Laser-induced nonsequential double ionization at and above the recollision-excitation-tunneling threshold

We perform a detailed analysis of the recollision-excitation-tunneling (RESI) mechanism in laser-induced nonsequential double ionization (NSDI), in which the first electron, upon return, promotes a second electron to an excited state, from which it subsequently tunnels, based on the strong-field approximation. We show that the shapes of the electron momentum distributions carry information about the bound-state with which the first electron collides, the bound state to which the second electron is excited, and the type of electron-electron interaction. Furthermore, one may define a driving-field intensity threshold for the RESI physical mechanism. At the threshold, the kinetic energy of the first electron, upon return, is just sufficient to excite the second electron. We compute the distributions for helium and argon in the threshold and above-threshold intensity regime. In the latter case, we relate our findings to existing experiments. The electron-momentum distributions encountered are symmetric with respect to all quadrants of the plane spanned by the momentum components parallel to the laser-field polarization, instead of concentrating on only the second and fourth quadrants.Comment: 14 pages, 7 figure

Into the realm of social capital for adolescents: a latent profile analysis

Background Recent reports of increasing prevalence of frequent health complaints and mental health problems among adolescents call for directing more attention on determinants of adolescent health. The relationship between health and social capital has gained increased attention since the early 2000’s and research at review level confirms the importance of social capital for health outcomes, despite methodological heterogeneity. The aim of this study was to identify distinct profiles of family, school and peer social capital in a nationally representative sample of adolescents and to explore health outcomes in those profiles. Method Cross-sectional data from the Swedish Health Behaviour of School-aged Children 2013/14 was used for this study. The analytical sample consisted of 7,804 adolescents aged 11-, 13- and 15-years. Items representing sense of belonging and emotional support were assessed in three contexts; family, school and among peers. Latent profile analyses (LPA) were run to determine social capital profiles. Health outcomes included frequent health complaints and life satisfaction, while socioeconomic status and genders were included as predictors. Results The results show that five distinct profiles best represent the data for 11- and 15-year olds, while a four-profile model was optimal for 13-year olds. Some profiles were recurrent between age groups but unique profiles were also found. Health outcomes were significantly different between profiles depending on levels of social capital in the different contexts. Conclusions This study provides novel insight into how social capital co-occurs among adolescents within the contexts of family, school and peers and how this translates into differences in health outcomes. The national representativeness of the sample increases the implications of the results and contributes to meaningful insights that help explain the interactions of social capital in multiple contexts, complementing what is previously known about the relationship with adolescent health. © 2019 Ahlborg et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Teachers’ perceptions of autonomy in the tensions between a subject focus and a cross-curricular school profile: A case study of a Finnish upper secondary school

Recent research has indicated global trends of decreasing teacher autonomy and increasing teacher accountability. Standardised national tests have been identifed as one of many factors constraining teacher autonomy. Another trend infuencing teachers’ scope of action is the profling and branding of schools that compete for students. This qualitative case study concerns the general upper secondary level in Finland, the only level of education in the country with a high-stakes fnal examination—the matriculation exam. The upper secondary level is generally regarded as Finland’s most subject-focused level of education. In contrast to this subject-focused tradition, the case school for this research has developed a cross-curricular profle emphasising creativity, boundary crossing and an outward orientated approach. The study explores the teachers’ perceptions of how their autonomy is constrained in this context characterised by tensions between the cross-curricular school profle on one hand, and the subject-focused tradition and student evaluations on the other. Although one might expect these tensions to constrain teacher autonomy, the results show that the teachers, in fact, experience the cross-curricular school profle as increasing their individual autonomy. The study demonstrates that upper secondary teachers can experience extensive autonomy despite global trends of increasing teacher accountability and diminishing teacher autonomy.publishedVersio

How sensorimotor interaction shapes and supports young children's gestural communication around science

