The CO reaction on metals is of great interest experimentally and theoretically because it serves as a model system to understand molecular chemisorption and catalyzed reactions on metals. This thesis aims at progressing along the general trends of surface science: bridging the pressure and material gaps in the study of catalysts. Sum Frequency Generation (SFG) is at the heart of this work. It involves a nonlinear optical process with an IR pulse induced coherent first-order polarization up-converted by a visible pulse into a second-order polarization at the sum frequency. In this thesis it is used to record CO vibrational spectra on the Pd nanoparticles (NP)/MgO/Ag(100) to understand the adsorption and oxidation thanks to its specific advantages in surface science: sensitivity and surface selectivity. The questions proposed are the possible roles of the adsorption sites which only exist on the NPs, the effect of the size of NPs and the presence of oxygen on the CO adsorption and catalytic reactivity, the effect of adsorption of oxygen (from 'normal' - dissociative chemisorption to 'sub-surface'), the variation of reactivity of CO in the different sites when pressure and temperature increase. (1) We have studied CO adsorption on Pd(100) as a reference. Below a CO coverage of 0.5 ML SFG results confirm previous IRAS studies. Above 0.5 ML, we have observed in much more details than previously two vibrational bands assigned to CO at compressed and uncompressed bridge sites, of which we have measured the frequency and intensity and the decoherence time T₂ as a function of coverage. (2) Pd NP size effect on CO adsorption is studied (from Pd(100) to particles with about 300 atoms). At pressures below 10⁻³ mbar the CO spectra on a coalesced layer and on large NPs are dominated by the same bridge band as on Pd(100). The CO singleton frequency decreases with coverage, revealing the evolution of chemisorption with size. DFT calculations done at ENS Lyon reveal that the main mechanism is the strain induced by the substrate which increases the Pd-Pd bondlength, favors electron back donation to CO, weakens the CO bond and probably reinforce the CO-metal bond. (3) Because of a limit of our maximal temperature, we have to study the CO catalytic oxidation in an excess of oxygen to avoid self-poisoning by CO. The results strongly suggest that bridge sites are the key sites in catalysis in our experimental condition. However, while a fraction of bridge sites are more reactive on NPs, a large fraction of them seem less reactive with respect to Pd(100). The reactivity of CO on (100) facet decreases at smaller NP size. It emerges the ideal that the reaction proceeds by the most reactive sites, and that the other sites are only reservoir in reactivity, if the diffusion between sites are high enough. Oxygen modifies the adsorption of co-reactants. In the case of CO + O / Pd NPs / MgO, below 10⁻⁴ mbar oxygen does not seem to influence significantly CO adsorption; between 10⁻³ and 10⁻¹ mbar the spectroscopic signature of CO compression disappears, and above 1 mbar a new class of a top sites appears, suggesting that some oxygen species (perhaps "subsurface") favors CO adsorption on linear sites. A pump-probe experiment has been done to compare the effect of pump on different adsorption sites. All this confirms the interest of SFG vibrational spectroscopy for catalysis. An additional contribution of this thesis to SFG is the study of the spectro-temporal aspects of SFG emission. SFG spectra containing several bands are modeled in details based on an ODT/Au system and compared to experimental spectra, showing that in SFG spectra are affected by the spectro-temporal shape of the visible laser. The standard deconvolution method used in the literature is only approximate. Accurate spectro-temporal spectrum modeling is required to evaluate precisely the relative intensities when several bands are present.

