Jacques Derouard

CF2 KINETICS AND RELATED MECHANISMS IN THE PRESENCE OF POLYMERS IN FLUOROCARBON PLASMAS

Laser-induced fluorescence was used to characterize the fluorocarbon plasma that was used to modify the wettability of hexatriacontane (HTC), a polymer model. The plasma volume downstream from a microwave plasma source was examined by monitoring the CF2 concentration as well as the CF2 decay rate (in the afterglow of a pulsed discharge) during treatment of the polymer surface (in 20–100 mTorr CF4). In parallel, the behavior of F atoms was monitored by means of actinometric optical emission. Pulsed modulation of the discharge allowed the effects of variations in the loss and production rates for CF2 to be distinguished, in the presence of hexatriacontane. Our observations are consistent with enhanced production of CF2, possibly by an ion-assisted surface mechanism, in the presence of HTC.

In vivo 129Xe NMR in rat brain during intra-arterial injection of hyperpolarized 129Xe dissolved in a lipid emulsion

Hyperpolarized 129Xe was dissolved in a lipid emulsion and administered to anaesthetized rats by manual injections into the carotid (approximately 1-1.5 mL in a maximum time of 30 s). During injection, 129Xe NMR brain spectra at 2.35 T were recorded over 51 s, with a repetition time of 253 ms. Two peaks assigned to dissolved 129Xe were observed (the larger at 194 +/- 1 ppm assigned to intravascular xenon and the smaller at 199 +/- 1 ppm to xenon dissolved in the brain tissue). Their kinetics revealed a rapid intensity increase, followed by a plateau (approximately 15 s duration) and then a decrease over 5 s. This behaviour was attributed to combined influences of the T1 relaxation of the tracer, of radiofrequency sampling, and of the tracer perfusion rate in rat brain. Similar kinetics were observed in experiments carried out on a simple micro-vessel phantom. An identical experimental set-up was used to acquire a series of 2D projection 129Xe images on the phantom and the rat brain.

Spatial fluorescence cross-correlation spectroscopy

We present an alternative method for diffusion measurements of fluorescent species in solution by use of confocal microscopy and fluorescence correlation spectroscopy techniques. It consists of making a time and spatial dual correlation in which one detects the fluorescence signals from two nearby separate confocal volumes and cross correlates them. To improve the spatial discrimination between the two confocal volumes we propose filtering of fluorescence photocounts by rejecting the fluorescence background, which corresponds to particles located far from the center of the detection volumes.

Absolute concentration measurements by pulsed laser-induced fluorescence in low-pressure gases: allowing for saturation effects

Abstract We present a simple method to account for the inevitable partial saturation of the excitation process in pulsed laser-induced fluorescence. The method allows the absolute concentration of a transitory species in a low-pressure gas to be determined by comparing the signal with that from a calibrating (stable) species of known concentration: the two species are excited with different laser powers, in the inverse ratio of their transition strenghts. The validity of the method is justified both theoretically and experimentally, and is applied to the measurement of CF radicals in a fluorocarbon discharge.

Photobleaching, mobility, and compartmentalisation: inferences in fluorescence correlation spectroscopy.

In living cells the transport and diffusion of molecules is constrained by compartments of various sizes. This paper is an attempt to show that the size of these compartments can in principle be estimated experimentally from Fluorescence Correlation Spectroscopy (FCS) combined with the measurement of the photobleaching rate. In this work, confocal fluorescence microscopy experiments have been carried out on giant unilamellar vesicles, a system that mimics cellular compartmentalisation. We have developed numerical and analytical models to describe the fluorescence decay due to photobleaching in this geometry, which has enabled us to point out two regimes depending on the value of the parameter pB = σ P/D (where σB is the photobleaching cross section of the dye, D its diffusion constant, and P the laser power (in photon/s)). In particular, when pB ≪ 1 (i.e. in the fast diffusion regime), the photobleaching rate is independent of the diffusion constant and scales like σBP/R2, in agreement with the experimental results. On the other hand, the standard diffusion models used to analyse the FCS data do not take into account the effects of the fluorescence decay on the autocorrelation curve. We show here how to correct the raw data for these drawbacks.

