres diferentes abordajes: 1) prevalencia de alguna actividad fĂsica de ocio; 2) prevalencia de algĂșn traslado activo para la escuela; 3) prevalencia de compromiso de por lo menos 300 minutos por semana en la combinaciĂłn de ambos (1 y 2). Variables independientes incluyeron caracterĂsticas demogrĂĄficas, socioeconĂłmicas, comportamentales, sociales y biolĂłgicas y nĂșmero de actividades fĂsicas practicadas en el tiempo de ocio. Los anĂĄlisis estadĂsticos fueron hechos por la regresiĂłn de Poisson. RESULTADOS: La proporciĂłn de adolescentes envueltos en alguna actividad fĂsica de ocio fue 75,6% mientras que 73,4% presentaron alguna forma de traslado activo para la escuela. La prevalencia de actividad fĂsica total (escore Âł 300 min/sem) fue 48,2% siendo mayor para los niños (62,6%) que para las niñas (34,5%). AdemĂĄs, la prevalencia aumentĂł de acuerdo con el nĂșmero de actividades fĂsicas practicadas (pOBJETIVO: Estimar a prevalĂȘncia de atividade fĂsica entre adolescentes e identificar fatores associados. MĂTODOS: Estudo transversal aninhado em uma coorte com 4.325 indivĂduos de 14-15 anos em Pelotas, RS, em 2008. A atividade fĂsica foi analisada por meio de trĂȘs diferentes abordagens: 1) prevalĂȘncia de alguma atividade fĂsica de lazer; 2) prevalĂȘncia de algum deslocamento ativo para a escola; 3) prevalĂȘncia de engajamento em pelo menos 300 minutos por semana da combinação de ambos (1 e 2). VariĂĄveis independentes incluĂram caracterĂsticas demogrĂĄficas, socioeconĂŽmicas, comportamentais, sociais e biolĂłgicas e nĂșmero de atividades fĂsicas praticadas no tempo de lazer. As anĂĄlises estatĂsticas foram feitas pela regressĂŁo de Poisson. RESULTADOS: A proporção de adolescentes envolvidos em alguma atividade fĂsica de lazer foi 75,6%, enquanto 73,4% apresentou alguma forma de deslocamento ativo para a escola. A prevalĂȘncia de atividade fĂsica total (escore >; 300 min/sem) foi 48,2%, sendo maior para os meninos (62,6%) do que para as meninas (34,5%). AlĂ©m disso, a prevalĂȘncia aumentou de acordo com o nĂșmero de atividades fĂsicas praticadas (p < 0,001). Os fatores associados Ă maior prĂĄtica de atividade fĂsica (lazer + deslocamento) nos nĂveis recomendados foram: cor de pele nĂŁo-branca, ter repetido de ano na escola e jogar videogame. As variĂĄveis menor nĂvel socioeconĂŽmico, maior tempo de uso de computador e atividade fĂsica dos pais estiveram associadas ao desfecho apenas entre as meninas. CONCLUSĂES: Menos da metade dos adolescentes atingiu as recomendaçÔes para a prĂĄtica de atividade fĂsica, e essa proporção tende a diminuir entre os sujeitos de maior nĂvel socioeconĂŽmico. Os fatores associados diferiram entre lazer e deslocamento. Engajamento em uma ampla variedade de atividades fĂsicas deve ser encorajado desde a infĂąncia
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