Estudos de interface têm presenciado um recente crescimento devido as novas propriedades físico-químicas, que puderam ser explorados com o advento de técnicas com resolução ao nível molecular/atômico. Dessas técnicas, dois ramos que merecem destaque são as Microscopias de Varredura por Ponta de Prova e Espectroscopias Óticas. Neste trabalho fazemos uso de algumas destas técnicas (o Microscópio de Tunelamento de Elétrons e Espectroscopia Vibracional por Geração de Soma de Freqüências) para estudar a adsorção de moléculas alifáticas e aromáticas em Au. Dois casos são abordados: como a rugosidade do substrato influencia no mecanismo de automontagem de monocamadas de alcanotióis e a automontagem de derivados de uracil em interfaces eletroquímicas. No primeiro caso, mostra-se que a quantidade de defeitos moleculares na monocamada adsorvida e extremamente sensível a rugosidade do substrato utilizado. Unem-se os resultados das técnicas acima aos resultados de sondas eletroquímica para se chegar a um modelo. Também e estudado a dependência das etapas de fisisorção e quimisorção em função da concentração da solução de alcanotiól. Para o segundo caso, um derivado halogenado do uracil (5-fluorouracil), mostra-se como a substituição química na base nitrogenada leva a diferentes mecanismos de formação de monocamadas na interface eletroquímica. Imagens de Microscopia de Tunelamento de Elétrons com resolução atômica e molecular mostram que em densidade de cargas negativas as moléculas estão fisisorvidas, porém não formam estruturas periódicas em contraste com uracil e timina, entretanto em densidades de cargas positivas formam estruturas periódicas quimisorvidas, assim como uracil e timina. E discutido como são diferentes os mecanismos de interação intermolecular: no caso dos alcanotiois preponderando às interações de van der Waals e no caso dos derivados de uracil pelas ligações via pontes de hidrogênio são dominantes. / Interface science has experienced a new rebirth since the development of new probes with atomic/molecular resolution, giving new insights about the physical-chemical properties, which differ substantially from the bulk. Among these techniques, two branches deserve special attention: the Scanning Probe Microscopies and Optical Spectroscopy. In this work, two derivatives of theses techniques (the Scanning Tunnelling Microscopy and Vibrational Spectroscopy by Sum-Frequency Generation) are combined giving new insights about the molecular adsorption onto Au. Two examples are focused: how roughness plays a key role in the structure of self-assembled alkanethiol monolayer and the uracil derivatives self-assembling at electrochemical interfaces. For the former, it has been shown that the amount of defects on the adsorbed monolayer is highly sensitive to substrate roughness. Combining the results of each technique with a well-known electrochemical probe, a physical model is proposed. The physisorbed and chemisorbed states are studied as a function of alkanethiol solution concentration as well. For the later case, the chemical substitution of uracil leads to drastically different results for the physisorbed phase (negative charge densities), compared to uracil and thymine. In the chemisorbed phase (positive charge densities) imaging with molecular resolution is achieved showing a quasi-hexagonal structure, similar to the structure of thymine and uracil. It is discussed what are the main driving forces for the self-assembling mechanism: van der Waal interactions for the alkanethiols and hydrogen bonding for uracil derivatives.

La réaction de CO sur métaux est d'un grand intérêt, car il sert de système modèle pour comprendre la chimisorption et les réactions catalytiques sur les métaux. Cette thèse se place dans la démarche générale de la science des surfaces de franchir les « fossés » de pression et de materiaux pour l’étude de la catalyse. La Génération de Somme de Fréquences (SFG) est au cœur de ce travail. Elle implique un processus optique non linéaire créé par une impulsion IR qui induit une polarisation cohérente du premier ordre, convertie par une impulsion visible en une polarisation du second ordre à la fréquence somme. La SFG est utilisée pour mesurer les spectres vibrationnels de CO sur Pd nanoparticule (NP)/MgO/Ag(100) grâce à des avantages spécifiques en science de la surface de SFG : sensibilité, sélectivité de surface. Les questions posées sont les rôles possibles des sites d'adsorption qui n'existent que sur les NP, l'effet de taille des NP, l'adsorption de l'oxygène (de « normal » - chimisorption