Les pel·lícules del mes de febrer

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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

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

Spectroscopie vibrationnelle de l’interface H-Pt par génération de la fréquence somme

Les propriétés vibrationnelles de l'interface platine-hydrogène en milieu sulfurique sont étudiées par génération de la fréquence somme (GFS), par mixage non linéaire d'un faisceau laser visible à fréquence fixe et d'un faisceau laser infrarouge accordable. Les spectres vibrationnels de la liaison H-Pt sont mesurés sur un large domaine de potentiel s'étendant de part et d'autre du seuil de la réaction de dégagement de l'hydrogène. Les signatures vibrationnelles de l'hydrogène adsorbé en sous tension sont sensibles à l'orientation cristallographique de la surface de l'électrode et sont influencées par les interactions entre l'hydrogène adsorbé et les molécules d'eau de l'interface. Dans le domaine de surtension de l'hydrogène une nouvelle résonance est détectée vers 1770 cm-1. Elle n'est pas sensible à la structure superficielle de l'électrode et peut être attribuée à l'intermédiaire de la réaction de dégagement de l’hydrogène

Investigation of the adsorption of 4-cyanopyridine on Au(111) by in situ visible-infrared sum frequency generation

We have investigated the configuration of 4-cyanopyridine on Au(111) electrodes in perchlorate solution by in situ visible–IR sum frequency generation (SFG). Thanks to the use of two IR tunable lasers (the free electron laser CLIO and a table laser based on an optical parametric oscillator) we have obtained SFG spectra of the system in the spectral range of both the aromatic cycle and the CN stretching modes. We present herein the first SFG spectra ever obtained under electrochemical conditions in the 9–12 μm range. Our results show a potential-dependent orientation of the adsorbed 4-cyanopyridine from a configuration where the molecule is adsorbed perpendicular to the electrode via the nitrogen of the pyridine ring at negative potentials, to a flat adsorption configuration at intermediate potentials, and finally a perpendicular orientation again where the molecule is bound through the nitrogen of the cyanide end

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

Etude de l’interface électrochimique par excitation des plasmons de surface en lumière convergente. II – Système argent-électrolyte

Nous présentons ici les principaux résultats obtenus sur le système argent-électrolyte aqueux, par excitation optique des plasmons de surface en lumière convergente. Nous définissons un schéma optique de l’interface idéalement polarisable qui rend compte des différentes perturbations induites par le champ électrique statique dans la phase ionique et la phase métallique par des films minces effectifs équivalents. Les effets de la polarisation sont alors exprimés en terme de modifications de la résonance des plasmons de surface (déplacement de la résonance ΔK1 et amortissement ΔK2). Les études effectuées avec différentes structures de l’électrode et différents ions nous permettent d’aboutir aux conclusions suivantes :