Jean-Paul Mosnier

In-package atmospheric pressure cold plasma treatment of cherry tomatoes

Cold plasma is increasingly under research for decontamination of foods, especially fresh fruits and vegetables. The effect of cold plasma on food quality, however, remains under researched. This study investigates the effects of cold plasma generated within a sealed package from a dielectric barrier discharge on the physical quality parameters and respiration rates of cherry tomatoes. Respiration rates and weight loss were monitored continuously, while other parameters are reported at the end of storage period. Differences among weight loss, pH and firmness for control and treated cherry tomatoes were insignificant towards the end of storage life. Changes in respiration rates and colour of tomatoes were recorded as a function of treatment, which were not drastic. The results implicate that cold plasma could be employed as a means for decontamination of cherry tomatoes while retaining product quality.

Correlation of Raman and X-ray diffraction measurements of annealed pulsed laser deposited ZnO thin films

Abstract Raman spectroscopy, X-ray diffractometry and atomic force microscopy have been used to characterise ZnO thin films grown by pulsed laser deposition as a function of the post-growth annealing temperature. The results show substantial enhancement and broadening of certain Raman features which correlate excellently with the change in width of the X-ray diffraction peaks. The 570 cm−1 Raman feature showed pronounced asymmetry and enhanced intensity in the unannealed sample. An increase in grain size observed after subsequent annealing produced a substantial reduction in both the asymmetry and intensity of this peak. Our experimental data suggest that electric fields, due to charge trapping at grain boundaries, in conjunction with localised and surface phonon modes are the cause of the intensity enhancement and asymmetry of this feature.

X-UV Absorption Spectroscopy with Laser-Produced Plasmas: A Review

Exteme-ultraviolet (X-UV) absorption spectroscopy has gained impetus in recent years, spurred on by a growing interest in the physics of core excited atomic, molecular and solid species. Progress in this field has been helped in no small measure by developments in X-UV light sources. One such source is the laser-produced plasma which, by suitable choice of target material, can be used to produce a clean, line free continuum virtually throughout the X-UV and V-UV spectral regions from 40-2000 ?. We briefly address here the issues surrounding the origin and characteristics of laser-plasma continuum light sources. In addition we review progress to date on the application of these sources to the study of the X-UV photoabsorption spectra of atoms, ions and molecules.

Emission characteristics and dynamics of the stagnation layer in colliding laser produced plasmas

The expansion dynamics of ion and neutral species in laterally colliding laser produced aluminum plasmas have been investigated using time and space resolved optical emission spectroscopies and spectrally and angularly resolved fast imaging. The emission results highlight a difference in neutral atom and ion distributions in the stagnation layer where, at a time delay of 80 ns, the neutral atoms are localized in the vicinity of the target surface (<1 mm from the target surface) while singly and doubly charged ions lie predominantly at larger distances, <1.5 and <2 mm, respectively. The imaging results show that the ions were found to form a well defined, but compressed, stagnation layer at the collision front between the two seed plasmas at early times (Δt<80 ns). On the other hand, the excited neutrals were observed to form a V-shaped emission feature at the outer regions of the collision front with enhanced neutral emission in the less dense, cooler regions of the stagnation layer.

Study of photoluminescence at 3.310 and 3.368 eV in GaN/sapphire(0001) and GaN/GaAs(001) grown by liquid-target pulsed-laser deposition

Epitaxial GaN films of thickness ∼1 μm have been grown on sapphire(0001) and GaAs(001) substrates using the liquid-target pulsed-laser-deposition technique in a 5 Torr nitrogen atmosphere. Detailed x-ray diffraction and photoluminescence studies were carried out for both types of samples. Significantly enhanced low-temperature photoluminescence emissions at 3.368 eV (I3) and 3.310 eV (I4) were observed for the material deposited on a GaAs(001) substrate at ∼800 °C. We propose a model to explain the emission mechanism for both lines in which the electrons and holes are confined in cubic inclusions within the hexagonal material, analogously to a type-I quantum well.

Diagnostic of an expanding laser-produced lithium plasma using ICCD frame photography and shadowgraphy

Abstract The expansion of a laser-produced lithium plasma is characterized using two different high-speed imaging techniques. Firstly, a sequence of frames of the luminous plume is recorded using an interference filter/gated ICCD camera combination. Expansion velocities are estimated from these images. The conditions, in which the radial distributions of emitters could be recovered using Abel inversion, are discussed. Secondly, shadowgraphs obtained with a synchronized tunable dye laser light source are recorded at different probe wavelengths in the vicinity of the Li 0 670.7-nm resonance. The fringe patterns observed in these images are interpreted in terms of strong refractive index gradients within the plasma. The effect of anomalous dispersion is observed and strongly modifies the appearance of the shadowgraphs.

Extreme-ultraviolet studies with laser-produced plasmas

A new multilaser multichannel spectrometer system has been developed that allows a wide range of investigations of the interaction of extreme-ultraviolet (EUV) photons with matter in the form of free atoms or ions, e.g., in gases or plasmas or bound as in solids. The EUV photons are generated by a laser-produced plasma. Applications to the study of photoabsorption by thin foils, gases, and ground- and excited-state atoms and ions are described. The design and performance of a collimated, quasi-monochromatic, intense source of EUV radiation based on the combination of a laser-produced plasma with a EUV multilayer mirror is also reported.

Effects of excitonic diffusion on stimulated emission in nanocrystalline ZnO

We present optically pumped emission data for ZnO, showing that high excitation effects and stimulated emission/lasing are observed in nanocrystalline ZnO thin films at room temperature, although such effects are not seen in bulk material of better optical quality. A simple model of exciton density profiles is developed which explains our results and those of other authors. Inhibition of exciton diffusion in nanocrystalline samples compared to bulk significantly increases exciton densities in the former, leading—via the nonlinear dependence of emission in the exciton bands on the pump intensity—to large increases in emission and to stimulated emission.

Expansion dynamics and equilibrium conditions in a laser ablation plume of lithium: Modeling and experiment

The gas dynamics and atomic kinetics of a laser ablation plume of lithium, expanding adiabatically in vacuum, are included in a numerical model, using isothermal and isentropic self-similar analytical solutions and steady-state collisional radiative equations, respectively. Measurements of plume expansion dynamics using ultrafast imaging for various laser wavelengths (266–1064nm), fluences (2–6.5Jcm−2), and spot sizes (50–1000μm) are performed to provide input parameters for the model and, thereby, study the influence of laser spot size, wavelength, and fluence, respectively, on both the plume expansion dynamics and atomic kinetics. Target recoil pressure, which clearly affects plume dynamics, is included in the model. The effects of laser wavelength and spot size on plume dynamics are discussed in terms of plasma absorption of laser light. A transition from isothermal to isentropic behavior for spot sizes greater than 50μm is clearly evidenced. Equilibrium conditions are found to exist only up to 300ns a...

NEW HIGH-RESOLUTION MEASUREMENTS OF DOUBLY EXCITED STATES OF LI+

We present new results for the doubly excited states Li+(nln´l´) of singly ionized lithium which arise from the measurements of the Auger decay of these states to Li++ ground state ions. Primary excitation of the doubly excited states is achieved by photon impact on neutral lithium atoms and therefore the experiment probes directly the double-shakeup photoionization process h+LiLi+**+e-. Detection of the individual Li+ Auger decay lines provides information on the energies and widths of many doubly excited levels of the ion, including the lowest lying (3l,3l´) state. Classifications are made using recent theoretical calculations.