dissociative - à « sub-surface »), sur l'adsorption de CO et la réactivité catalytique, la variation de la réactivité de CO dans les différents sites lors de l'augmentation de la pression et de la température. (1) Nous avons étudié l’adsorption de CO sur Pd (100) comme une référence. En dessous d’une couverture de 0.5 ML de CO, les résultats de SFG confirment les études IRAS antérieures. Au-dessus de 0.5 ML, nous avons observé deux bandes vibrationnelles attribuées à CO dans des sites pontés « comprimés » et « non comprimés », dont nous avons mesuré la fréquence et l’intensité en fonction de la couverture, ainsi que le temps de décohérence T₂. (2) L’effet de taille des NP de Pd sur l'adsorption de CO a été observé (depuis Pd(100) à NP d’environ 300 atomes). Aux pressions ≤ 10⁻³ mbar, les spectres de CO sur une couche coalescées et sur des NP larges sont dominés par la même bande de sites pontés que sur Pd (100). La fréquence « singleton » de CO diminue avec la taille des NP, ce qui révèle l'évolution de la chimisorption avec la taille des NP. Des calculs DFT faits à l'ENS Lyon révèlent que le mécanisme principal est la contrainte induite par le substrat qui augmente la longueur de liaison Pd-Pd, favorise la rétrodonation d’électrons vers CO, affaiblit la liaison interne de CO et probablement renforce la liaison CO-métal. (3) Pour CO oxidation catalytic, les résultats suggèrent fortement que les sites pontés sont les sites clé dans la catalyse dans nos conditions expérimentales. Cependant, tandis qu'une fraction des sites pontés sont plus réactifs sur les NP, une grande fraction sont moins réactifs par rapport à Pd(100). La réactivité de CO sur les facettes (100) diminue à plus petite taille des NP. Il se dégage l’idée que la réaction procède par les sites les plus réactifs, et que les autres sites servent seulement de réservoirs en réactifs, à condition que la diffusion entre sites soit suffisamment élevée. L’oxygène modifie l'adsorption de co-réactifs. Dans le cas de CO+O/NP de Pd/MgO, au-dessus de 1 mbar, une nouvelle classe de sites linéaires apparaît, qui est probablement due à "sub-surface" oxygen. Une expérience pompe-sonde a été faite pour comparer l’effet de pompe sur les différents sites d’adsorption. Tous ces résultats confirment l'intérêt de spectroscopie vibrationnelle de SFG pour l’étude de la catalyse. Une contribution supplémentaire de cette thèse est l'étude des aspects spectro-temporels de l’émission SFG. Des spectres SFG qui contiennent plusieurs bandes sont modélisés en détail dans le cas du système modèle ODT/Au, et comparés à des spectres expérimentaux. Les spectres SFG sont affectées par la forme spectro-temporel du laser visible. La comparaison montre que l’interprétation quantitative des intensités relatives des spectres SFG obtenus avec des impulsions femtosecondes nécessite une analyse spectro-temporelle et pas seulement spectrale. La méthode de déconvolution standard utilisée dans la littérature est approximative. / The CO reaction on metals is of great interest experimentally and theoretically because it serves as a model system to understand molecular chemisorption and catalyzed reactions on metals. This thesis aims at progressing along the general trends of surface science: bridging the pressure and material gaps in the study of catalysts. Sum Frequency Generation (SFG) is at the heart of this work. It involves a nonlinear optical process with an IR pulse induced coherent first-order polarization up-converted by a visible pulse into a second-order polarization at the sum frequency. In this thesis it is used to record CO vibrational spectra on the Pd nanoparticles (NP)/MgO/Ag(100) to understand the adsorption and oxidation thanks to its specific advantages in surface science: sensitivity and surface selectivity. The questions proposed are the possible roles of the adsorption sites which only exist on the NPs, the effect of the size of NPs and the presence of oxygen on the CO adsorption and catalytic reactivity, the effect of adsorption of oxygen (from ‘normal’ – dissociative chemisorption to ‘sub-surface’), the variation of reactivity of CO in the different sites when pressure and temperature increase. (1) We have studied CO adsorption on Pd(100) as a reference. Below a CO coverage of 0.5 ML SFG results confirm previous IRAS studies. Above 0.5 ML, we have observed in much more details than previously two vibrational bands assigned to CO at compressed and uncompressed bridge sites, of which we have measured the frequency and intensity and the decoherence time T₂ as a function of coverage. (2) Pd NP size effect on CO adsorption is studied (from Pd(100) to particles with about 300 atoms). At pressures below 10⁻³ mbar the CO spectra on a coalesced layer and on large NPs are dominated by the same bridge band as on Pd(100). The CO singleton frequency decreases with coverage, revealing the evolution of chemisorption with size. DFT calculations done at ENS Lyon reveal that the main mechanism is the strain induced by the substrate which increases the Pd-Pd bondlength, favors electron back donation to CO, weakens the CO bond and probably reinforce the CO-metal bond. (3) Because of a limit of our maximal temperature, we have to study the CO catalytic oxidation in an excess of oxygen to avoid self-poisoning by CO. The results strongly suggest that bridge sites are the key sites in catalysis in our experimental condition. However, while a fraction of bridge sites are more reactive on NPs, a large fraction of them seem less reactive with respect to Pd(100). The reactivity of CO on (100) facet decreases at smaller NP size. It emerges the ideal that the reaction proceeds by the most reactive sites, and that the other sites are only reservoir in reactivity, if the diffusion between sites are high enough. Oxygen modifies the adsorption of co-reactants. In the case of CO + O / Pd NPs / MgO, below 10⁻⁴ mbar oxygen does not seem to influence significantly CO adsorption; between 10⁻³ and 10⁻¹ mbar the spectroscopic signature of CO compression disappears, and above 1 mbar a new class of a top sites appears, suggesting that some oxygen species (perhaps “subsurface”) favors CO adsorption on linear sites. A pump-probe experiment has been done to compare the effect of pump on different adsorption sites. All this confirms the interest of SFG vibrational spectroscopy for catalysis. An additional contribution of this thesis to SFG is the study of the spectro-temporal aspects of SFG emission. SFG spectra containing several bands are modeled in details based on an ODT/Au system and compared to experimental spectra, showing that in SFG spectra are affected by the spectro-temporal shape of the visible laser. The standard deconvolution method used in the literature is only approximate. Accurate spectro-temporal spectrum modeling is required to evaluate precisely the relative intensities when several bands are present.

Les processus physico-chimiques se produisant au sein des nanoparticules que sont les boîtes quantiques semiconductrices (QDs) sont à l'origine d'une nouvelle classe de sondes fluorescentes trouvant des applications en catalyse, en reconnaissance moléculaire et en imagerie. Le confinement quantique des électrons aux sein de ces objets luminescents, qui donne lieu à leur structure excitonique si particulière, permet de tirer simultanément profit de leurs propriétés optiques d'absorption et d'émission dans la gamme spectrale visible, et ce, dans le but de faciliter la détection et l’identification des espèces chimiques situées dans leur environnement proche. Dans ce contexte, nous nous sommes intéressés à des QDs de 3 à 4 nm de diamètre, composées d’un alliage ternaire de cadmium, de tellure et de soufre, et fonctionnalisées par des ligands mercaptocarboxyliques. De manière à déterminer l’ensemble de leurs propriétés structurales, chimiques et optoélectroniques, nous les avons tout d’abord caractérisées à l’état de solutions colloïdales par diverses techniques expérimentales : microscopie électronique, zêta-métrie, analyse par diffusion dynamique de la lumière, spectroscopies de rayons X, d’absorption UV-visible et d’émission de fluorescence. Ceci nous a permis de déduire la composition chimique des nanocristaux, leur structure cristalline, leur taille, leur dispersion en taille, la composition chimique de leurs ligands, les énergies propres de leurs états électroniques, leur moments dipolaires de transition et leur section efficace d’absorption. Fort de ces connaissances, nous avons pu développer un modèle analytique pour calculer la susceptibilité diélectrique des QDs et extraire de cette manière leur fonction de réponse linéaire, véritable carte d’identité optoélectronique. Nous avons ensuite optimisé la conception par voie chimique d’interfaces composées de QDs et de différentes espèces moléculaires organiques, dépôts réalisés sous forme de monocouches ou de films épais sur des substrats solides plans de silicium, de verre et de fluorure de calcium fonctionnalisés par des organosilanes. Ces interfaces substrat/QDs/molécules ont alors été étudiées par spectroscopie linéaire