Behavior of molecules and molecular ions near a field emitter

The cold emission of particles from surfaces under intense electric fields is a process which underpins a variety of applications including atom probe tomography (APT), an analytical microscopy technique with near-atomic spatial resolution. Increasingly relying on fast laser pulsing to trigger the emission, APT experiments often incorporate the detection of molecular ions emitted from the specimen, in particular from covalently or ionically bonded materials. Notably, it has been proposed that neutral molecules can also be emitted during this process. However, this remains a contentious issue. To investigate the validity of this hypothesis, a careful review of the literature is combined with the development of new methods to treat experimental APT data, the modeling of ion trajectories, and the application of density-functional theory simulations to derive molecular ion energetics. It is shown that the direct thermal emission of neutral molecules is extremely unlikely. However, neutrals can still be formed in the course of an APT experiment by dissociation of metastable molecular ions

A Photonic Atom Probe coupling 3D Atomic Scale Analysis with in situ Photoluminescence Spectroscopy

Laser enhanced field evaporation of surface atoms in Laser-assisted Atom Probe Tomography (La-APT) can simultaneously excite phtotoluminescence in semiconductor or insulating specimens. An atom probe equipped with appropriate focalization and collection optics has been coupled with an in-situ micro-Photoluminescence ({\mu}PL) bench that can be operated during APT analysis. The Photonic Atom Probe instrument we have developped operates at frequencies up to 500 kHz and is controlled by 150 fs laser pulses tunable in energy in a large spectral range (spanning from deep UV to near IR). Micro-PL spectroscopy is performed using a 320 mm focal length spectrometer equipped with a CCD camera for time-integrated and with a streak camera for time-resolved acquisitions. An exemple of application of this instrument on a multi-quantum well oxide heterostructure sample illustrates the potential of this new generation of tomographic atom probe.Comment: 22 pages, 4 figures. The following article has been accepted by the Review of Scientific Instruments. After it is published, it will be found at https://publishing.aip.org/resources/librarians/products/journals

Three-dimensional nanoscale study of Al segregation and quantum dot formation in GaAs/AlGaAs core-shell nanowires

GaAs/Al-GaAs core-shell nanowires fabricated by molecular beam epitaxy contain quantum confining structures susceptible of producing narrow photoluminescence (PL) and single photons. The nanoscale chemical mapping of these structures is analyzed in 3D by atom probe tomography (APT). The study allows us to confirm that Al atoms tend to segregate within the AlGaAs shells towards the vertices of the hexagons defining the nanowire cross section. We also find strong alloy fluctuations remaining AlGaAs shell, leading occasionally to the formation of quantum dots (QDs). The PL emission energies predicted in the framework of a 3D effective mass model for a QD analyzed by APT and the PL spectra measured on other nanowires from the same growth batch are consistent within the experimental uncertainties. (C) 2014 AIP Publishing LLC

The Shift from Local to Global Visual Processing in 6-Year-Old Children Is Associated with Grey Matter Loss

International audienceBackground: A real-world visual scene consists of local elements (e.g. trees) that are arranged coherently into a global configuration (e.g. a forest). Children show psychological evolution from a preference for local visual information to an adult-like preference for global visual information, with the transition in visual preference occurring around 6 years of age. The brain regions involved in this shift in visual preference have not been described. Methods and Results: We used voxel-based morphometry (VBM) to study children during this developmental window to investigate changes in gray matter that underlie the shift from a bias for local to global visual information. Six-year-old children were assigned to groups according to their judgment on a global/local task. The first group included children who still presented with local visual processing biases, and the second group included children who showed global visual processing biases. VBM results indicated that compared to children with local visual processing biases, children with global visual processing biases had a loss of gray matter in the right occipital and parietal visuospatial areas. Conclusions: These anatomical findings are in agreement with previous findings in children with neurodevelopmental disorders and represent the first structural identification of brain regions that allow healthy children to develop a global perception of the visual world

