Ecosystemes steppiques(Cas de Brezina– El Bayadh)

Postprint (published version

Contribution a la comprehension de la qualité des sols de la steppe algerienne (Cas du secteur El Bayadh-Brezina, ouest de l'Algerie)

Postprint (published version

Adsorption of 4-cyanopyridine on Au(111) : ab initio calculations and SFG measurement

International audienceThe adsorption of 4-cyanopyridine (4-CP) on a gold single crystal electrode in sodium perchlorate supporting electrolyte has been investigated by voltammetry, sum frequency generation (SFG) and ab initio calculations. Combination of in situ vibrational SFG spectra and DFT, MP2 and MP2 + SQM calculations allows us to obtain fundamental information on the behavior of adsorbed 4-CP as a function of the electrode potential. The configuration of the adsorbed 4-CP is dependent on the electrode potential. 4-CP is adsorbed perpendicular to the electrode surface through the pyridine nitrogen at negative potential, and through the nitrile nitrogen at positive potential. In the intermediate potential range, the molecule is adsorbed parallel to the electrode surface, through the electrons of the aromatic ring

SFG experiment and ab initio study of the chemisorption of CN- on low-index platinum surfaces

A dual analysis is proposed in order to have a better understanding of the adsorption of the cyanide ions on a platinum electrode. The SFG (Sum Frequency Generation) spectroscopy allows the in situ vibrational study and the SFG spectra of the CN− species adsorbed on single crystal Pt electrode allow a systematic study of the low-index platinum surfaces. This experimental work is supported by ab initio calculations using density functional theory and cluster models. For each surface orientation and each geometry, a cluster model of 20-30 Pt atoms has been built in order to interpret the chemisorption of the CN− ions through four kinds of adsorption geometry: on-top or bridge site, bonding via C or N atoms. Geometries have been optimized and adsorption energies, electronic properties and vibrational frequencies have been computed. From the electronic properties, we can propose an analysis of the bonding mechanism for each studied kind of adsorption. The SFG spectra of the CN−/Pt(1 1 1) system present an unique resonance owing to the top C adsorption. It is mainly the same for the CN−/Pt(1 0 0) system. It is also the case for the SFG spectra of the CN−/Pt(1 1 0) system recorded at negative electrochemical voltage; at more positive voltage, a second resonance appears at a lower frequency, owing to the top N adsorption. Experimental and theoretical values of the C-N stretching frequencies are in excellent agreement

SFG experiment and ab initio study of the chemisorption of CN- on low-index platinum surfaces

A dual analysis is proposed in order to have a better understanding of the adsorption of the cyanide ions on a platinum electrode. The SFG (Sum Frequency Generation) spectroscopy allows the in situ vibrational study and the SFG spectra of the CN− species adsorbed on single crystal Pt electrode allow a systematic study of the low-index platinum surfaces. This experimental work is supported by ab initio calculations using density functional theory and cluster models. For each surface orientation and each geometry, a cluster model of 20-30 Pt atoms has been built in order to interpret the chemisorption of the CN− ions through four kinds of adsorption geometry: on-top or bridge site, bonding via C or N atoms. Geometries have been optimized and adsorption energies, electronic properties and vibrational frequencies have been computed. From the electronic properties, we can propose an analysis of the bonding mechanism for each studied kind of adsorption. The SFG spectra of the CN−/Pt(1 1 1) system present an unique resonance owing to the top C adsorption. It is mainly the same for the CN−/Pt(1 0 0) system. It is also the case for the SFG spectra of the CN−/Pt(1 1 0) system recorded at negative electrochemical voltage; at more positive voltage, a second resonance appears at a lower frequency, owing to the top N adsorption. Experimental and theoretical values of the C-N stretching frequencies are in excellent agreement

Ab initio interpretation of sum frequency generation spectra of 4-cyanopyridine adsorbed on Au(1 1 1)