ELECTRIC FIELD MEASUREMENTS IN DISCHARGES BY 2+1 PHOTON LASER STARK SPECTROSCOPY OF ATOMIC HYDROGEN

We have developed a new method for measuring electric field magnitude using 2+1 photon laser‐induced fluorescence of ground‐state hydrogen atoms. The atoms are excited to the n=2 level by two 243‐nm photons, then further excited to the n=6 level by one photon at 410 nm and detected by cascaded Hα fluorescence. The electric field magnitude is deduced from the Stark splitting of the n=6 level. The method was calibrated using H atoms from a distant source flowing through a region of known electric field. Results are presented for field profiles in dc discharges in 0.3 and 1 Torr pure H2.

Mass Spectrometric Detection of F Atoms and CFx Radicals in CF 4 Plasmas

Threshold ionization mass spectrometry method has been applied to the study of the neutral radicals present in a microwave generated CF 4 plasma. Calibration of the mass spectrometry signals by comparison with the ion signals produced by the dissociative ionization of CF 4 allows the estimation of the absolute concentration of the radicals in a 15 mTorr plasma, with an emphasis on the cases of atomic fluorine and CF x (x = 1-3) radicals. The effect, on the gas phase composition, of the introduction of a silicon sample in the plasma chamber is also investigated. Among these effects, the introduction of silicon leads to decrease the concentration of the F and H atoms as well as HF molecules, and to increase the concentration of the SiF 3 and CF x radicals.

Treatment of organic polymer surfaces by CF4 plasmas: Etching by fluorine atoms and influence of vacuum ultraviolet radiation

Previous works have shown that atomic fluorine is the main etching agent of organic polymer surfaces subjected to fluorinated plasma treatments. In this work the etching probability per F atom impinging on a polymer-like material (hexatriacontane C36H74, a model molecule for high density polyethylene) has been estimated from direct measurements of the etching rate using a quartz crystal microbalance and from the absolute F atom concentration using threshold mass spectrometry. This etching probability has roughly the same value as the sticking probability of F atoms on this surface, which is in the range of 1%–3%. It has been pointed out that the reactivity of the surface with respect to F atoms is enhanced when the polymer surface is seen by the plasma. This could be due to either ions or vacuum ultraviolet (VUV) radiation. To more fully understand this point, we have built an external independent VUV source to irradiate the sample and we have used threshold ionization mass spectrometry to monitor the F a...

Time‐resolved electric‐field measurements in 30 kHz hydrogen discharges by optical emission Stark spectroscopy

The temporal behavior of the cathode sheath in 30 kHz 0.4–1 Torr H2 discharges has been investigated by optical emission spectroscopy. Analysis of the Stark splitting of plasma‐induced H Balmer delta emission was used to measure the electric field with spatial and temporal resolution in the instantaneous cathode sheath. The location of the plasma/sheath boundary was determined from the position of the maximum of the H2 d 3Πu→a 3Σg+ (0,0) Q1 emission at 622.5 nm. Both methods showed that the sheath width increases as the cathode voltage becomes more negative, whereas the width remains constant as the applied voltage drops off. Analysis of the electric‐field profile provided information on the time evolution of the ion density close to the electrode during the cathode half‐cycle, in agreement with recent numerical calculations. At the beginning of the anodic half‐cycle an intense flash of plasma‐induced emission was observed, localized within 3 mm from the electrode.

Time-domain diffuse optical tomography using silicon photomultipliers: feasibility study

Abstract. Silicon photomultipliers (SiPMs) have been very recently introduced as the most promising detectors in the field of diffuse optics, in particular due to the inherent low cost and large active area. We also demonstrate the suitability of SiPMs for time-domain diffuse optical tomography (DOT). The study is based on both simulations and experimental measurements. Results clearly show excellent performances in terms of spatial localization of an absorbing perturbation, thus opening the way to the use of SiPMs for DOT, with the possibility to conceive a new generation of low-cost and reliable multichannel tomographic systems.