d’absorption UV-visible et par spectroscopie optique non-linéaire de génération de fréquence-somme (SFG). La première nous a permis de déterminer la densité superficielle des QDs déposés et d’en caractériser la stabilité temporelle, et la seconde, qui combine deux lasers visible et infrarouge, d’identifier la signature vibrationnelle des ligands recouvrant les QDs. Grâce à ces échantillons, nous avons alors montré par spectroscopie SFG deux couleurs l’existence d’un couplage vibroélectronique entre les QDs et leur environnement moléculaire. En particulier, nous avons démontré que l’amplitude de vibration des modes moléculaires associés aux ligands des QDs et aux organosilanes greffés sur les substrats est maximale lorsque les QDs sont eux-mêmes stimulés par la lumière visible dans leur premier état excitonique. Cette démonstration expérimentale s’accompagne par ailleurs d’une démonstration théorique : en utilisant les diagrammes de Feynman dans l’espace des fréquences imaginaires de Matsubara, nous avons déterminé l’expression analytique de la susceptibilité non-linéaire d’ordre 2 du complexe QD/molécule. Nous avons alors vérifié que l’hypothèse d’un couplage dipolaire entre QDs et molécules menait à une modélisation de la réponse vibrationnelle SFG compatible avec les mesures expérimentales. De cette manière, l’existence d’un couplage vibroélectronique de nature dipolaire entre boîtes quantiques et molécules est attesté. / The different physico-chemical processes occurring within semiconductor quantum dots (QDs) give rise to a new class of fluorescent probes and a wide range of applications in catalysis, molecular recognition and imaging. Within these luminescent nanoparticles, the quantum confinement of electrons, which leads to their very special excitonic structure, allows us to benefit from both their absorption and emission optical properties, with the specific aim of fostering the detection and the identification of the chemical species located in their direct environment. Within this framework, we were interested in 3 to 4-nm-sized QDs composed of ternary alloys of cadmium, telluride and sulfur, and functionalized by mercaptocarboxylic ligands. In order to determine their structural, chemical and optoelectronic properties, we first characterized them thanks to several experimental techniques: electron microscopy, zeta potentiel measurements, dynamic light scattering analysis, X-ray, UV-visible and fluorescence spectroscopies. This enabled us to deduce the chemical composition of the nanocrystals, their crystal structure, size, size-dispersion, the chemical composition of their ligands, the eigenenergies of their electronic states, their transition dipole moments and absorption cross-sections. Given all those results, we succeeded in deriving an analytical model of the QD dielectric susceptibility and extracting in this way their linear response function. Then, we optimized the chemical synthesis of nanostructured interfaces made of QDs and various molecular species through the use of flat solid substrates of silicon, glass and calcium fluoride functionalized with organosilanes. These substrate/QDs/molecules interfaces were studied by linear UV-visible absorption spectroscopy and by sum-frequency generation non-linear optical spectroscopy (SFG). The former allowed us to determine the surface density of the deposited QDs and to characterize their stability over time, while the later, which combines two visible and infrared lasers, enabled us to identify the vibrational signature of the QD ligands. Thanks to those samples probed by two-colour SFG spectroscopy, we therefore shew the existence of a vibroelectronic coupling between QDs and their molecular surroundings. Especially, we demonstrated that the vibration amplitudes associated to the molecular modes of the QD ligands and the organosilanes grafted on the substrates are maximum when the QDs are excited by visible light into their first excitonic state. This experimental demonstration is further supported by theoretical considerations: Feynman diagrams in Matsubara imaginary-time representation were used to determine the analytical expression of the second-order nonlinear susceptibility of the QD/molecule bipartite system. We thus verified that the hypothesis of a dipolar coupling between QDs and molecules resulted in a modeling of the vibrational SFG response which proved to be in complete agreement with the experimental measurements. Thus, we evidenced the existence of a dipolar vibroelectronic coupling between quantum dots and molecules.

