Etude experimentale du seuil d'emission photoelectrique du Cu2S massif

L'étude expérimentale de l'émission photoelectrique du Cu₂S massif poli mécaniquement a été faite entre 4 et 28 eV. La mesure du rendement quantique a permis de déterminer la valeur du seuil d'émission. Nous avons mis en évidence deux mécanismes d'excitation des électrons qui se traduisent par deux pentes et deux seuils de photoémission, l'un à 5,7 eV et l'autre à 7,1 eV.     

Low temperature kinetics, crossed beam dynamics and theoretical studies of the reaction S((1)D) + CH4 and low temperature kinetics of S((1)D) + C2H2

The reaction between sulfur atoms in the first electronically excited state, S((1)D), and methane (CH(4)), has been investigated in a complementary fashion in (a) crossed-beam dynamics experiments with mass spectrometric detection and time-of-flight (TOF) analysis at two collision energies (30.4 and 33.6 kJ mol(-1)), (b) low temperature kinetics experiments ranging from 298 K down to 23 K, and (c) electronic structure calculations of stationary points and product energetics on the CH(4)S singlet potential energy surface. The rate coefficients for total loss of S((1)D) are found to be very large (ca. 2 × 10(-10) cm(3) molec(-1) s(-1)) down to very low temperatures indicating that the overall reaction is barrier-less. Similar measurements are also performed for S((1)D) + C(2)H(2), and also for this system the rate coefficients are found to be very large (ca. 3 × 10(-10) cm(3) molec(-1) s(-1)) down to very low temperatures. From laboratory angular and TOF distributions at different product masses for the reaction S((1)D) + CH(4), it is found that the only open reaction channel at the investigated collision energies is that leading to SH + CH(3). The product angular, T(θ), and translational energy, P(E'(T)), distributions in the center-of-mass frame are derived. The reaction dynamics are discussed in terms of two different micromechanisms: a dominant long-lived complex mechanism at small and intermediate impact parameters with a strongly polarized T(θ), and a direct pickup-type (stripping) mechanism occurring at large impact parameters with a strongly forward peaked T(θ). Interpretation of the experimental results on the S((1)D) + CH(4) reaction kinetics and dynamics is assisted by high-level theoretical calculations on the CH(4)S singlet potential energy surface. The dynamics of the SH + CH(3) forming channel are compared with those of the corresponding channel (leading to OH + CH(3)) in the related O((1)D) + CH(4) reaction, previously investigated in crossed-beams in other laboratories at comparable collision energies. The possible astrophysical relevance of S((1)D) reactions with hydrocarbons, especially in the chemistry of cometary comae, is discussed.     

Quantitative IBBCEAS measurements of I2 in the presence of aerosols

An instrument implementing Incoherent BroadBand Cavity-Enhanced Absorption Spectroscopy (IBBCEAS) for quantitative measurements of molecular iodine (I₂) at ambient pressure and in the presence of aerosols is presented. The instrument is based on a LED emitting in the green spectral region around 500–550 nm, exciting a multitude of rovibronic transitions in the I₂ B ← X electronic absorption spectrum. I₂ was generated using a permeation device and was mixed with a dilution flow of Ar and another flow containing NaCl aerosols. Retrieval of the mirror reflectivity, necessary for quantitative IBBCEAS measurements, was not pursued in this study. Instead, calibrated I₂ flows were used, and a differential optical absorption spectroscopy type of algorithm was implemented for the data analysis. This approach has several benefits, in particular when light extinction due to aerosols is substantial. Estimated detection limits are roughly 0.04 nmol/l without aerosols and 0.4 nmol/l in the presence of aerosols (10¹⁰ particles/l with 60–70 nm mean diameter, leading to I ₀/I _aerosols ≈ 5.4). A drop in measured I₂ concentration in relation to the expected mixing ratio was observed when using the highest aerosol concentration, likely due to adsorption of I₂ onto aerosols.