Momentum space tomographic imaging of photoelectrons

We apply tomography, a general method for reconstructing 3-D distributions from multiple projections, to reconstruct the momentum distribution of electrons produced via strong field photoionization. The projections are obtained by rotating the electron distribution via the polarization of the ionizing laser beam and recording a momentum spectrum at each angle with a 2-D velocity map imaging spectrometer. For linearly polarized light the tomographic reconstruction agrees with the distribution obtained using an Abel inversion. Electron tomography, which can be applied to any polarization, will simplify the technology of electron imaging. The method can be directly generalized to other charged particles.Comment: Accepted by J. Phys.

Analyse de sensibilité paramétrique d'un modèle simulant le transport de pesticide dans le sol

Afin de protéger l'eau souterraine des pesticides épandus à la surface des sols agricoles, on doit être capable de prédire et d'évaluer a priori les risques de contamination. La modélisation mathématique, qui est basée sur la représentation des processus, s'avère être un outil à privilégier pour une telle prédiction. Cependant, la fiabilité des résultats de ces modèles est fonction de la précision et de la représentativité des différents paramètres d'entrée. A l'aide d'une analyse de sensibilité, il est possible d'évaluer l'impact de la variabilité de ces paramètres sur les résultats de la modélisation. Une étude de sensibilité menée avec le modèle Pesticide Rosit Zone Model (PRZM) a permis d'identifier les paramètres physiques d'entrée dont la variation apporte le plus de changements au niveau des principaux résultats, c'est-à-dire les paramètres d'entrée auxquels le modèle est le plus sensible. L'utilisation du coefficient de sensibilité relatif s'avère être à cet égard un outil de comparaison fort efficace dans le cadre d'une telle étude. Les paramètres d'apport en eau (précipitations) et en pesticide (taux d'application) sont ceux dont les variations provoquent le plus d'impact au niveau des résultais de la simulation. Egalement, les résultats des simulations sont aussi sensibles aux variations de la capacité au champ et de la densité du sol. La température, le point de flétrissement, ta profondeur d'évaporation et la dispersion sont des paramètres auxquels la modélisation est peu sensible.The ever increasing number of cases of groundwater contamination by pesticides has recently given rise to numerous experimental studies on the tale of these compounds within the soil-water system. In parallel with these experimental studies, the quick and intensive development of numerous simulation models bas emphasized the importance of the various factors and processes controlling the transport of pesticides in the unsaturated zone. The calibration and the validation of such deterministic models, which are especially used in groundwater management, requires the evaluation of several parameters related to the nature of the pesticide as well as to the pedologic and hydrogeological conditions of a given site. The reliability of results predicted by those models is mainly a function of the precision and the representativeness in the evaluation of those parameters at a specific site.The aim of this study was to evaluate the sensitivity of a simulation modes of pesticide movement in the unsaturated zone with regards to the variation in soil physical properties. More particularly, a sensitivity analysis was performed in order to determine the importance of the variation in these parameters with respect to simulation results obtained from the Pesticide Root Zone Model (PRZM). The spatial variability of the composition and structure of the soil, which comes from the site pedogenesis, is the main cause of the variability of the soil water distribution and of the soil transient properties. The sensitivity analysis of a model with respect to the variation in these parameters allows the evaluation of the impact of their representativeness on the model output results. Such an analysis thus allows the determination of an acceptable level of precision (or error) for which an increase of precision in the evaluation of a parameter does not anymore correspond to a significant gain in the representativeness of the model results. It also permits the estimation of the impact of a potential variation of a parameter on the prediction of pesticide transport in the unsaturated zone.The deterministic PRZM modal used in this study has been developed by the US-EPA and devoted to pesticide application on agricultural sites. This modal evaluates the pesticide leaching towards groundwater with respect to the type of culture and pesticide used, the climatic conditions, the soil characteristics and some agricultural practices. Output results from the model can he expressed in concentrations or masses of pesticide, in fluxes or cumulated quantities. Simulations were performed using characteristics and data of the Portneuf region (Quebec). This area of potato farming was until recently one of the Quebec most important sites for aldicarb application before the recommendation of its non use. Cultivated surfaces, which are rather homogeneous, consist of medium-size sand. Simulations were performed river a ten year period (1974 to 1984), implying one application of pesticide a1 the seed-time, and a trop for each year.The parameters of water and pesticide inputs (rain and application rates) are those that induce the higher impact on the simulation results. The simulation results were also influenced by the variation of the field capacity and of the soil bulk density. Temperature, wilting point, maximum evaporation depth and dispersion are the input parameters for which the PRZM model is the less sensitive. The simulation results of pesticide transport, which ultimately consist in predicting the groundwater contamination, are very sensitive to the variations of some physical parameters for which the precision and the representativeness in the measured values are thus very important for the reliability of the results. Considering the spatial variability of a site characteristics, the representativeness of these results is very uncertain if a limited number of data is used in order to determine the mean value. A particular attention has to be focused on the parameters that induce the higher sensitivity of the model. Finally, such a sensitivity analysis shows that a stochastic approach in modelling the solute transport through soil can be a good alternative to take into account the variability of parameters encountered in field situations

