Low-temperature reactions: Tunnelling in space.

International audienceChemical reactions with activation barriers generally slow to a halt in the extreme cold of dense interstellar clouds. Low-temperature experiments on the reaction of OH with methanol have now shown that below 200 K there is a major acceleration in the rate that can only be explained by enhanced quantum mechanical tunnelling through the barrier

Application of advanced (S)TEM methods for the study of nanostructured porous functional surfaces: A few working examples

Nanostructured films offer the ability of modifying surface properties, even more, when they can generate layers with controlled porosity. The lower implicit integrity of these (multi)layers when compared to their compact counterparts, hinders the attainment of electron-transparent sections of submicron thicknesses (lamellae), which becomes one of the main reason for the scarcity of studies thorough (scanning-)transmission electron microscopy ((S)TEM). Aware of this opportunity, this report provides an overview of the possibilities offered by the application of a variety of (S)TEM techniques for the study of nanostructured and porous photonic surfaces. A few working examples are presented to illustrate the type of information that can be obtained in the case of mesoporous films prepared either by at oblique angles physical processes as well as nitride nanowire arrays prepared by epitaxy methods. It will be demonstrated that this approach enables the realization of several pioneering works, which are essential to complete the characterization of such porosity-controlled coatings. Topics as diverse as the preparation of electron-transparent specimens and the advanced characterization of their structures, morphologies, interfaces and compositions are addressed thanks to the implementation of new breakthroughs in (S)TEM, which allow to obtain high-resolution imaging, spectroscopies, or tomography, at both microscopic and nanoscopic levels. Finally, establishing (S)TEM as a reference tool for the advanced structural, chemical and morphological characterization of porous nanostructured skins, will open new horizons, providing better and new insights and thus allowing the optimization of the fabrication and design of such architectures

Cognitive engagement in the problem-based learning classroom

The objective of the present study was to examine to what extent autonomy in problem-based learning (PBL) results in cognitive engagement with the topic at hand. To that end, a short self-report instrument was devised and validated. Moreover, it was examined how cognitive engagement develops as a function of the learning process and the extent to which cognitive engagement determines subsequent levels of cognitive engagement during a one-day PBL event. Data were analyzed by means of confirmatory factor analysis, repeated measures ANOVA, and path analysis. The results showed that the new measure of situational cognitive engagement is valid and reliable. Furthermore, the results revealed that students’ cognitive engagement significantly increased as a function of the learning event. Implications of these findings for PBL are discussed

La vitesse de la lumière dans l'air et dans le vide

Since 1950, the velocity of propagation of electromagnetic waves in vacuum has been taken as approximately 299 793,1 km/s, the error being less than 1 km/s. Experiments which are now in course should give a much better precision (0,01 km/s). In this paper, the methods of measurement selected are classified according to the nature of the measured velocity : direct measurements (in air, group or signal velocity ; in a vacuum, phase velocity), indirect measurements (generally, interferentiel measurements of phase velocity). In the opinion of the author, there is no point in taking a weighted mean as has been done in most of the recent reviews. The collected results may raise the question of changing the fundamental units of the systems.Depuis 1950, on admet que la vitesse de propagation des ondes électromagnétiques dans le vide est aux environs de 299 793,1 km/s, l'erreur étant inférieure à 1 km/s. Les expériences en cours doivent permettre d'atteindre des précisions bien supérieures (0,01 km/s). Dans cet article, le classement des méthodes de mesures est fait en fonction de la nature de la vitesse mesurée : mesures directes (dans l'air, vitesse de groupe ou de signal ; dans le vide, vitesse de phase), mesures indirectes (mesures généralement interférentielles, vitesse de phase). L'auteur ne pense pas qu'il y ait intérêt à faire une moyenne pondérée, comme dans la plupart des études bibliographiques récentes. L'ensemble des résultats peut poser le problème du changement des unités fondamentales des systèmes

Interféromètre dont les miroirs sont des filtres interférentiels transparents pour une même radiation σ0

This interferometer is characterised by a coeffcient of "sharpness" and contrast varying rapidly into the pass-band of the filters ; contrast and " sharpness " are low inside this pass-band, and higher outside. When the apparatus is used to scan a line near an intense parasitical line, the parasitical one decreases the contrast but not the "sharpness" of the principal one to be analysed. This allows the slits of the monochromator, to be opened more widely thus giving more light.Cet interféromètre est caractérisé par une finesse et un contraste rapidement variables dans la région spectrale de la bande de transparence ; à l'intérieur de cette bande le contraste et la finesse sont petits, à l'extérieur ils sont plus grands. Lorsque l'appareil analyse une raie accompagnée d'une raie parasite importante située dans la bande de transparence, cette raie parasite affaiblit le contraste de l'enregistrement sans altérer la finesse d'analyse de la raie principale. Dans ces conditions on peut ouvrir davantage les fentes du monochromateur et obtenir ainsi plus de lumière

Utilisation d'un interféromètre enregistreur associé à un monochromateur à prismes pour l'étude des structures de raies Raman

The relatively large distance between components of the Raman spectrum of a given substance (about some ten cm-1) is a reason for using coupled prism-monochromator — recording interferometer ; this gives an important luminosity improvement, compared to the prism-spectrometer having the same resolving power. A record of a ν1 line from GeCl 4 obtained with this device is given. There arise some difficulties, however, for lines near the incident line, or for relatively weak ones. There must be explained by superposition of incident and Raman lines. Methods are given to solve these difficulties : use of double monochromator, liquid filters, interference filters or double interferometer.La distance relativement grande des différentes raies de diffusion du spectre Raman d'un corps (de l'ordre de quelques dizaines de cm-1) est un élément favorable pour l'utilisation d'un ensemble monochromateur à prismes-interféromètre enregistreur qui permet un gain de lumière important sur l'utilisation d'un spectromètre à prisme de résolution identique. Nous donnons un enregistrement d'une raie ν 1 de GeCl4 obtenu avec ce montage. Quelques difficultés apparaissent cependant pour des raies très proches de la raie excitatrice, ou pour des raies relativement peu intenses ; on est obligé de tenir compte de la superposition de la raie excitatrice et des raies Raman. On donne des solutions à ces difficultés et on les justifie : utilisation d'un monochromateur double, de filtres à liquides, de filtres interférentiels ou d'un interféromètre double

Étude de procédés électriques de « balayage » pour des interféromètres enregistreurs

"Pressure" scanning, with pressure varying from 0 to 1 atmosphere, is not efficient for thicknesses less than 0,2 mm. " Mechanical " scanning is ordinarily used. Application of magnetostriction and reverse piezoelectric effect has been studied. The second effect is, a priori, more attractive, for it needs only very low power consumption, but large crystals of barium titanate must be prepared (available ceramics show some lack of similitarity and fidelity).Le balayage, dit « de pression », avec variation de pression de 0 à 1 atmosphère n'est pas utilisable en dessous de 0,2 mm d'épaisseur. On emploie habituellement le balayage « mécanique ». Nous avons étudié l'application de la magnétostriction et de l'effet piézoélectrique inverse. Le deuxième effet est plus séduisant, a priori, parce qu'il ne demande qu'une puissance minime, mais il nécessite la fabrication de grandes lames cristallines de titanate de baryum (les céramiques utilisées actuellement son trop peu semblables et leur fidélité est peu sûre)