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Transition metal oxides having high dielectric constants and wide band gaps find very important and interesting technological applications in surface physics. In particular, titania is the most commonly used material in heterogeneous catalysis because of its stable and flat surfaces. Having Ti cations at different charge states within the system brings about various novel electronic properties which are mainly surface related. Adsorption of catalytically important or chemically useful molecules on titania surfaces are investigated, electronic energy bands and charge densities are calculated from first principles using the density functional theory in the GGA scheme. The comparisons with the leading theories and existing experimental data are maid.

In the first part of the Thesis, a new type of active system for SERS and SERRS of hydrophobic molecules, namely a 3-dimensional (3D) nanosponge aggregate with incorporated hydrophobic molecules has been developed, and tested by fullerene C60 and hydrophobic free- base tetraphenylporfine (H2TPP). The SERS and SERRS (surface enhanced /resonance/ Raman scattering) limits of detection (LODs) of C60 at four excitation wavelengths spanning the visible spectral region were found to be by one order of magnitude lower than in the reference system, which mimics the previously reported ways of utilization of Ag nanosponges as substrates for SERS and SERRS. The superiority of the newly developed sample is attributed to the efficient localization of the hydrophobic molecules into hot spots in 2D fractal aggregates of Ag nanoparticles (NPs). Diprotonation of H2TPP during the procedure using HCl as the preaggregation agent has been eliminated by employment of NaCl. On the other hand, investigation of the mechanism of H2TPP protonation during the former preparation procedure opened a possibility to employ Ag nanosponge aggregate as nanoreactor. In the second part of the Thesis, 2D assemblies of AgNPs were found to be better substrates for SERS of single layer graphene (SLG) than the 3D ones. In particular, the 2D...

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In this thesis, I have investigated gas phase infrared spectroscopy of environmentally as well as astrophysical important large organic molecules such as naphthalene, methy-lated naphthalene, fluorine, methyalted fluorine etc. which are commonly known as polycyclic aromatic hydrocarbons (PAHs). Depending upon the molecular weight these organic molecules can exist both in gaseous as well as in the particulate state at room temperature hence they are the major environmental pollutants. They are also responsible for the unidentified infrared emission bands in the interstellar medium. Chapter 1 provides a brief introduction to my thesis work. A detailed literature survey on the importance, abundance of the PAHs in the environment as well as various spectroscopic techniques useful for identifying the PAHs has been done. Since the objective of my thesis work is to assign the observed fundamental infrared bands of large organic molecules with the help of high level quantum mechanical calculations, a brief introduction to the various high level quantum mechanical techniques that I have used in assigning the bands have been described in this chapter. In Chapter 2 I have presented the experimental and the theoretical methodologies in details. The chapter begins with a detailed description of the experimental procedure used for recording the infrared spectrum of these molecules followed by the theoretical methodologies used for the assignment of the observed infrared bands as well as for identifying the Fermi resonances. In Chapters 3 and 4, of this thesis I have recorded infrared spectrum of 1-and 2-methylnaphthalene (1-and 2-MN), fluorine (FL), 1-methylfluorene (1-MFL) and 1,8-dimethylfluorene (1,8-DMF) in the gas phase. The observed bands were assigned with the help of scaled harmonic frequency, scaled quantum mechanical harmonic force field (SQMFF) and enharmonic frequency calculations. The first two methods are based on the harmonic approximation, whereas the enharmonic frequency calculation is based on the standard second order perturbation theory. All these calculations gave me a partial fit to the fundamental bands in both aromatic and aliphatic C-H stretching as well as in the non C-H stretching region. At the end of both the chapters an error analysis in fitting the spectrum from all the three different calculations have been presented. Evidently the non linear least square fitting method employed in SQMFF calculation gives much better agreement between the experiment and theory than the other two methods. It has been observed in the experimental spectrum of methylated naphthalene that the band structure near the C-H stretch around 3000 cm−1 is very complicated and many bands and shoulders remain unassigned by the methods described in Chapters 3 and 4. Fermi resonance is one of the potential reason for the complicated band structure in this region. In Chapter 5, I have taken naphthalene and have investigated the Fermi resonance around the C-H stretching region using an effective vibrational hamiltonian (EVH) approach. In this method I have constructed an EVH consisting of 8 C-H stretches and 8 H-C-C in-plane bend overtones and 28 H-C-C in-plane bend combination modes as the basis. Both type 1 (stretch overtone) and type 2 (stretch combination) Fermi resonances were investigated. Calculated frequencies belonging to B1u and B2u irreducible representation were compared with the observed bands. Many bands and shoulders have been assigned as the overtone and combination modes of low frequency H-C-C bend motion obtained from the EVH approach. How-ever some bands remain unassigned in this method. This is perhaps due to the neglect of the carbon framework motion in the construction of the EVH. To improve upon the results obtained from the EVH formalism I included the carbon frame degrees of freedom and have carried out a full variation treatment in curvilinear coordinates. I have considered the 8 C-H stretches and 8 H-C-C in-plane bends of naphthalene as local mode oscillators and 17 coordinates belonging to the carbon framework motion as curvilinear normal mode oscillators. A quartic hamiltonian in a mixed local mode -normal mode basis was constructed including up to three body terms in both kinetic and potential energy part. The hamiltonian was subsequently recast into the ladder operator form and diagonal zed in a symmetry adapted basis with polyad constraints. Frequencies so obtained were compared to the experiment All these findings have been presented in Chapter 6 of this thesis. The concluding remark of the thesis and the future direction is presented in Chapter 7

