Comptes Rendus de l'Académie des Sciences - Series IV - Physics-Astrophysics最新文献

All solid-state laser based Lidar systems allow today a direct access to the air pollution physico-chemistry. As an example, we present the characterization of an ozone episode in Paris in summer 1999, using Lidar 3D-mappings over 33 hours.

In this Note, we present a detection criterion for exo-planets to be used with the space mission COROT. This criterion is based on the transit method, which suggests the observation of star dimming caused by partial occultations by planetary companions. When at least three transits are observed, we show that a cross-correlation technique can yield a detection threshold, thus enabling the evaluation of the number of possible detections assuming a model for the stellar population in the Galaxy.

Concentration measurements using femtosecond Raman Induced Polarization Spectroscopy (RIPS) are performed in binary gas mixtures CO₂–N₂ and CO₂–N₂O at room temperature. The principle of these measurements is based on the nonlinear rotational time response of each molecular component of the mixture. The general form of this molecular response is a series of periodic transients with a period related to the rotational constant B_e. The relative strength of the individual responses allows an accurate determination of the concentration. Two techniques are presented using either two pulses (one pump and one probe) or three pulses (two pumps and one probe).

Electrostriction, population transfer caused by absorption of laser energy and subsequent thermalization are the most important mechanisms responsible for the formation of laser-induced gratings. Applying time resolved light diffraction from electrostrictive gratings allows for the measurements of thermodynamic properties like temperature, gas composition, and flow velocity. Thermal gratings can be employed as a very sensitive spectroscopic tool.

The Two-Colour Four-Wave Mixing (TCFWM) signal formed by a non-resonant pump laser and a resonant probe laser in an acetylene/air diffusion flame comprises of contributions due to soot and C₂, the latter is generated by vaporising the soot. Profile measurements across the flame have shown that the signal composition changes within the flame.

Intermediate level labelling by Two-Colour Resonant Four-Wave Mixing Spectroscopy (TC-RFWM) is applied to reduce the spectral complexity of molecular spectra. In addition, we observe extra resonances forbidden by a conventional three-level scheme in OH- and NH-containing flames. A detailed investigation shows that the newly observed TC-RFWM resonances are induced by collisional energy transfer within the population, alignment and orientation gratings formed in both electronic states that are coupled to the laser fields.

The ultrasensitivity of the cavity ring down spectroscopy technique has been used to detect the minor CN species in a premixed rich (φ=1.2) methane/air flame stabilized at 53 hPa. The CN radical has been probed around 387 nm on the B–X (0–0) band. A peak concentration of 4.7·10⁹ cm⁻³ has been measured which corresponds to a mole fraction of 0.022 ppm. A detection limit of a few ppb has been reached despite the presence of an important off-resonance background due to the flame.

A summary of the main results obtained by the two groups in the field of H₂ vibrational spectral line signatures for various mixtures, in connection with CARS diagnostics of H₂–O₂ combustion systems, is presented. H₂–X systems may have specific large inhomogeneous spectral features, due to the dependence of the line broadening and line shifting on the (H₂) radiator speed, particularly at high temperature. Thus, careful attention has to be paid to rigorously analyze such features, both from the experimental point of view (Dijon) and from the theoretical one (Besançon). Applications of the present results to high-pressure H₂/air flame thermometry are also briefly described. They present an approach aiming to include the more recent basic results on coherent Raman line shape in CARS diagnostics, in order to improve the accuracy of temperature measurements.

Soot formation is still one of the most pressing problems in combustion. Chemical mechanisms have been established which need to be examined in detail under laboratory conditions. Some of the main pathways concerning the formation of soot precursors are still under debate. While it seems commonly accepted that some of the dominant routes may be fuel-specific, experimental data and their comparison with kinetic models for fuels with more than 3 C atoms are scarce. This article will present an overview of the work pursued in Bielefeld on the characterisation of fuel-rich flames by a combination of laser spectroscopy and mass spectrometry.

In this paper, we report about an application of the two-color laser-induced thermal gratings technique in the visible region to the detection of nitrogen dioxide in mixtures of nitrogen and argon at total pressure ranging up to 1 bar. We have investigated the dependence of the signal on the gas mixture parameters (pressure, NO₂ concentration, collision partners) and on the laser properties (beams polarization, pulse energy, bandwidth) at various instants during the temporal evolution of the thermal grating. Comparison of experimental results and theoretical predictions is performed. Finally, the detectivity of 4 ppm was obtained at normal conditions of pressure (1 bar) and temperature (300 K).