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Diese Arbeit konzentriert sich auf die Anwendbarkeit eines hochmodernen 100 kHz BB-VSFG-Spektrometers, das kürzlich im SALSA Photonics Lab entwickelt wurde, für die Analyse der Grenzflächenstruktur von Alkylketten, des sie umgebenden Wassers und der Phosphatkopfgruppen von Phospholipidschichten. Zunächst wurden Phospholipid-Doppelschichten, die mehrere Komponenten enthalten, bei Laserwiederholraten von 5, 10, 50 und 100 kHz mit konstanter Pulsenergie untersucht. Die BB-VSFG-Spektren legen nahe, dass die Phospholipid-Doppelschichten während der Messungen ohne wärmeinduzierte Veränderungen stabil waren. Darüber hinaus bot die Erhöhung der Laserwiederholungsrate eine praktikable Möglichkeit, Spektren in kurzen Datenerfassungszeiten zu erhalten, ohne dass das Signal-Rausch-Verhältnis beeinträchtigt wurde. Die extrem kurze Aufnahmezeit von 500 ms, die hohe spektrale Auflösung und alle verwendeten Pulsparameter sorgen dafür, dass bei Messungen unter Umgebungsbedingungen keine thermisch bedingten Photoschäden auftreten. Es wurde eine systematische Untersuchung von ein- und zweikomponentigen Phospholipid-Monoschichten in Abhängigkeit von der Oberflächenspannung und dem Mischungsverhältnis für verschiedene Kombinationen an Polarisationen durchgeführt und die Abhängigkeit der Schwingungsspektren untersucht. Die Struktur von Alkylketten und umgebendem Wasser wurde anhand derselben Modellsystemen analysiert. Bislang nicht beobachtete Schwingungsbanden und Spektren von Monolagen mit geringer Oberflächenbedeckung wurden mit einem bisher nicht erreichten Signal-Rausch-Verhältnis gemessen und beschrieben. Die Struktur von Phospholipid-Monolagen mit identischen Kopfgruppen und unterschiedlichen Ketten wurde analysiert und verglichen. Die Spektren bestätigten die Anwesenheit von Wassermolekülen in der Nähe der Phosphat- und Cholingruppen der Phospholipid-Monolagen. / This thesis focuses on the applicability of a state-of-the-art 100 kHz BB-VSFG spectrometer recently developed at the SALSA Photonics Lab and on the analysis of the interfacial structure of alkyl chains, surrounding water, and the phosphate head groups of phospholipid layers. First, multi-component phospholipid bilayers were studied at laser repetition rates of 5, 10, 50, and 100 kHz at constant pulse energy. The spectra suggest that the phospholipid bilayers were stable during the measurements with no heat-induced distortions. Moreover, an increase in the laser repetition rate provided a feasible route to obtain spectra in short data acquisition times without compromising the signal-to-noise ratio. The extremely short acquisition time of 500 ms, the high spectral resolution, and all applied pulse parameters ensured no thermal induced photodamages occur during the measurements. A systematic study of one- and two-component phospholipid monolayers as a function of surface tension and mixture ratio at different polarization combinations was performed and the dependence of the vibrational spectra was explored. The structure of alkyl chains and surrounding water was analyzed using the same model systems. Vibrational modes that were previously unseen and spectra of monolayers at low surface coverage were reported for the first time with an unprecedented signal-to-noise ratio. The structure of phospholipid monolayers containing identical head groups and different chains was analyzed and compared. The order of the phospholipid molecules as a function of the composition of the monolayers was inferred from the spectral data. The influence of the hydration and/or changes in the orientation of the phosphate group was visible from the spectra as well.

Monolayers of the cationic surfactant hexadecyltrimethylammonium halide (CTAX, where X = F¯, Cl¯, Br¯, and I¯) have been studied at the air-water and oilwater interfaces. At the air-water interface, the effects of the halide counterion and the addition of counterion were investigated. Sum-frequency spectroscopy (SFS), ellipsometry, and surface tensiometry indicated that the counterion changed the efficiency and effectiveness of the surfactant, both decreasing in the order of Br¯> Cl¯>F¯. The addition of salt in the form of 0.1 M KX was found to reduce the cmc but had little effect on the limiting area per molecule attained at the cmc, which increased from 44 Ų for CTAB to 65 Ų for CTAC and ca. 94 Ų for CTAF. Neither SFS nor ellipsometry provided any firm evidence for specific effects of the halide ions on the structure of the surfactant monolayers. For CTAB monolayers in the absence of excess electrolyte, the effect of area per molecule on the sum-frequency (SF) spectra was studied. Mixed monolayers of CTAB and tetradecane at the air-water interface exhibit a first-order phase transition from a conformationally disordered to a conformationally ordered state as the temperature is lowered. The phase transition occurs ca. 11 °C above the bulk melting point of tetradecane. A new experimental arrangement is described for acquiring SF spectra from surfactants at the oil-water interface. The key features of this approach are the stabilisation of a thin oil film between a sapphire prism and an aqueous phase, and the use of total internal reflection to enhance the total signal and discriminate against signals from other interfaces in the system. With this new methodology, the first SF vibrational spectra of surfactant monolayers at an alkane-water interface were obtained. Surface tensiometry was used to characterise the monolayers further. The structure of CTAB monolayers at the hexadecane-water interface was determined by SFS and compared with monolayers of CTAB at the air-water interface. At low concentrations, CTAB/hexadecane showed the expected features in the C-H stretching region, characteristic of a conformationally disordered monolayer. As the bulk concentration approached the critical micelle concentration, the spectra changed to one characteristic of a more ordered, upright conformation. Ellipsometric measurements supported this conclusion. This qualitative structural change is not observed in analogous monolayers at the air-water interface or CCl₄-water interface, or in surfactant solutions in contact with a hydrophobic solid surface.