When What's Left Is Right: Visuomotor Transformations in an Aged Population

Background: There has been little consensus as to whether age-related visuomotor adaptation effects are readily observable. Some studies have found slower adaptation, and/or reduced overall levels. In contrast, other methodologically similar studies have found no such evidence of aging effects on visuomotor adaptation. A crucial early step in successful adaptation is the ability to perform the necessary transformation to complete the task at hand. The present study describes the use of a viewing window paradigm to examine the effects of aging in a visuomotor transformation task. Methods: Two groups of participants, a young adult control group (age range 18–33 years old, mean age = 22) and an older adult group (age range 62–74, mean age = 68) completed a viewing window task that was controlled by the user via a computer touchscreen. Four visuomotor ‘‘flip’ ’ conditions were created by varying the relationship between the participant’s movement, and the resultant on-screen movement of the viewing window: 1) No flip 2) X-Axis and Y-axis body movements resulted in the opposite direction of movement of the viewing window. In each of the 3) Flip-X and 4) Flip-Y conditions, the solitary X- or Y-axes were reversed. Response times and movement of the window were recorded. Conclusions: Older participants demonstrated impairments in performing a required visuomotor transformation, as evidenced by more complex scanning patterns and longer scanning times when compared to younger control participants. These results provide additional evidence that the mechanisms involved in visuomotor transformation are negatively affected by age

Developmental Issues in School-Based Aggression Prevention from a Social-Cognitive Perspective

Contemporary research on the development and prevention of aggressive behavior in childhood and adolescence emphasizes the importance of social-cognitive factors such as perceptual biases, problem-solving skills, and social-moral beliefs in the maintenance of aggression. Indeed, school-based social-cognitive intervention approaches have been identified as best practices by the Centers for Disease Control and Prevention. However, because child age is an important covariate of both intervention effectiveness and social-cognitive ability, school-based prevention program designers should keep in mind a number of issues identified through developmental research. In this paper, we review the social-cognitive model of aggressive behavior development as applied to prevention programming. We then discuss some of the ways in which the broader developmental research base can inform the design of aggression prevention programs.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45098/1/10935_2005_Article_5.pd

Extension de l'exploration visuelle et nombre d'éléments présents sur des stimulus, dans une tâche de différenciation perceptive

Summary The extent of visual scanning was recorded during a comparison task between drawings of houses, and the variation of the scanning in function of age and the number of windows was measured. It appeared that many children between 5 and 7 years of age did not explore the stimuli exhaustively before judging them identical. The proportion of exhaustive children increases dramatically between 5;0 and 6;6, regardless of the number of windows present. The more numerous the windows, the more windows are observed by non exhaustive children but the ratio between available information (number of windows present) and gathered information (number of windows fixated) remains constant at every age level. The authors conclude that the evolution from limited scanning to exhaustive scanning results from a modification in the decision criteria, related to the level of cognitive development. When a child is not exhaustive, the extent of his scanning seems to depend on the interest generated by the material or the task, the complexity of which is a powerful factor in eliciting interest.Résumé On a étudié les variations de l'extension de l'exploration oculaire en fonction du nombre de fenêtres présentes sur des dessins de maisons et del'âge des sujets (5 et 6;6 ans), dans une tâche de comparaison par paire avec réponse « pareil » ou « pas pareil ». Il est apparu que certains enfants seulement explorent exhaustivement les stimulus avant de répondre pareil. Le nombre des enfants exhaustifs augmente fortement entre 5 et 6;6 ans. Les enfants non exhaustifs regardent d'autant plus de fenêtres qu'il y en a plus sur les maisons, mais le rapport entre la quantité d'information prélevée (nombre de fenêtres regardées) et l'information disponible (nombre de fenêtres présentes) demeure constant à chaque âge. Les auteurs concluent que le passage d'une exploration non exhaustive à une exploration exhaustive marque un changement de règle de décision lié au niveau de développement intellectuel. Lorsqu'un enfant est non exhaustif, l'extension de son exploration paraît liée à l'intérêt éveillé par le matériel et la tâche, et entre autres par son niveau de complexité.E. Vurpillot, Castelo R., Renard Catherine. Extension de l'exploration visuelle et nombre d'éléments présents sur des stimulus, dans une tâche de différenciation perceptive. In: L'année psychologique. 1975 vol. 75, n°2. pp. 355-373