International audienceThe sum frequency generation spectroscopy allows the in situ vibrational study of 4-cyanopyridine (4-CP) adsorbed on a gold electrode. Our (DFT, MP2, MP2 + SQM) calculations permit to propose: (1) the whole vibrational structure of the 4-CP (vibrational frequencies, infrared and Raman intensities) and (2) the interpretation of the adsorption phenomenon. The Au(1 1 1) surface has been modelled by a cluster composed of 28 Au atoms. Our results show that the most stable adsorption configuration is the vertical one when the 4-CP is linking on-top (s axial bond, via N of the pyridine ring). Moreover, the introduction of the electric field in the calculations confirms the observation of the spectra obtained by electrochemical potential scanning: (i) At negative values, the 4-CP is in its stable adsorption geometry; (ii) then, it turns over to a flat configuration (p bond); (iii) at large positive values of the potential, the 4-CP is still vertically linked (s axial bond via N of the cyano group). © 2001 Elsevier Science B.V

The influence of plasma waves on the dispersion of surface plasmons : experimental evidence

Surface plasmon excitation by attenuated total reflection in Otto geometry has been carried out in a wide spectral range between 1.9 and 5 eV. Samples were Al-films covered by thin (5 nm) continuous Ag-layers. Results were discussed in the framework of a hydrodynamic model of the conduction electrons both in the film and in the substrate using additional boundary conditions. The calculated and measured dispersion relation and reflectivity curves show that longitudinal plasma waves have to be taken into account. A new branch of the surface plamon dispersion is found experimentally between 4.2 and 4.6 eV. It is attributed to coupled plasma wave-surface plasmon modes localized in the tail of the electronic charge density at the interfaces.Des mesures d'excitation des plasmons de surface par réflexion totale atténuée ont été réalisées dans un large domaine spectral, entre 1,9 et 5 eV sur des substrats d'aluminium recouverts de films minces continus d'argent. Les résultats sont discutés sur la base du modèle hydrodynamique des électrons libres aussi bien dans le film que dans le substrat, ce qui conduit à introduire des conditions aux limites supplémentaires. L'interprétation des résultats expérimentaux (relation de dispersion et courbes de réflectivité) nécessite la prise en compte des ondes longitudinales de plasma. Une nouvelle branche de la courbe de dispersion des plasmons de surface est mise en évidence entre 4,2 et 4,6 eV. Elle est attribuée au couplage entre l'onde de plasma et les modes de surface localisés dans la queue de la densité de charge aux interfaces

ELLIPSOMÉTRIEÉTUDE DE L'INTERFACE OR-ÉLECTROLYTE PAR ELLIPSOMÉTRIE AVEC EXCITATION DE PLASMONS DE SURFACE

Les interfaces Au/H2SO4 et Au/HClO4 ont été étudiés par ellipsométrie avec excitation de plasmons de surface dans une plage de potentiel comprenant la zone de couche double et la zone de l'oxyde. Dans la zone de couche double, nous avons pu séparer la contribution au signal optique de la surface du métal de celle de la couche double électrolytique. La profondeur de pénétration du champ électrique dans le métal a été déterminée pour des charges inférieures à 20 µC.cm-2. La comparaison des deux électrolytes a permis d'évaluer l'effet optique de l'adsorption de SO-4. Dans la zone de l'oxyde, nous avons calculé la constante diélectrique complexe du film d'oxyde pour chaque potentiel au cours de l'oxydation et de la réduction ; ceci nous a permis de préciser le mécanisme de ces deux réactions.Au-H2SO4 and Au-HClO4 interfaces have been studied by surface plasmons waves ellipsometry in the double layer and oxide range. The effects of the metal surface and the inner layer of potential, on the optical parameters have been separated. The screening depth into the metal has been determined for surface charges smaller than 20 µC.cm-2. By comparing the results obtained with the two electrolytes, we have reached the optical effect of SO-4 adsorption. The variations against potential of the oxide film complex dielectric constant have been determined during oxidation and reduction processes. This allows us to precise oxidation and reduction mechanisms