Ce travail de thèse traite d'expériences d'astrophysique de laboratoire sur des matériaux aromatiques, étudiés pour la plupart dans des conditions proches de celles rencontrées dans les milieux interstellaire et circumstellaire, comprenant rayons cosmiques et irradiations UV. Ces dernières sont à l’origine de bandes d'émission dans l'infrarouge moyen, dont les porteurs supposés sont principalement les Hydrocarbures Aromatiques Polycycliques (PAHs) et les nanoparticules aromatiques. Un nouveau spectromètre, FIREFLY (Fluorescence in the InfraRed from Excited FLYing molecules), contenant une réplique des filtres circulaires variables à bord de l'instrument ISOCAM du satellite ISO, a été mis au point et caractérisé au cours de cette thèse. Cet instrument a permis de mesurer la désexcitation infrarouge dans la région des modes d'élongations CH (3.3 µm, soit 3000 cm-1) des dérivés du benzène et du naphtalène à température ambiante, suite à l’absorption d'un photon UV. Ceci montre, avec l’appui de la modélisation, que la spectroscopie d’émission IR est un outil puissant permettant de comprendre les effets d'anharmonicité liés à l'énergie interne, l’isomérisation voire même la dynamique intramoléculaire non-adiabatique, au travers de la spectroscopie d'excitation de fluorescence infrarouge (une nouvelle technique). Ce travail est préliminaire à la future mesure de fluorescence infrarouge de nanoparticules aromatiques en phase gazeuse et à basse température produites par une flamme basse pression, dans le but de comparer les spectres de laboratoire avec les observations astrophysiques. Le dépôt d'énergie par les rayons cosmiques a été étudié grâce à une autre expérience, l'irradiation ionique d'analogues de poussières interstellaires et circumstellaires (suies produites par la flamme basse pression), mettant en évidence une réorganisation chimique. Les suies sont caractérisées par différents diagnostics complémentaires tels que la Microscopie Electronique en Transmission à Haute Résolution et les spectroscopies infrarouge à Transformée de Fourier et Raman. L'ensemble des informations tirées permet de mieux cerner la nanostructuration des analogues et ainsi mieux identifier les différentes signatures spectrales astrophysiques (interprétation de la bande à 7.7 µm comme une bande de défauts). Grâce à ces expériences, nous espérons améliorer notre compréhension de la structure, croissance et évolution de la poussière, d'un point de vue astrophysique. / The work presented in this thesis deals with laboratory astrophysics experiments of aromatic compounds, mostly studied in conditions similar to those encountered in interstellar and circumstellar environments, including cosmic rays and UV irradiations. These are the source of mid-infrared emission bands whose carriers are supposed to be composed mainly of Polycyclic Aromatic Hydrocarbons (PAHs) and aromatic nanoparticles.A new spectrometer, FIREFLY (Fluorescence in the InfraRed from Excited FLYing molecules), containing a replica of the circular variable filters of the ISOCAM instrument on-board the ISO satellite, was developed and characterized in this thesis. This instrument was used to measure the infrared de-excitation in the CH stretching mode region (3.3 µm or 3000 cm-1) from benzene derivatives and from naphthalene at room temperature, following the absorption of a single UV photon. With the support of modelling, IR emission spectroscopy is a powerful tool for understanding the effects of anharmonicity related to the internal energy, isomerization and also the non-adiabatic intramolecular dynamics through the new technique of IR fluorescence excitation spectroscopy. This work is preliminary to the infrared fluorescence measurement of low-temperature gas phase aromatic nanoparticles produced by a low pressure flame, in order to compare the laboratory spectra with astrophysical observations.Cosmic-ray energy deposition has been studied with another experiment: the ionic irradiation of interstellar and circumstellar dust analogues (soot produced by the low pressure flame) highlights a chemical reorganization. Soot is characterized by complementary diagnostics such as High Resolution Transmission Electron Microscopy, Fourier Transform Infrared spectroscopy and Raman spectroscopy. The results help identify the analogue nanostructures and better identify the astrophysical spectral signatures (interpretation of the 7.7 µm band as a defect band). Through these experiences, we hope to improve our understanding of the structure, growth and evolution of the dust, in the astrophysical context.