Research has demonstrated that gesture produced during conversation can provide insights into scientific thinking and can aid scientific communication in adults and school-aged children. However, to date, there has been a limited exploration into the role of gesture in supporting young children’s science communication, and how this is underpinned and shaped by their sensorimotor experiences. This study examines, identifies and conceptualises ways in which children spontaneously used gesture during their interaction-orientated discourse and how this mapped to their action experiences at a water table. Findings show how gestural communication in children under 5 years of age can convey different levels of complexity related to science thinking

Thermoelectric properties of Ba8Ga16Ge30 with TiO2 nanoinclusions

The effects on thermal and electrical properties of adding small amounts of TiO2 nanoinclusions to bulk Ba8Ga16Ge30 clathrate have been investigated. The thermal properties were analysed using the transient plane source technique and the analysis showed a significant decrease in thermal conductivity as the volume fraction of TiO2 increased from 0 vol. % to 1.2 vol. %. The introduction of TiO2 nanoparticles caused a shift in the peak value of the Seebeck coefficient towards lower temperatures. The maximum value of the Seebeck coefficient was, however, only little affected. The introduction of TiO2 nanoparticles into the bulk Ba8Ga16Ge30 resulted in an increased electrical resistivity of the sample, thus simultaneously reducing the charge carrier contribution to the thermal conductivity, partly explaining the decrease in total thermal conductivity. Due to the large increase in resistivity of the samples, ZT was only somewhat improved for the material with 0.4 vol. % TiO2 while the ZT values of the other materials were lower than for the reference Ba8Ga16Ge30 material without TiO2 nanoparticles. The combined results are consistent with a scenario where the nanoparticle introduction causes a light doping of the semiconductor matrix and an increased concentration of phonon scattering centres

Application of Tomographic Inversion in Studying Airglow in the Mesopause Region

It is pointed out that observations of periodic nightglow structures give excellent information on atmospheric gravity waves in the mesosphere and lower thermosphere. The periods, the horizontal wavelengths and the phase speeds of the waves can be determined from airglow images and, using several cameras, the approximate altitude of the luminous layer can also be determined by triangulation. In this paper the possibility of applying tomographic methods for reconstructing the airglow structures is investigated using numerical simulations. A ground-based chain of cameras is assumed, two-dimensional airglow models in the vertical plane above the chain are constructed, and simulated data are calculated by integrating the models along a great number of rays with different elevation angles for each camera. After addition of random noise, these data are then inverted to obtain reconstructions of the models. A tomographic analysis package originally designed for satellite radiotomography is used in the inversion. The package is based on a formulation of stochastic inversion which allows the input of a priori information to the solver in terms of regularization variances. The reconstruction is carried out in two stages. In the first inversion, constant regularization variances are used within a wide altitude range. The results are used in determining the approximate altitude range of the airglow structures. Then, in the second inversion, constant non-zero regularization variances are used inside this region and zero variances outside it. With this method reliable reconstructions of the models are obtained. The number of cameras as well as their separations are varied in order to find out the limitations of the method

Time-delayed nonsequential double ionization with few-cycle laser pulses: importance of the carrier-envelope phase

We perform theoretical investigations of laser-induced nonsequential double ionization with few cycle pulses, with particular emphasis on the dependence of the electron-momentum distributions on the carrier-envelope phase. We focus on the recollision-excitation with subsequent tunneling ionization (RESI) pathway, in which a released electron, upon return to its parent ion, gives part of its kinetic energy to promote a second electron to an excited state. At a subsequent time, the second electron is freed through tunneling ionization. We show that the RESI electron-momentum distributions vary dramatically with regard to the carrier-envelope phase. By performing a detailed analysis of the dynamics of the two active electrons in terms of quantum orbits, we relate the shapes and the momentum regions populated by such distributions to the dominant set of orbits along which rescattering of the first electron and ionization of the second electron occurs. These orbits can be manipulated by varying the carrier-envelope phase. This opens a wide range of possibilities for controlling correlated attosecond electron emission by an adequate pulse choice.Comment: 12 pages, 7 figures, 1 tabl