Characterization of the Electrical Behaviour of Thin Dielectric Films at Nanoscale using Methods Derived from Atomic Force Microscopy: Application to Plasma Deposited AgNPs-Based Nanocomposites

International audienceRecent advances in the development of micro-and nano-devices call for applications of thin nanocomposite dielectric films (thickness less than few tens of nanometers) with tuneable electrical properties. For optimization purposes, their behaviour under electrical stress needs to be probed at relevant scale, i.e. nanoscale. To that end electrical modes derived from Atomic Force Microscopy (AFM) appear the best methods due to their nanoscale resolution and non-destructive nature which permits in-situ characterization. The potentialities of electrical modes derived from AFM are presented in this work. The samples under study consist of plasma processed thin dielectric silica layers with embedded silver nanoparticles (AgNPs). Charge injection at local scale, performed by using AFM tip, is investigated by Kelvin Probe Force Microscopy (KPFM). Modulation of the local permittivity induced by the presence of AgNPs is assessed by Electrostatic Force Microscopy (EFM)

Alignment dependent enhancement of the photo-electron cutoff for multi-photon ionization of molecules

The multiphoton ionization rate of molecules depends on the alignment of the molecular axis with respect to the ionizing laser polarization. By studying molecular frame photo-electron angular distributions from N$_2$, O$_2$ and benzene, we illustrate how the angle-dependent ionization rate affects the photo-electron cutoff energy. We find alignment can enhance the high energy cutoff of the photo-electron spectrum when probing along a nodal plane or when ionization is otherwise suppressed. This is supported by calculations using a tunneling model with a single ion state.Comment: 4 pages, 4 figure

Methodology for extraction of space charge density profiles at nanoscale from Kelvin probe force microscopy measurements

International audienceTo understand the physical phenomena occurring at metal/dielectric interfaces, determination of the charge density profile at nanoscale is crucial. To deal with this issue, charges were injected applying a DC voltage on lateral Al-electrodes embedded in a SiN x thin dielectric layer. The surface potential induced by the injected charges was probed by Kelvin probe force microscopy (KPFM). It was found that the KPFM frequency mode is a better adapted method to probe accurately the charge profile. To extract the charge density profile from the surface potential two numerical approaches based on the solution to Poisson's equation for electrostatics were investigated: the second derivative model method, already reported in the literature, and a new 2D method based on the finite element method (FEM). Results highlight that the FEM is more robust to noise or artifacts in the case of a non-flat initial surface potential. Moreover, according to theoretical study the FEM appears to be a good candidate for determining charge density in dielectric films with thicknesses in the range from 10 nm to 10 μm. By applying this method, the charge density profile was determined at nanoscale, highlighting that the charge cloud remains close to the interface

Handling Geometric Features in Nanoscale Characterization of Charge Injection and Transport in thin Dielectric Films

International audienceDue to miniaturization and attractiveness of nanosized and/or nanostructured dielectric layers, characterization at the local scale of charge injection and transport phenomena comes to the fore. To that end the electric modes derived from Atomic Force Microscopy (AFM) are more and more frequently used. In this study, the influence of AFM tip-plane system configuration on the electric field distribution is investigated for homogeneous and heterogeneous (nanostructured) thin dielectric layers. The experimental and computing results reveal that the radial component of the electric field conveys the charge lateral spreading whereas the axial component of the electric field governs the amount of injected charges. The electric field distribution is slightly influenced by the heterogeneity of the material. Moreover, the interpretation of the current measurements requires consideration of the entire electric field distribution and not only the computed field at the contact point

Simulation of cellular irradiation with the CENBG microbeam line using GEANT4

Light-ion microbeams provide a unique opportunity to irradiate biological samples at the cellular level and to investigate radiobiological effects at low doses of high LET ionising radiation. Since 1998 a single-ion irradiation facility has been developed on the focused horizontal microbeam line of the CENBG 3.5 MV Van de Graaff accelerator. This setup delivers in air single protons and alpha particles of a few MeV onto cultured cells, with a spatial resolution of a few microns, allowing subcellular targeting. In this paper, we present results from the use of the GEANT4 toolkit to simulate cellular irradiation with the CENBG microbeam line, from the entrance to the microprobe up to the cellular medium.Comment: 6 pages, 8 figures, presented at the 2003 IEEE-NSS conference, Portland, OR, USA, October 20-24, 200

Generation of broad XUV continuous high harmonic spectra and isolated attosecond pulses with intense mid-infrared lasers

We present experimental results showing the appearance of a near-continuum in the high-order harmonic generation (HHG) spectra of atomic and molecular species as the driving laser intensity of an infrared pulse increases. Detailed macroscopic simulations reveal that these near-continuum spectra are capable of producing IAPs in the far field if a proper spatial filter is applied. Further, our simulations show that the near-continuum spectra and the IAPs are a product of strong temporal and spatial reshaping (blue shift and defocusing) of the driving field. This offers a possibility of producing IAPs with a broad range of photon energy, including plateau harmonics, by mid-IR laser pulses even without carrier-envelope phase stabilization.Comment: 7 pages, 5 figures, submitted to J.Phys. B (Oct 2011