Die vorliegende Arbeit widmet sich der Untersuchung von 4 Modellsystemen der Oberflächenforschung. Die verwendeten experimentellen Methoden sind Summenfrequenzerzeugungsspekroskopie (SFG), Thermische Desorptionsspektroskopie (TDS), Beugung niederenergetischer Elektronen (LEED), Augerelektronenspektroskopie (AES), Infrarotadsorptionsspektrokopie (IRAS) und Rastertunnelmikroskopie (STM). Durch die Verwendung von SFG waren Messungen bis zu 50 mbar möglich. Die untersuchten Systeme sind: CO auf Pt(111), Wasser auf Ag(001) und MgO/Ag(001), CO auf Au/MgO/Ag(001) und CO auf Au-Pd/MgO/Ag(001). Bei den Messungen von CO auf Pt(111) tritt unter bestimmten Druck- und Temperaturbedingungen eine bisher nicht näher untersuchte Doppelpeakstruktur auf, die genauer charakterisiert wurde. Die Untersuchungen von Wasser auf MgO/Ag(001) und Ag(001) zeigen, dass sich auf MgO/Ag(001) zuerst eine Monolage Wasser mit einer darauffolgenden Multilage ausbildet, wohingegen es auf Ag(001) von Beginn an in einer Multilagenschicht wächst. Die Monolage kann unter der Multilage gemessen werden und einige Resonanzen identifiziert werden. Für Au/MgO/Ag(001) zeigte sich mittels STM eine Abhängigkeit des Au-Wachstums von der Schichtdicke, die allerdings nicht spektroskopisch in Erscheinung tritt. Bei den Messungen an gemischten Au-Pd-Teilchen auf MgO/Ag(001) zeigen sich Unterschiede im Adsorptionsverhalten zwischen reinen Metallteilchen und gemischten Teilchen, die auf eine Wechselwirkung zwischen den beiden Metallen zurückzuführen ist. Nach Heizschritten auf 600 K reichern sich die Au-Atome im Mantel des Teilchens an, das Pd bildet den Kern. Die Ergebnisse aller Messungen werden unter Berücksichtigung neuer Veröffentlichungen diskutiert. / This thesis focuses on 4 different model systems of surface science. The experimental techniques used for the measurements include sum frequency generation (SFG), thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), infrared adsorption spectrosocopy (IRAS) and scanning tunneling microscopy (STM). By using SFG, measurements could be performed up to a pressure of 50 mbar. The systems under investigation were: CO on Pt(111), water on Ag(001) and on MgO/Ag(001), CO on Au/MgO/Ag(001), and CO on Au-Pd/MgO/Ag(001). The system of CO on Pt(111) exhibits a two peak-pattern under certain pressure and temperature conditions which has not been studied so far. Various experiments helped to elucidate the origin of this distinct behaviour. The measurements of water on Ag(001) and MgO/Ag(001) show that on MgO, water first adsorbs as a monolayer with a following multilayer, whereas on Ag(001) it adsorbs as a multilayer from the beginning. The monolayer can be studied below the multilayer and some resonances can be identified. For the case of Au/MgO/Ag(001), STM shows that the growth mode of Au depends on the thickness of the supporting MgO film, which can not be seen with spectroscopic methods. For mixed Au-Pd particles on MgO/Ag(001) a clear difference in the adsorption behaviour between pure metal particles and mixed particles can be seen, which is explained by an interaction between these metals. Annealing the mixed particles to 600 K leads to a segregation of the metals, where the Au atoms diffuse to the shell and the Pd atoms make up the core. The results of all these measurements are discussed in the light of recent publications.